DHCP-Server Konfigurieren unter CentOS 7.x

Für die Zuweisung der Netzwerkkonfiguration an den Klienten durch unseren Server bedienen wir uns des DHCP1), DHCP ist eine Ergänzung und Erweiterung von BOOTP2). DHCP wurde im RFC 2131 definiert und bekam von der Internet Assigned Numbers Authority die beiden UDP-Ports 67 und 68 zugewiesen.

Mittels DHCP ist die automatische Einbindung eines neuen Klienten in unser bestehendes Netzwerk ohne große manuelle Konfiguration möglich. Am Klient muss daher nur der automatische Bezug der IP-Adresse eingestellt sein. Beim Start des Klienten am Netz kann dieser die IP-Adresse, die Netzmaske, das Gateway, DNS-Server und weitere Konfigurationsparameter vom DHCP-Server beziehen. Neben diesen klassischen Parametern zählen hierzu auch die Verwendung einer Reihe von weiteren IP-Variablen, wie z.B.: X-Display- , Time-, Swap-, NIS-Server und die Unterstützung von Vendor-Code-Identifiern zum Einsatz im Bereich PXE3) finden.

Beim Starten eines Klienten frägt dieser über einen Broadcast im gesamten Netzwerk nach (s)einer IP-Adresse. Als Antwort auf seinen Broadcast erhält er die beiden wichtigsten Parameter:

  • IP-Adresse
  • Lease-Time

Darüber hinaus können optional noch weitere Parameter mit übergeben werden, wie z.B.:

  • Default-Route
  • Netzmaske
  • DNS-Server-Adressen
  • WINS-Server
  • Broadcast-Adresse
  • IP-Varialen
  • sowie noch weitere Parameter

Der grundsätzliche Ablauf bei der Adress-Anfrage folgt dabei folgendem Schema. Die Kommunikation zwischen dem Server (Port 67) und dem Klienten (Port 68) erfolgte mittels UPD4).

Beim Booten des Klienten frägt dieser mit einer DHCPDISCOVER-Nachricht via Broadcast nach seiner Konfiguration. Zu diesem Zeitpunkt besitzt er noch keine eigene IP-Adresse und er kennt auch noch nicht, in welchem Netz er sich befindet. Lediglich seine MAC5)-Adresse seines Netzwerkinterfaces ist ihm bekannt. Aus diesem Grund sendet er ein Broadcastpaket mit der Quelladresse 0.0.0.0 und an die Zieladresse 255.255.255.255.

1. Stufe der DHCP-Adressanfrage - DHCPDISCOVER

 Sep 12 21:34:12 nss dhcpd: DHCPDISCOVER from 00:04:13:23:3f:b5 via eth0

Dieses Broadcast-Paket beantwortet nun der DHCP-Server mit einer DHCPOFFER-Nachricht. Das Antwortpaket beinhaltet bereits als Zieladresse die IP, welche der Klient in Zukunft bekommen soll. Da bei der vorherigen Anfrage des Klienten, dieser seine eigene MAC-Adresse mitschickte, kann nun auf diese Weise die DHCPOFFER-Nachricht ihr Ziel finden.

2. Stufe der DHCP-Adressanfrage - DHCPOFFER

 Sep 12 21:34:12 nss dhcpd: DHCPOFFER on 192.168.10.61 to 00:04:13:23:3f:b5 via eth0

Der Klient hat also vom DHCP-Server ein sogenanntes Angebot (offer) bekommen und entscheidet nun, ob es für ihn so in Ordnung ist. Trifft dies zu, sendet er eine DHCPREQUEST-Nachricht, an den DHCP-Server um diesen mitzuteilen, daß er diese Konfiguration nutzen will.

3. Stufe der DHCP-Adressanfrage - DHCPREQUEST

 Sep 12 21:34:12 nss dhcpd: DHCPREQUEST for 192.168.10.61 (192.168.10.1) from 00:04:13:23:3f:b5 via eth0

Der DHCP-Server bestätigt dies und sendet eine DHCPACK-Nachricht, somit besitzt der Klient nun seine eigene IP-Adresse und kennt ggf. noch weitere Parameter für seine weitere Netzwerkkommunkation.

3. Stufe der DHCP-Adressanfrage - DHCPACK

 Sep 12 21:34:12 nss dhcpd: DHCPACK on 192.168.10.61 to 00:04:13:23:3f:b5 via eth0

Der gesamte erfolgreiche Ablauf aus Sicht des DHCP-Servers entspricht folgendem Diagramm.

erfolgreiche Ablauf aus Sicht des DHCP-Serverserfolgreiche Ablauf aus Sicht des DHCP-Servers  DHCP - SERVER  DHCP - SERVER  Client  Client (Port 67) DHCPDISCOVERDHCPDISCOVER mitMAC 00:04:13:23:3f:b5DHCPOFFER (Port 68)DHCPOFFER mit Angabeder IP 192.168.10.61an MAC 00:04:13:23:3f:b5(Port 67) DHCPREQUESTDHCPREQUEST mit Angabeder IP 192.168.10.61und MAC 00:04:13:23:3f:b5DHCPACK (Port 68)DHCPACK mit Angabeder IP 192.168.10.61und der MAC 00:04:13:23:3f:b5

Im syslog des DHCP-Servers wird der Ablauf wie folgt festgehalten:

Sep 12 21:34:12 nss dhcpd: DHCPDISCOVER from 00:04:13:23:3f:b5 via eth0
Sep 12 21:34:12 nss dhcpd: DHCPOFFER on 192.168.10.61 to 00:04:13:23:3f:b5 via eth0
Sep 12 21:34:12 nss dhcpd: DHCPREQUEST for 192.168.10.61 (192.168.10.1) from 00:04:13:23:3f:b5 via eth0
Sep 12 21:34:12 nss dhcpd: DHCPACK on 192.168.10.61 to 00:04:13:23:3f:b5 via eth0

Sollte die ganze Prozedur Fehl schlagen, z.B. weil der Klient herausgefunden hat, daß die IP-Adresse doppelt vergeben ist, sendet er eine DHCPDECLINE-Nachricht an der Server. Im Falle einer DHCPDECLINE-Nachricht, sperrt der Server die Adresse für die interne Vergabe und die gesamte Vergabeprozedur beginnt erneut von vorne.

Zusammen mit seiner IP-Adresse erhält der Klient in der DHCPACK-Nachricht auch eine Lease-Time mitgeteilt, welche ihm mitteilt, wie lange die IP-Adresse für ihn reserviert ist. Im RFC Standard wurde definiert, daß der Klient nach der Hälfte der Lease-Time einen erneuten DHCPREQUEST sendet. So teilt er dem Server mit, daß er weiterhin die für ihn reservierte IP-Adresse behalten möchte. Nach Erhalt dieser Nachricht sendet der DHCP-Server eine identische DHCPACK-Nachricht an den Client zurück, in der dann die aktuelle neue Lease-Time mitgeteilt wird. Die IP-Adresse ist somit verlängert und der DCHP-Refresh ist komplett. Sollte der Klient es versäumen eine Verlängerung zu beantragen, muss er die Konfiguration des Netzwerkinterfaces verwerfen und der DHCP-Request startet erneut mit einer DHCPDISCOVER-Nachricht.

Beim Herunterfahren eines Klienten kann dieser dem Server mit einer DHCPRELEASE-Nachricht den Server informieren, damit dieser die Adresse wieder freigeben kann.

 Sep 12 21:58:17 nss dhcpd: DHCPRELEASE of 192.168.10.238 from 00:17:a4:7d:26:1a (hpc6180) via eth0 (found)

Der Klient hat aber auch die Möglichkeit, seine zuletzt zugewiesene IP-Adresse über den Reboot hinweg zu „merken“. Dies kann z.B. dann der Fall sein, wenn die Lease-Time, noch nicht abgelaufen ist, oder dem Klienten eine feste IP-Adresse zugeteilt wurde. Dann entfallen die Initialisierungsschritte und der Klient schickt direkt eine DHCPREQUEST-Nachricht an den DHCP-Server. Dieser bestätigt entweder die Anfrage oder sendet eine DHCPNAK-Nachricht um dem Klienten mitzuteilen, daß dieser seine gespeicherten Konfigurationen zu löschen, und die Anfrage komplett von vorne zu beginnen hat.

 Sep 12 22:01:13 nss dhcpd: DHCPREQUEST for 192.168.10.15 from 00:17:a4:7d:26:1a via eth0
 Sep 12 22:01:13 nss dhcpd: DHCPACK on 192.168.10.15 to 00:17:a4:7d:26:1a via eth0

Die Installation und Konfiguration des DHCP-Servers gestaltet sich relativ einfach.

Zuerst ist via yum der dhcp-Server zu installieren.

 # yum install dhcp -y

Um sich einen ersten Überblick zum gerade eben installiertem RPM-Paket zu verschaffen, rufen wir den Befehl rpm mit der option -qil auf.

 # rpm -qil dhcp
Name        : dhcp
Epoch       : 12
Version     : 4.2.5
Release     : 42.el7.centos
Architecture: x86_64
Install Date: Thu 11 Feb 2016 10:43:32 AM CET
Group       : System Environment/Daemons
Size        : 1452508
License     : ISC
Signature   : RSA/SHA256, Wed 25 Nov 2015 03:25:28 PM CET, Key ID 24c6a8a7f4a80eb5
Source RPM  : dhcp-4.2.5-42.el7.centos.src.rpm
Build Date  : Thu 19 Nov 2015 10:40:07 PM CET
Build Host  : worker1.bsys.centos.org
Relocations : (not relocatable)
Packager    : CentOS BuildSystem <http://bugs.centos.org>
Vendor      : CentOS
URL         : http://isc.org/products/DHCP/
Summary     : Dynamic host configuration protocol software
Description :
DHCP (Dynamic Host Configuration Protocol) is a protocol which allows
individual devices on an IP network to get their own network
configuration information (IP address, subnetmask, broadcast address,
etc.) from a DHCP server. The overall purpose of DHCP is to make it
easier to administer a large network.

To use DHCP on your network, install a DHCP service (or relay agent),
and on clients run a DHCP client daemon.  The dhcp package provides
the ISC DHCP service and relay agent.
/etc/NetworkManager
/etc/NetworkManager/dispatcher.d
/etc/NetworkManager/dispatcher.d/12-dhcpd
/etc/dhcp
/etc/dhcp/dhcpd.conf
/etc/dhcp/dhcpd6.conf
/etc/openldap/schema/dhcp.schema
/etc/sysconfig/dhcpd
/usr/bin/omshell
/usr/lib/systemd/system/dhcpd.service
/usr/lib/systemd/system/dhcpd6.service
/usr/lib/systemd/system/dhcrelay.service
/usr/sbin/dhcpd
/usr/sbin/dhcrelay
/usr/share/doc/dhcp-4.2.5
/usr/share/doc/dhcp-4.2.5/dhcpd.conf.example
/usr/share/doc/dhcp-4.2.5/dhcpd6.conf.example
/usr/share/doc/dhcp-4.2.5/ldap
/usr/share/doc/dhcp-4.2.5/ldap/README.ldap
/usr/share/doc/dhcp-4.2.5/ldap/dhcp.schema
/usr/share/doc/dhcp-4.2.5/ldap/dhcpd-conf-to-ldap
/usr/share/man/man1/omshell.1.gz
/usr/share/man/man5/dhcpd.conf.5.gz
/usr/share/man/man5/dhcpd.leases.5.gz
/usr/share/man/man8/dhcpd.8.gz
/usr/share/man/man8/dhcrelay.8.gz
/usr/share/systemtap/tapset/dhcpd.stp
/var/lib/dhcpd
/var/lib/dhcpd/dhcpd.leases
/var/lib/dhcpd/dhcpd6.leases

Die Konfiguration eines DHCP-Servers erfolgt für die beiden Adress-Varianten IPv4 und IPv6 in zwei getrennten Daemon. Im ersten Schritt werden wir nun einen DHCP-Daemon für IPv4 aufsetzen.

dhcp manpage

 # man 5 dhcpd.conf
dhcpd.conf(5)                             File Formats Manual                             dhcpd.conf(5)



NAME
       dhcpd.conf - dhcpd configuration file

DESCRIPTION
       The  dhcpd.conf  file contains configuration information for dhcpd, the Internet Systems Consor‐
       tium DHCP Server.

       The dhcpd.conf file is a free-form ASCII text file.   It  is  parsed  by  the  recursive-descent
       parser  built into dhcpd.  The file may contain extra tabs and newlines for formatting purposes.
       Keywords in the file are case-insensitive.  Comments may be  placed  anywhere  within  the  file
       (except within quotes).  Comments begin with the # character and end at the end of the line.

       The  file  essentially  consists  of a list of statements.  Statements fall into two broad cate‐
       gories - parameters and declarations.

       Parameter statements either say how to do something (e.g., how long a lease to  offer),  whether
       to do something (e.g., should dhcpd provide addresses to unknown clients), or what parameters to
       provide to the client (e.g., use gateway 220.177.244.7).

       Declarations are used to describe the topology of the network, to describe clients on  the  net‐
       work, to provide addresses that can be assigned to clients, or to apply a group of parameters to
       a group of declarations.  In any group of parameters and declarations, all  parameters  must  be
       specified before any declarations which depend on those parameters may be specified.

       Declarations  about network topology include the shared-network and the subnet declarations.  If
       clients on a subnet are to be assigned addresses dynamically, a range  declaration  must  appear
       within the subnet declaration.  For clients with statically assigned addresses, or for installa‐
       tions where only known clients will be served, each such client must have  a  host  declaration.
       If  parameters  are to be applied to a group of declarations which are not related strictly on a
       per-subnet basis, the group declaration can be used.

       For every subnet which will be served, and for every subnet to which the  dhcp  server  is  con‐
       nected, there must be one subnet declaration, which tells dhcpd how to recognize that an address
       is on that subnet.  A subnet declaration is required for each subnet even if no  addresses  will
       be dynamically allocated on that subnet.

       Some  installations have physical networks on which more than one IP subnet operates.  For exam‐
       ple, if there is a site-wide requirement that 8-bit subnet masks be used, but a department  with
       a single physical ethernet network expands to the point where it has more than 254 nodes, it may
       be necessary to run two 8-bit subnets on the same ethernet until such time  as  a  new  physical
       network  can  be  added.   In  this case, the subnet declarations for these two networks must be
       enclosed in a shared-network declaration.

       Note that even when the shared-network declaration is absent, an empty one  is  created  by  the
       server  to contain the subnet (and any scoped parameters included in the subnet).  For practical
       purposes, this means that "stateless" DHCP clients, which are not tied to addresses (and  there‐
       fore subnets) will receive the same configuration as stateful ones.

       Some sites may have departments which have clients on more than one subnet, but it may be desir‐
       able to offer those clients a uniform set of parameters which are different than what  would  be
       offered  to  clients  from  other  departments  on  the  same subnet.  For clients which will be
       declared explicitly with host declarations, these declarations can be enclosed in a group decla‐
       ration  along  with  the  parameters  which  are  common  to that department.  For clients whose
       addresses will be dynamically assigned, class declarations and conditional declarations  may  be
       used to group parameter assignments based on information the client sends.

       When  a  client  is to be booted, its boot parameters are determined by consulting that client's
       host declaration (if any), and then consulting any class declarations matching the client,  fol‐
       lowed  by  the  pool,  subnet and shared-network declarations for the IP address assigned to the
       client.  Each of these declarations itself appears within a lexical scope, and all  declarations
       at  less  specific lexical scopes are also consulted for client option declarations.  Scopes are
       never considered twice, and if parameters are declared in more than  one  scope,  the  parameter
       declared in the most specific scope is the one that is used.

       When  dhcpd tries to find a host declaration for a client, it first looks for a host declaration
       which has a fixed-address declaration that lists an IP address that is valid for the  subnet  or
       shared  network  on which the client is booting.  If it doesn't find any such entry, it tries to
       find an entry which has no fixed-address declaration.

EXAMPLES
       A typical dhcpd.conf file will look something like this:

       global parameters...

       subnet 204.254.239.0 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.10 204.254.239.30;
       }

       subnet 204.254.239.32 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.42 204.254.239.62;
       }

       subnet 204.254.239.64 netmask 255.255.255.224 {
         subnet-specific parameters...
         range 204.254.239.74 204.254.239.94;
       }

       group {
         group-specific parameters...
         host zappo.test.isc.org {
           host-specific parameters...
         }
         host beppo.test.isc.org {
           host-specific parameters...
         }
         host harpo.test.isc.org {
           host-specific parameters...
         }
       }

                                                   Figure 1


       Notice that at the beginning of the file, there's a place for global parameters.  These might be
       things  like the organization's domain name, the addresses of the name servers (if they are com‐
       mon to the entire organization), and so on.  So, for example:

            option domain-name "isc.org";
            option domain-name-servers ns1.isc.org, ns2.isc.org;

                                                   Figure 2

       As you can see in Figure 2, you can specify host addresses  in  parameters  using  their  domain
       names  rather than their numeric IP addresses.  If a given hostname resolves to more than one IP
       address (for example, if that host has two  ethernet  interfaces),  then  where  possible,  both
       addresses are supplied to the client.

       The  most obvious reason for having subnet-specific parameters as shown in Figure 1 is that each
       subnet, of necessity, has its own router.  So for the first subnet, for example, there should be
       something like:

            option routers 204.254.239.1;

       Note  that the address here is specified numerically.  This is not required - if you have a dif‐
       ferent domain name for each interface on your router,  it's  perfectly  legitimate  to  use  the
       domain name for that interface instead of the numeric address.  However, in many cases there may
       be only one domain name for all of a router's IP addresses, and it would not be  appropriate  to
       use that name here.

       In Figure 1 there is also a group statement, which provides common parameters for a set of three
       hosts - zappo, beppo and harpo.  As you can see, these hosts are all in the test.isc.org domain,
       so  it  might  make sense for a group-specific parameter to override the domain name supplied to
       these hosts:

            option domain-name "test.isc.org";

       Also, given the domain they're in, these are probably test machines.  If we wanted to  test  the
       DHCP leasing mechanism, we might set the lease timeout somewhat shorter than the default:

            max-lease-time 120;
            default-lease-time 120;

       You  may  have  noticed  that  while some parameters start with the option keyword, some do not.
       Parameters starting with the option keyword correspond to actual DHCP options, while  parameters
       that  do not start with the option keyword either control the behavior of the DHCP server (e.g.,
       how long a lease dhcpd will give out), or specify client parameters that are not optional in the
       DHCP protocol (for example, server-name and filename).

       In  Figure  1,  each  host had host-specific parameters.  These could include such things as the
       hostname option, the name of a file to upload (the filename parameter) and the  address  of  the
       server from which to upload the file (the next-server parameter).  In general, any parameter can
       appear anywhere that parameters are allowed, and will be applied according to the scope in which
       the parameter appears.

       Imagine  that  you have a site with a lot of NCD X-Terminals.  These terminals come in a variety
       of models, and you want to specify the boot files for each model.  One way to do this  would  be
       to have host declarations for each server and group them by model:

       group {
         filename "Xncd19r";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
         host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
         host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }
       }

       group {
         filename "Xncd19c";
         next-server ncd-booter;

         host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
         host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }
       }

       group {
         filename "XncdHMX";
         next-server ncd-booter;

         host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
         host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
         host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }
       }

ADDRESS POOLS
       The pool declaration can be used to specify a pool of addresses that will be treated differently
       than another pool of addresses, even on the same network segment or subnet.   For  example,  you
       may  want to provide a large set of addresses that can be assigned to DHCP clients that are reg‐
       istered to your DHCP server, while providing a smaller set of  addresses,  possibly  with  short
       lease times, that are available for unknown clients.  If you have a firewall, you may be able to
       arrange for addresses from one pool to be allowed access to the  Internet,  while  addresses  in
       another  pool  are  not, thus encouraging users to register their DHCP clients.  To do this, you
       would set up a pair of pool declarations:

       subnet 10.0.0.0 netmask 255.255.255.0 {
         option routers 10.0.0.254;

         # Unknown clients get this pool.
         pool {
           option domain-name-servers bogus.example.com;
           max-lease-time 300;
           range 10.0.0.200 10.0.0.253;
           allow unknown-clients;
         }

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           max-lease-time 28800;
           range 10.0.0.5 10.0.0.199;
           deny unknown-clients;
         }
       }

       It is also possible to set up entirely different subnets for known and unknown clients - address
       pools  exist  at the level of shared networks, so address ranges within pool declarations can be
       on different subnets.

       As you can see in the preceding example, pools can have permit lists that control which  clients
       are  allowed  access to the pool and which aren't.  Each entry in a pool's permit list is intro‐
       duced with the allow or deny keyword.  If a pool has a permit list, then only those clients that
       match  specific  entries  on  the permit list will be eligible to be assigned addresses from the
       pool.  If a pool has a deny list, then only those clients that do not match any entries  on  the
       deny  list will be eligible.   If both permit and deny lists exist for a pool, then only clients
       that match the permit list and do not match the deny list will be allowed access.

DYNAMIC ADDRESS ALLOCATION
       Address allocation is actually only done when a client is in the  INIT  state  and  has  sent  a
       DHCPDISCOVER message.  If the client thinks it has a valid lease and sends a DHCPREQUEST to ini‐
       tiate or renew that lease, the server has only three choices - it can  ignore  the  DHCPREQUEST,
       send  a  DHCPNAK to tell the client it should stop using the address, or send a DHCPACK, telling
       the client to go ahead and use the address for a while.

       If the server finds the address the client is requesting, and that address is available  to  the
       client,  the  server  will send a DHCPACK.  If the address is no longer available, or the client
       isn't permitted to have it, the server will send a DHCPNAK.  If the server knows  nothing  about
       the  address,  it will remain silent, unless the address is incorrect for the network segment to
       which the client has been attached and the server is authoritative for that network segment,  in
       which case the server will send a DHCPNAK even though it doesn't know about the address.

       There  may be a host declaration matching the client's identification.  If that host declaration
       contains a fixed-address declaration that lists an IP address that is valid for the network seg‐
       ment  to  which  the  client  is connected.  In this case, the DHCP server will never do dynamic
       address allocation.  In this case, the client is required to take the address specified  in  the
       host  declaration.   If  the  client sends a DHCPREQUEST for some other address, the server will
       respond with a DHCPNAK.

       When the DHCP server allocates a new address for a client (remember, this only  happens  if  the
       client  has  sent a DHCPDISCOVER), it first looks to see if the client already has a valid lease
       on an IP address, or if there is an old IP address the client had before that  hasn't  yet  been
       reassigned.   In  that case, the server will take that address and check it to see if the client
       is still permitted to use it.  If the client is no longer permitted to  use  it,  the  lease  is
       freed  if  the server thought it was still in use - the fact that the client has sent a DHCPDIS‐
       COVER proves to the server that the client is no longer using the lease.

       If no existing lease is found, or if the client is forbidden to receive the existing lease, then
       the server will look in the list of address pools for the network segment to which the client is
       attached for a lease that is not in use and that the client is  permitted  to  have.   It  looks
       through  each  pool  declaration in sequence (all range declarations that appear outside of pool
       declarations are grouped into a single pool with no permit list).  If the permit  list  for  the
       pool allows the client to be allocated an address from that pool, the pool is examined to see if
       there is an address available.  If so, then the client is  tentatively  assigned  that  address.
       Otherwise,  the  next  pool  is  tested.   If no addresses are found that can be assigned to the
       client, no response is sent to the client.

       If an address is found that the client is permitted to have, and that has never been assigned to
       any client before, the address is immediately allocated to the client.  If the address is avail‐
       able for allocation but has been previously assigned to a different client, the server will keep
       looking in hopes of finding an address that has never before been assigned to a client.

       The DHCP server generates the list of available IP addresses from a hash table.  This means that
       the addresses are not sorted in any particular order, and so it is not possible to  predict  the
       order  in  which  the DHCP server will allocate IP addresses.  Users of previous versions of the
       ISC DHCP server may have become accustomed to the DHCP server allocating IP addresses in ascend‐
       ing  order,  but this is no longer possible, and there is no way to configure this behavior with
       version 3 of the ISC DHCP server.

IP ADDRESS CONFLICT PREVENTION
       The DHCP server checks IP addresses to see if they are in use before allocating them to clients.
       It  does  this by sending an ICMP Echo request message to the IP address being allocated.  If no
       ICMP Echo reply is received within a second, the address is assumed to be free.   This  is  only
       done for leases that have been specified in range statements, and only when the lease is thought
       by the DHCP server to be free - i.e., the DHCP server or its failover peer has  not  listed  the
       lease as in use.

       If  a response is received to an ICMP Echo request, the DHCP server assumes that there is a con‐
       figuration error - the IP address is in use by some host on the  network  that  is  not  a  DHCP
       client.  It marks the address as abandoned, and will not assign it to clients.

       If  a DHCP client tries to get an IP address, but none are available, but there are abandoned IP
       addresses, then the DHCP server will attempt to reclaim an abandoned IP address.  It  marks  one
       IP  address  as  free,  and then does the same ICMP Echo request check described previously.  If
       there is no answer to the ICMP Echo request, the address is assigned to the client.

       The DHCP server does not cycle through abandoned IP addresses if the first IP address  it  tries
       to  reclaim  is  free.   Rather,  when  the  next DHCPDISCOVER comes in from the client, it will
       attempt a new allocation using the same method described here, and will typically try a  new  IP
       address.

DHCP FAILOVER
       This  version of the ISC DHCP server supports the DHCP failover protocol as documented in draft-
       ietf-dhc-failover-12.txt.  This is not a final protocol document, and we have not done  interop‐
       erability  testing  with other vendors' implementations of this protocol, so you must not assume
       that this implementation conforms to the standard.  If you wish to use  the  failover  protocol,
       make sure that both failover peers are running the same version of the ISC DHCP server.

       The  failover  protocol allows two DHCP servers (and no more than two) to share a common address
       pool.  Each server will have about half of the available IP addresses in the pool at  any  given
       time for allocation.  If one server fails, the other server will continue to renew leases out of
       the pool, and will allocate new addresses out of the roughly half of available addresses that it
       had when communications with the other server were lost.

       It  is possible during a prolonged failure to tell the remaining server that the other server is
       down, in which case the remaining server will (over time) reclaim all the  addresses  the  other
       server had available for allocation, and begin to reuse them.  This is called putting the server
       into the PARTNER-DOWN state.

       You can put the server into the PARTNER-DOWN state either by using the omshell (1) command or by
       stopping the server, editing the last failover state declaration in the lease file, and restart‐
       ing the server.  If you use this last method, change the "my state" line to:

       failover peer name state {
       my state partner-down;
       peer state state at date;
       }

       It is only required to change "my state" as shown above.

       When the other server comes back online, it should automatically detect that it has been offline
       and  request  a  complete update from the server that was running in the PARTNER-DOWN state, and
       then both servers will resume processing together.

       It is possible to get into a dangerous situation: if you put one server  into  the  PARTNER-DOWN
       state,  and  then  *that* server goes down, and the other server comes back up, the other server
       will not know that the first server was in the PARTNER-DOWN state, and may issue addresses  pre‐
       viously  issued  by  the  other  server to different clients, resulting in IP address conflicts.
       Before putting a server into PARTNER-DOWN state, therefore, make sure that the other server will
       not restart automatically.

       The failover protocol defines a primary server role and a secondary server role.  There are some
       differences in how primaries and secondaries act, but most of the differences simply have to  do
       with  providing a way for each peer to behave in the opposite way from the other.  So one server
       must be configured as primary, and the other must be configured as  secondary,  and  it  doesn't
       matter too much which one is which.

FAILOVER STARTUP
       When a server starts that has not previously communicated with its failover peer, it must estab‐
       lish communications with its failover peer and synchronize with it before it can serve  clients.
       This  can  happen  either because you have just configured your DHCP servers to perform failover
       for the first time, or because one of your failover servers has failed catastrophically and lost
       its database.

       The  initial  recovery process is designed to ensure that when one failover peer loses its data‐
       base and then resynchronizes, any leases that the failed server gave out before it  failed  will
       be  honored.   When the failed server starts up, it notices that it has no saved failover state,
       and attempts to contact its peer.

       When it has established contact, it asks the peer for a complete copy its peer's lease database.
       The  peer then sends its complete database, and sends a message indicating that it is done.  The
       failed server then waits until MCLT has passed, and once MCLT has passed both servers  make  the
       transition  back  into normal operation.  This waiting period ensures that any leases the failed
       server may have given out while out of contact with its partner will have expired.

       While the failed server is recovering, its partner remains  in  the  partner-down  state,  which
       means  that  it  is  serving  all clients.  The failed server provides no service at all to DHCP
       clients until it has made the transition into normal operation.

       In the case where both servers detect that they have never before communicated with their  part‐
       ner,  they  both come up in this recovery state and follow the procedure we have just described.
       In this case, no service will be provided to DHCP clients until MCLT has expired.

CONFIGURING FAILOVER
       In order to configure failover, you need  to  write  a  peer  declaration  that  configures  the
       failover  protocol, and you need to write peer references in each pool declaration for which you
       want to do failover.  You do not have to do failover for all pools on a given  network  segment.
       You must not tell one server it's doing failover on a particular address pool and tell the other
       it is not.  You must not have any common address pools on which you are not doing  failover.   A
       pool declaration that utilizes failover would look like this:

       pool {
            failover peer "foo";
            pool specific parameters
       };

       Dynamic  BOOTP leases are not compatible with failover, and, as such, you need to disallow BOOTP
       in pools that you are using failover for.

       The  server currently  does very  little  sanity checking,  so if  you configure  it  wrong,  it
       will  just  fail in odd ways.  I would recommend therefore that you either do  failover or don't
       do failover, but don't do any mixed pools.  Also,  use the same master  configuration  file  for
       both   servers,  and  have  a  separate file  that  contains  the  peer declaration and includes
       the master file.  This will help you to avoid configuration  mismatches.  As our  implementation
       evolves,   this will become  less of  a  problem.  A  basic  sample dhcpd.conf  file for  a pri‐
       mary server might look like this:

       failover peer "foo" {
         primary;
         address anthrax.rc.vix.com;
         port 647;
         peer address trantor.rc.vix.com;
         peer port 847;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;
       }

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

         [ primary | secondary ];

         This determines whether the server is primary or secondary, as described  earlier  under  DHCP
         FAILOVER.

       The address statement

         address address;

         The  address  statement  declares the IP address or DNS name on which the server should listen
         for connections from its failover peer, and also the value to use for the DHCP Failover Proto‐
         col server identifier.  Because this value is used as an identifier, it may not be omitted.

       The peer address statement

         peer address address;

         The peer address statement declares the IP address or DNS name to which the server should con‐
         nect to reach its failover peer for failover messages.

       The port statement

         port port-number;

         The port statement declares the TCP port on which the server  should  listen  for  connections
         from  its  failover peer.  This statement may be omitted, in which case the IANA assigned port
         number 647 will be used by default.

       The peer port statement

         peer port port-number;

         The peer port statement declares the TCP port to which the server should connect to reach  its
         failover  peer  for  failover messages.  This statement may be omitted, in which case the IANA
         assigned port number 647 will be used by default.

       The max-response-delay statement

         max-response-delay seconds;

         The max-response-delay statement tells the DHCP server  how  many  seconds  may  pass  without
         receiving a message from its failover peer before it assumes that connection has failed.  This
         number should be small enough that a transient network failure that breaks the connection will
         not  result  in  the servers being out of communication for a long time, but large enough that
         the server isn't constantly making and breaking connections.  This parameter  must  be  speci‐
         fied.

       The max-unacked-updates statement

         max-unacked-updates count;

         The max-unacked-updates statement tells the remote DHCP server how many BNDUPD messages it can
         send before it receives a BNDACK from the local system.   We  don't  have  enough  operational
         experience to say what a good value for this is, but 10 seems to work.  This parameter must be
         specified.

       The mclt statement

         mclt seconds;

         The mclt statement defines the Maximum Client Lead Time.  It must be specified on the primary,
         and  may  not be specified on the secondary.  This is the length of time for which a lease may
         be renewed by either failover peer without contacting the other.  The longer you set this, the
         longer  it will take for the running server to recover IP addresses after moving into PARTNER-
         DOWN state.  The shorter you set it, the more load your servers will experience when they  are
         not  communicating.   A value of something like 3600 is probably reasonable, but again bear in
         mind that we have no real operational experience with this.

       The split statement

         split index;

         The split statement specifies the split between the primary and secondary for the purposes  of
         load  balancing.   Whenever  a client makes a DHCP request, the DHCP server runs a hash on the
         client identification, resulting in value from 0 to 255.  This is used as an index into a  256
         bit  field.   If the bit at that index is set, the primary is responsible.  If the bit at that
         index is not set, the secondary is responsible.  The split value determines how  many  of  the
         leading  bits  are set to one.  So, in practice, higher split values will cause the primary to
         serve more clients than the secondary.  Lower split values, the converse.   Legal  values  are
         between 0 and 255, of which the most reasonable is 128.

       The hba statement

         hba colon-separated-hex-list;

         The  hba  statement  specifies  the split between the primary and secondary as a bitmap rather
         than a cutoff, which theoretically allows for finer-grained control.  In  practice,  there  is
         probably no need for such fine-grained control, however.  An example hba statement:

           hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
               00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00;

         This  is  equivalent to a split 128; statement, and identical.  The following two examples are
         also equivalent to a split of 128, but are not identical:

           hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:
               aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa;

           hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:
               55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55;

         They are equivalent, because half the bits are set to 0, half are set to 1 (0xa  and  0x5  are
         1010  and  0101  binary  respectively)  and consequently this would roughly divide the clients
         equally between the servers.  They are not identical, because the actual peers this would load
         balance to each server are different for each example.

         You must only have split or hba defined, never both.  For most cases, the fine-grained control
         that hba offers isn't necessary, and split should be used.

       The load balance max seconds statement

         load balance max seconds seconds;

         This statement allows you to configure a cutoff after which load balancing is  disabled.   The
         cutoff  is  based  on  the  number  of seconds since the client sent its first DHCPDISCOVER or
         DHCPREQUEST message, and only works with clients that correctly implement  the  secs  field  -
         fortunately  most clients do.  We recommend setting this to something like 3 or 5.  The effect
         of this is that if one of the failover peers gets into a  state  where  it  is  responding  to
         failover  messages  but  not  responding to some client requests, the other failover peer will
         take over its client load automatically as the clients retry.

       The auto-partner-down statement

         auto-partner-down seconds;

         This statement instructs the server to initiate a timed delay  upon  entering  the  communica‐
         tions-interrupted state (any situation of being out-of-contact with the remote failover peer).
         At the conclusion of the timer, the server will automatically enter  the  partner-down  state.
         This  permits  the  server  to  allocate  leases  from  the partner's free lease pool after an
         STOS+MCLT timer expires, which can be dangerous if the partner is in  fact  operating  at  the
         time (the two servers will give conflicting bindings).

         Think very carefully before enabling this feature.  The partner-down and communications-inter‐
         rupted states are intentionally segregated because there do exist situations where a  failover
         server  can  fail to communicate with its peer, but still has the ability to receive and reply
         to requests from DHCP clients.  In general, this feature should only be used in those  deploy‐
         ments where the failover servers are directly connected to one another, such as by a dedicated
         hardwired link ("a heartbeat cable").

         A zero value disables the auto-partner-down feature (also the default), and any positive value
         indicates the time in seconds to wait before automatically entering partner-down.

       The Failover pool balance statements.

          max-lease-misbalance percentage;
          max-lease-ownership percentage;
          min-balance seconds;
          max-balance seconds;

         This version of the DHCP Server evaluates pool balance on a schedule, rather than on demand as
         leases are allocated.  The latter approach proved to be slightly klunky when pool  misbalanced
         reach total saturation...when any server ran out of leases to assign, it also lost its ability
         to notice it had run dry.

         In order to understand pool balance, some elements of its operation first need to be  defined.
         First,  there  are  ´free´  and ´backup´ leases.  Both of these are referred to as ´free state
         leases´.  ´free´ and ´backup´ are ´the free states´ for the purpose  of  this  document.   The
         difference  is that only the primary may allocate from ´free´ leases unless under special cir‐
         cumstances, and only the secondary may allocate ´backup´ leases.

         When pool balance is performed, the only plausible expectation is to provide a 50/50 split  of
         the  free  state  leases  between  the  two servers.  This is because no one can predict which
         server will fail, regardless of the relative load placed upon the two servers, so giving  each
         server  half the leases gives both servers the same amount of ´failure endurance´.  Therefore,
         there is no way to configure any different behaviour, outside of some very  small  windows  we
         will describe shortly.

         The first thing calculated on any pool balance run is a value referred to as ´lts´, or "Leases
         To Send".  This, simply, is the difference in the count of free and backup leases, divided  by
         two.   For  the secondary, it is the difference in the backup and free leases, divided by two.
         The resulting value is signed: if it is positive, the local server is  expected  to  hand  out
         leases  to  retain  a  50/50 balance.  If it is negative, the remote server would need to send
         leases to balance the pool.  Once the lts value reaches zero, the pool is  perfectly  balanced
         (give or take one lease in the case of an odd number of total free state leases).

         The  current  approach is still something of a hybrid of the old approach, marked by the pres‐
         ence of the max-lease-misbalance statement.  This parameter configures what used to be  a  10%
         fixed  value in previous versions: if lts is less than free+backup * max-lease-misbalance per‐
         cent, then the server will skip balancing a given pool (it won't  bother  moving  any  leases,
         even  if  some  leases  "should"  be moved).  The meaning of this value is also somewhat over‐
         loaded, however, in that it also governs the estimation of when to attempt to balance the pool
         (which  may  then  also be skipped over).  The oldest leases in the free and backup states are
         examined.  The time they have resided in their respective queues is used  as  an  estimate  to
         indicate  how  much time it is probable it would take before the leases at the top of the list
         would be consumed (and thus, how long it would take to use all leases in  that  state).   This
         percentage  is  directly multiplied by this time, and fit into the schedule if it falls within
         the min-balance and max-balance configured values.  The scheduled  pool  check  time  is  only
         moved in a downwards direction, it is never increased.  Lastly, if the lts is more than double
         this number in the negative direction, the local server will ´panic´ and transmit  a  Failover
         protocol POOLREQ message, in the hopes that the remote system will be woken up into action.

         Once  the  lts value exceeds the max-lease-misbalance percentage of total free state leases as
         described above, leases are moved to the remote server.  This is done in two passes.

         In the first pass, only leases whose most recent bound client would have been  served  by  the
         remote server - according to the Load Balance Algorithm (see above split and hba configuration
         statements) - are given away to the peer.  This first pass will happily continue to give  away
         leases,  decrementing the lts value by one for each, until the lts value has reached the nega‐
         tive of the total number of leases multiplied by the max-lease-ownership percentage.  So it is
         through this value that you can permit a small misbalance of the lease pools - for the purpose
         of giving the peer more than a 50/50 share of leases in the hopes  that  their  clients  might
         some  day  return and be allocated by the peer (operating normally).  This process is referred
         to as ´MAC Address Affinity´, but this is somewhat misnamed: it applies equally to DHCP Client
         Identifier  options.   Note  also that affinity is applied to leases when they enter the state
         ´free´ from ´expired´ or ´released´.  In this case also, leases will not be moved from free to
         backup if the secondary already has more than its share.

         The  second pass is only entered into if the first pass fails to reduce the lts underneath the
         total number of free state leases multiplied by the max-lease-ownership percentage.   In  this
         pass,  the oldest leases are given over to the peer without second thought about the Load Bal‐
         ance Algorithm, and this continues until the lts falls under this value.   In  this  way,  the
         local  server will also happily keep a small percentage of the leases that would normally load
         balance to itself.

         So, the max-lease-misbalance value acts as a behavioural gate.  Smaller values will cause more
         leases  to  transition  states to balance the pools over time, higher values will decrease the
         amount of change (but may lead to pool starvation if there's a run on leases).

         The max-lease-ownership value permits a small (percentage) skew in the lease balance of a per‐
         centage of the total number of free state leases.

         Finally, the min-balance and max-balance make certain that a scheduled rebalance event happens
         within a reasonable timeframe (not to be thrown off by, for example, a 7 year old free lease).

         Plausible values for the percentages lie between 0 and 100, inclusive, but values over 50  are
         indistinguishable  from one another (once lts exceeds 50% of the free state leases, one server
         must therefore have 100% of the leases in its respective free state).  It  is  recommended  to
         select  a  max-lease-ownership  value that is lower than the value selected for the max-lease-
         misbalance value.  max-lease-ownership defaults to 10, and  max-lease-misbalance  defaults  to
         15.

         Plausible  values  for the min-balance and max-balance times also range from 0 to (2^32)-1 (or
         the limit of your local time_t value), but default to values  60  and  3600  respectively  (to
         place balance events between 1 minute and 1 hour).

CLIENT CLASSING
       Clients  can be separated into classes, and treated differently depending on what class they are
       in.  This separation can be done either with a conditional statement, or with a match  statement
       within  the class declaration.  It is possible to specify a limit on the total number of clients
       within a particular class or subclass that may hold leases at one time, and it  is  possible  to
       specify automatic subclassing based on the contents of the client packet.

       To add clients to classes based on conditional evaluation, you can specify a matching expression
       in the class statement:

       class "ras-clients" {
         match if substring (option dhcp-client-identifier, 1, 3) = "RAS";
       }

       Note that whether you use matching expressions or add statements (or both) to classify  clients,
       you must always write a class declaration for any class that you use.  If there will be no match
       statement and no in-scope statements for a class, the declaration should look like this:

       class "ras-clients" {
       }

SUBCLASSES
       In addition to classes, it is possible to declare subclasses.  A subclass is a  class  with  the
       same  name as a regular class, but with a specific submatch expression which is hashed for quick
       matching.  This is essentially a speed hack - the main  difference  between  five  classes  with
       match expressions and one class with five subclasses is that it will be quicker to find the sub‐
       classes.  Subclasses work as follows:

       class "allocation-class-1" {
         match pick-first-value (option dhcp-client-identifier, hardware);
       }

       class "allocation-class-2" {
         match pick-first-value (option dhcp-client-identifier, hardware);
       }

       subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
       subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
       subclass "allocation-class-1" 1:0:0:c4:aa:29:44;

       subnet 10.0.0.0 netmask 255.255.255.0 {
         pool {
           allow members of "allocation-class-1";
           range 10.0.0.11 10.0.0.50;
         }
         pool {
           allow members of "allocation-class-2";
           range 10.0.0.51 10.0.0.100;
         }
       }

       The data following the class name in the subclass declaration is a  constant  value  to  use  in
       matching the match expression for the class.  When class matching is done, the server will eval‐
       uate the match expression and then look the result up in the hash table.  If it finds  a  match,
       the client is considered a member of both the class and the subclass.

       Subclasses can be declared with or without scope.  In the above example, the sole purpose of the
       subclass is to allow some clients access to one address pool,  while  other  clients  are  given
       access  to the other pool, so these subclasses are declared without scopes.  If part of the pur‐
       pose of the subclass were to define different parameter values for some clients, you might  want
       to declare some subclasses with scopes.

       In  the  above  example,  if  you had a single client that needed some configuration parameters,
       while most didn't, you might write the following subclass declaration for that client:

       subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
         option root-path "samsara:/var/diskless/alphapc";
         filename "/tftpboot/netbsd.alphapc-diskless";
       }

       In this example, we've used subclassing as a way to control address allocation on  a  per-client
       basis.   However, it's also possible to use subclassing in ways that are not specific to clients
       - for example, to use the value of the vendor-class-identifier option to determine  what  values
       to send in the vendor-encapsulated-options option.  An example of this is shown under the VENDOR
       ENCAPSULATED OPTIONS head in the dhcp-options(5) manual page.

PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION
       You may specify a limit to the number of clients in a class that can be  assigned  leases.   The
       effect of this will be to make it difficult for a new client in a class to get an address.  Once
       a class with such a limit has reached its limit, the only way a new client in that class can get
       a  lease  is  for an existing client to relinquish its lease, either by letting it expire, or by
       sending a DHCPRELEASE packet.  Classes with lease limits are specified as follows:

       class "limited-1" {
         lease limit 4;
       }

       This will produce a class in which a maximum of four members may hold a lease at one time.

SPAWNING CLASSES
       It is possible to declare a spawning class.  A spawning class is a class that automatically pro‐
       duces  subclasses based on what the client sends.  The reason that spawning classes were created
       was to make it possible to create lease-limited classes on the fly.  The envisioned  application
       is  a  cable-modem environment where the ISP wishes to provide clients at a particular site with
       more than one IP address, but does not wish to provide such clients with their own  subnet,  nor
       give  them  an  unlimited number of IP addresses from the network segment to which they are con‐
       nected.

       Many cable modem head-end systems can be configured to add a Relay Agent Information  option  to
       DHCP packets when relaying them to the DHCP server.  These systems typically add a circuit ID or
       remote ID option that uniquely identifies the customer site.  To take advantage of this, you can
       write a class declaration as follows:

       class "customer" {
         spawn with option agent.circuit-id;
         lease limit 4;
       }

       Now  whenever  a  request  comes  in from a customer site, the circuit ID option will be checked
       against the class's hash table.  If a subclass is found that matches the circuit ID, the  client
       will  be  classified in that subclass and treated accordingly.  If no subclass is found matching
       the circuit ID, a new one will be created and logged in the dhcpd.leases file,  and  the  client
       will  be  classified in this new class.  Once the client has been classified, it will be treated
       according to the rules of the class, including, in this case,  being  subject  to  the  per-site
       limit of four leases.

       The  use of the subclass spawning mechanism is not restricted to relay agent options - this par‐
       ticular example is given only because it is a fairly straightforward one.

COMBINING MATCH, MATCH IF AND SPAWN WITH
       In some cases, it may be useful to use one expression to assign a client to a particular  class,
       and  a second expression to put it into a subclass of that class.  This can be done by combining
       the match if and spawn with statements, or the match if and match statements.  For example:

       class "jr-cable-modems" {
         match if option dhcp-vendor-identifier = "jrcm";
         spawn with option agent.circuit-id;
         lease limit 4;
       }

       class "dv-dsl-modems" {
         match if option dhcp-vendor-identifier = "dvdsl";
         spawn with option agent.circuit-id;
         lease limit 16;
       }

       This allows you to have two classes that both have the same spawn with expression  without  get‐
       ting the clients in the two classes confused with each other.

DYNAMIC DNS UPDATES
       The  DHCP  server has the ability to dynamically update the Domain Name System.  Within the con‐
       figuration files, you can define how you want the Domain  Name  System  to  be  updated.   These
       updates  are  RFC  2136 compliant so any DNS server supporting RFC 2136 should be able to accept
       updates from the DHCP server.

       Two DNS update schemes are currently implemented, and another is planned.  The two that are cur‐
       rently  implemented  are  the  ad-hoc DNS update mode and the interim DHCP-DNS interaction draft
       update mode.  In the future we plan to add a third mode which will be the  standard  DNS  update
       method  based  on the RFCS for DHCP-DNS interaction and DHCID The DHCP server must be configured
       to use one of the two currently-supported methods, or not to do dns updates.  This can  be  done
       with the ddns-update-style configuration parameter.

THE AD-HOC DNS UPDATE SCHEME
       The ad-hoc Dynamic DNS update scheme is now deprecated and does not work.  In future releases of
       the ISC DHCP server, this scheme will not likely be available.  The interim scheme works, allows
       for  failover, and should now be used.  The following description is left here for informational
       purposes only.

       The ad-hoc Dynamic DNS update scheme implemented in this version of the ISC  DHCP  server  is  a
       prototype  design,  which does not have much to do with the standard update method that is being
       standardized in the IETF DHC working group, but rather implements some very basic,  yet  useful,
       update  capabilities.   This  mode  does not work with the failover protocol because it does not
       account for the possibility of two different DHCP servers updating the same set of DNS records.

       For the ad-hoc DNS update method, the client's FQDN is derived in two parts.  First,  the  host‐
       name is determined.  Then, the domain name is determined, and appended to the hostname.

       The DHCP server determines the client's hostname by first looking for a ddns-hostname configura‐
       tion option, and using that if it is present.  If no such option is present,  the  server  looks
       for a valid hostname in the FQDN option sent by the client.  If one is found, it is used; other‐
       wise, if the client sent a host-name option, that is used.  Otherwise, if there is a host decla‐
       ration  that  applies  to  the  client, the name from that declaration will be used.  If none of
       these applies, the server will not have a hostname for the client, and will not be able to do  a
       DNS update.

       The  domain  name is determined from the ddns-domainname configuration option.  The default con‐
       figuration for this option is:

         option server.ddns-domainname = config-option domain-name;

       So if this configuration option is not configured to a different value  (over-riding  the  above
       default),  or  if  a domain-name option has not been configured for the client's scope, then the
       server will not attempt to perform a DNS update.

       The client's fully-qualified domain name, derived as we have described, is used as the  name  on
       which  an  "A"  record will be stored.  The A record will contain the IP address that the client
       was assigned in its lease.  If there is already an A record  with  the  same  name  in  the  DNS
       server, no update of either the A or PTR records will occur - this prevents a client from claim‐
       ing that its hostname is the name of some network server.  For example, if you have a fileserver
       called  "fs.sneedville.edu",  and  the client claims its hostname is "fs", no DNS update will be
       done for that client, and an error message will be logged.

       If the A record update succeeds, a PTR record update for the assigned IP address will  be  done,
       pointing  to  the  A record.  This update is unconditional - it will be done even if another PTR
       record of the same name exists.  Since the IP address has been assigned to the DHCP server, this
       should be safe.

       Please  note  that  the current implementation assumes clients only have a single network inter‐
       face.  A client with two network interfaces will see unpredictable behavior.  This is considered
       a  bug,  and  will  be fixed in a later release.  It may be helpful to enable the one-lease-per-
       client parameter so that roaming clients do not trigger this same behavior.

       The DHCP protocol normally involves a four-packet exchange - first the client sends  a  DHCPDIS‐
       COVER  message, then the server sends a DHCPOFFER, then the client sends a DHCPREQUEST, then the
       server sends a DHCPACK.  In the current version of the server, the server will do a  DNS  update
       after  it  has  received the DHCPREQUEST, and before it has sent the DHCPACK.  It only sends the
       DNS update if it has not sent one for the client's address before,  in  order  to  minimize  the
       impact on the DHCP server.

       When  the  client's lease expires, the DHCP server (if it is operating at the time, or when next
       it operates) will remove the client's A and PTR records from the DNS database.   If  the  client
       releases  its  lease by sending a DHCPRELEASE message, the server will likewise remove the A and
       PTR records.

THE INTERIM DNS UPDATE SCHEME
       The interim DNS update scheme operates mostly according to  several  drafts  considered  by  the
       IETF.   While  the drafts have since become RFCs the code was written before they were finalized
       and there are some differences between our code and the final RFCs.  We plan to update our code,
       probably adding a standard DNS update option, at some time.  The basic framework is similar with
       the main material difference being that a DHCID RR was assigned in the  RFCs  whereas  our  code
       continues  to  use an experimental TXT record.  The format of the TXT record bears a resemblance
       to the DHCID RR but it is not equivalent (MD5 vs SHA1, field length differences etc).  The stan‐
       dard RFCs are:

                                         RFC 4701 (updated by RF5494)
                                                   RFC 4702
                                                   RFC 4703

       And the corresponding drafts were:

                                      draft-ietf-dnsext-dhcid-rr-??.txt
                                      draft-ietf-dhc-fqdn-option-??.txt
                                    draft-ietf-dhc-ddns-resolution-??.txt

       Because our implementation is slightly different than the standard, we will briefly document the
       operation of this update style here.

       The first point to understand about this style of DNS update is that unlike  the  ad-hoc  style,
       the  DHCP  server  does  not necessarily always update both the A and the PTR records.  The FQDN
       option includes a flag which, when sent by the client,  indicates  that  the  client  wishes  to
       update  its  own  A  record.   In  that  case,  the server can be configured either to honor the
       client's intentions or ignore them.  This is done with the statement  allow  client-updates;  or
       the statement ignore client-updates;.  By default, client updates are allowed.

       If  the server is configured to allow client updates, then if the client sends a fully-qualified
       domain name in the FQDN option, the server will use that name the client sent in the FQDN option
       to  update  the  PTR  record.   For  example,  let  us say that the client is a visitor from the
       "radish.org" domain, whose hostname is "jschmoe".  The server is for the  "example.org"  domain.
       The  DHCP  client  indicates in the FQDN option that its FQDN is "jschmoe.radish.org.".  It also
       indicates that it wants to update its own A record.  The DHCP server therefore does not  attempt
       to  set  up  an A record for the client, but does set up a PTR record for the IP address that it
       assigns the client, pointing at jschmoe.radish.org.  Once the DHCP client has an IP address,  it
       can update its own A record, assuming that the "radish.org" DNS server will allow it to do so.

       If the server is configured not to allow client updates, or if the client doesn't want to do its
       own update, the server will simply choose a name for the client from either the fqdn option  (if
       present)  or  the hostname option (if present).  It will use its own domain name for the client,
       just as in the ad-hoc update scheme.  It will then update both the A and PTR record,  using  the
       name  that  it  chose  for the client.  If the client sends a fully-qualified domain name in the
       fqdn option, the server uses only the leftmost part of the domain name - in the  example  above,
       "jschmoe" instead of "jschmoe.radish.org".

       Further,  if the ignore client-updates; directive is used, then the server will in addition send
       a response in the DHCP packet, using the FQDN Option, that implies to the client that it  should
       perform  its  own updates if it chooses to do so.  With deny client-updates;, a response is sent
       which indicates the client may not perform updates.

       Also, if the use-host-decl-names configuration option is enabled, then  the  host  declaration's
       hostname  will  be  used  in  place  of  the  hostname  option, and the same rules will apply as
       described above.

       The other difference between the ad-hoc scheme and the interim scheme is that with  the  interim
       scheme,  a method is used that allows more than one DHCP server to update the DNS database with‐
       out accidentally deleting A records that shouldn't be deleted nor failing to add A records  that
       should be added.  The scheme works as follows:

       When  the  DHCP server issues a client a new lease, it creates a text string that is an MD5 hash
       over the DHCP client's identification (see draft-ietf-dnsext-dhcid-rr-??.txt for details).   The
       update  adds  an  A record with the name the server chose and a TXT record containing the hashed
       identifier string (hashid).  If this update succeeds, the server is done.

       If the update fails because the A record already exists, then the DHCP server  attempts  to  add
       the A record with the prerequisite that there must be a TXT record in the same name as the new A
       record, and that TXT record's contents must be equal to hashid.  If this update  succeeds,  then
       the  client  has  its  A  record and PTR record.  If it fails, then the name the client has been
       assigned (or requested) is in use, and can't be used by the client.   At  this  point  the  DHCP
       server gives up trying to do a DNS update for the client until the client chooses a new name.

       The  interim DNS update scheme is called interim for two reasons.  First, it does not quite fol‐
       low the RFCs.  The RFCs call for a new DHCID RRtype while he interim DNS update  scheme  uses  a
       TXT  record.   The ddns-resolution draft called for the DHCP server to put a DHCID RR on the PTR
       record, but the interim update method does not do this.  In the final RFC this  requirement  was
       relaxed such that a server may add a DHCID RR to the PTR record.

       In  addition  to  these differences, the server also does not update very aggressively.  Because
       each DNS update involves a round trip to the DNS server, there is a cost associated  with  doing
       updates even if they do not actually modify the DNS database.  So the DHCP server tracks whether
       or not it has updated the record in the past (this information is stored on the lease) and  does
       not attempt to update records that it thinks it has already updated.

       This  can  lead  to  cases  where  the DHCP server adds a record, and then the record is deleted
       through some other mechanism, but the server never again updates the DNS because it  thinks  the
       data  is  already  there.   In this case the data can be removed from the lease through operator
       intervention, and once this has been done, the DNS will be updated  the  next  time  the  client
       renews.

DYNAMIC DNS UPDATE SECURITY
       When you set your DNS server up to allow updates from the DHCP server, you may be exposing it to
       unauthorized updates.  To avoid this, you should use TSIG signatures - a method of cryptographi‐
       cally  signing  updates  using  a shared secret key.  As long as you protect the secrecy of this
       key, your updates should also be secure.  Note, however, that the DHCP protocol itself  provides
       no  security,  and  that  clients can therefore provide information to the DHCP server which the
       DHCP server will then use in its updates, with the constraints described previously.

       The DNS server must be configured to allow updates for any zone that the  DHCP  server  will  be
       updating.   For  example,  let us say that clients in the sneedville.edu domain will be assigned
       addresses on the 10.10.17.0/24 subnet.  In that case, you will need a key  declaration  for  the
       TSIG  key  you  will  be  using,  and also two zone declarations - one for the zone containing A
       records that will be updates and one for the zone containing PTR records - for ISC  BIND,  some‐
       thing like this:

       key DHCP_UPDATER {
         algorithm hmac-md5;
         secret pRP5FapFoJ95JEL06sv4PQ==;
       };

       zone "example.org" {
            type master;
            file "example.org.db";
            allow-update { key DHCP_UPDATER; };
       };

       zone "17.10.10.in-addr.arpa" {
            type master;
            file "10.10.17.db";
            allow-update { key DHCP_UPDATER; };
       };

       You  will  also  have to configure your DHCP server to do updates to these zones.  To do so, you
       need to add something like this to your dhcpd.conf file:

       key DHCP_UPDATER {
         algorithm hmac-md5;
         secret pRP5FapFoJ95JEL06sv4PQ==;
       };

       zone EXAMPLE.ORG. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

       zone 17.127.10.in-addr.arpa. {
         primary 127.0.0.1;
         key DHCP_UPDATER;
       }

       The primary statement specifies the IP address of the name server whose zone information  is  to
       be  updated.   In  addition to the primary statement there are also the primary6 , secondary and
       secondary6 statements.  The primary6 statement specifies an IPv6 address for  the  name  server.
       The secondaries provide for additional addresses for name servers to be used if the primary does
       not respond.  The number of name servers the DDNS code will attempt to use before giving  up  is
       limited and is currently set to three.

       Note that the zone declarations have to correspond to authority records in your name server - in
       the above example, there must  be  an  SOA  record  for  "example.org."  and  for  "17.10.10.in-
       addr.arpa.".  For example, if there were a subdomain "foo.example.org" with no separate SOA, you
       could not write a zone declaration for "foo.example.org."  Also keep in mind that zone names  in
       your  DHCP  configuration  should end in a "."; this is the preferred syntax.  If you do not end
       your zone name in a ".", the DHCP server will figure it out.  Also note that in the DHCP config‐
       uration, zone names are not encapsulated in quotes where there are in the DNS configuration.

       You  should choose your own secret key, of course.  The ISC BIND 8 and 9 distributions come with
       a program for generating secret keys called dnssec-keygen.  The version that comes with  BIND  9
       is  likely  to produce a substantially more random key, so we recommend you use that one even if
       you are not using BIND 9 as your DNS server.  If you are using BIND 9's dnssec-keygen, the above
       key would be created as follows:

            dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER

       If you are using the BIND 8 dnskeygen program, the following command will generate a key as seen
       above:

            dnskeygen -H 128 -u -c -n DHCP_UPDATER

       You may wish to enable logging of DNS updates on your DNS server.  To do so, you might  write  a
       logging statement like the following:

       logging {
            channel update_debug {
                 file "/var/log/update-debug.log";
                 severity  debug 3;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;
            };
            channel security_info    {
                 file "/var/log/named-auth.info";
                 severity  info;
                 print-category yes;
                 print-severity yes;
                 print-time     yes;
            };

            category update { update_debug; };
            category security { security_info; };
       };

       You must create the /var/log/named-auth.info and /var/log/update-debug.log files before starting
       the name server.  For more information on configuring ISC BIND, consult the  documentation  that
       accompanies it.

REFERENCE: EVENTS
       There are three kinds of events that can happen regarding a lease, and it is possible to declare
       statements that occur when any of these events happen.  These events are the commit event,  when
       the  server  has  made  a commitment of a certain lease to a client, the release event, when the
       client has released the server from its commitment, and the expiry event,  when  the  commitment
       expires.

       To  declare a set of statements to execute when an event happens, you must use the on statement,
       followed by the name of the event, followed by a series of statements to execute when the  event
       happens, enclosed in braces.  Events are used to implement DNS updates, so you should not define
       your own event handlers if you are using the built-in DNS update mechanism.

       The built-in version of the DNS update mechanism  is  in  a  text  string  towards  the  top  of
       server/dhcpd.c.   If you want to use events for things other than DNS updates, and you also want
       DNS updates, you will have to start out by copying this code into your dhcpd.conf file and modi‐
       fying it.

REFERENCE: DECLARATIONS
       The include statement

        include "filename";

       The  include statement is used to read in a named file, and process the contents of that file as
       though it were entered in place of the include statement.

       The shared-network statement

        shared-network name {
          [ parameters ]
          [ declarations ]
        }

       The shared-network statement is used to inform the DHCP server that  some  IP  subnets  actually
       share  the  same  physical network.  Any subnets in a shared network should be declared within a
       shared-network statement.  Parameters specified in the shared-network  statement  will  be  used
       when  booting  clients  on  those subnets unless parameters provided at the subnet or host level
       override them.  If any subnet in a shared network has addresses available  for  dynamic  alloca‐
       tion,  those  addresses are collected into a common pool for that shared network and assigned to
       clients as needed.  There is no way to distinguish on which subnet of a shared network a  client
       should boot.

       Name  should  be the name of the shared network.  This name is used when printing debugging mes‐
       sages, so it should be descriptive for the shared network.  The name may have the  syntax  of  a
       valid  domain  name  (although  it will never be used as such), or it may be any arbitrary name,
       enclosed in quotes.

       The subnet statement

        subnet subnet-number netmask netmask {
          [ parameters ]
          [ declarations ]
        }

       The subnet statement is used to provide dhcpd with enough information to tell whether or not  an
       IP  address is on that subnet.  It may also be used to provide subnet-specific parameters and to
       specify what addresses may be dynamically allocated to clients booting  on  that  subnet.   Such
       addresses are specified using the range declaration.

       The  subnet-number should be an IP address or domain name which resolves to the subnet number of
       the subnet being described.  The netmask should be an IP address or domain name  which  resolves
       to the subnet mask of the subnet being described.  The subnet number, together with the netmask,
       are sufficient to determine whether any given IP address is on the specified subnet.

       Although a netmask must be given with every subnet declaration, it is recommended that if  there
       is any variance in subnet masks at a site, a subnet-mask option statement be used in each subnet
       declaration to set the desired subnet mask, since any subnet-mask option statement will override
       the subnet mask declared in the subnet statement.

       The subnet6 statement

        subnet6 subnet6-number {
          [ parameters ]
          [ declarations ]
        }

       The subnet6 statement is used to provide dhcpd with enough information to tell whether or not an
       IPv6 address is on that subnet6.  It may also be used to provide subnet-specific parameters  and
       to specify what addresses may be dynamically allocated to clients booting on that subnet.

       The subnet6-number should be an IPv6 network identifier, specified as ip6-address/bits.

       The range statement

       range [ dynamic-bootp ] low-address [ high-address];

       For any subnet on which addresses will be assigned dynamically, there must be at least one range
       statement.  The range statement gives the lowest and highest IP addresses in a  range.   All  IP
       addresses  in  the  range should be in the subnet in which the range statement is declared.  The
       dynamic-bootp flag may be specified if addresses in  the  specified  range  may  be  dynamically
       assigned  to  BOOTP  clients  as  well as DHCP clients.  When specifying a single address, high-
       address can be omitted.

       The range6 statement

       range6 low-address high-address;
       range6 subnet6-number;
       range6 subnet6-number temporary;
       range6 address temporary;

       For any IPv6 subnet6 on which addresses will be assigned dynamically, there must be at least one
       range6  statement. The range6 statement can either be the lowest and highest IPv6 addresses in a
       range6, or use CIDR notation, specified as ip6-address/bits. All  IP  addresses  in  the  range6
       should be in the subnet6 in which the range6 statement is declared.

       The  temporary  variant  makes  the  prefix (by default on 64 bits) available for temporary (RFC
       4941) addresses. A new address per prefix in the shared network is computed at each request with
       an IA_TA option. Release and Confirm ignores temporary addresses.

       Any  IPv6 addresses given to hosts with fixed-address6 are excluded from the range6, as are IPv6
       addresses on the server itself.

       The prefix6 statement

       prefix6 low-address high-address / bits;

       The prefix6 is the range6 equivalent for Prefix Delegation (RFC 3633). Prefixes of  bits  length
       are assigned between low-address and high-address.

       Any  IPv6 prefixes given to static entries (hosts) with fixed-prefix6 are excluded from the pre‐
       fix6.

       This statement is currently global but it should have a shared-network scope.

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]
        }

       The host declaration provides a scope in which to provide configuration information about a spe‐
       cific  client, and also provides a way to assign a client a fixed address.  The host declaration
       provides a way for the DHCP server to identify a DHCP or BOOTP client, and also a way to  assign
       the client a static IP address.

       If  it is desirable to be able to boot a DHCP or BOOTP client on more than one subnet with fixed
       addresses, more than one address may be specified in the fixed-address declaration, or more than
       one host statement may be specified matching the same client.

       If  client-specific  boot  parameters  must  change  based on the network to which the client is
       attached, then multiple host declarations should be used.  The host declarations will only match
       a  client  if  one of their fixed-address statements is viable on the subnet (or shared network)
       where the client is attached.  Conversely, for a host declaration to match a client being  allo‐
       cated  a dynamic address, it must not have any fixed-address statements.  You may therefore need
       a mixture of host declarations for any given client...some having fixed-address statements, oth‐
       ers without.

       hostname  should  be a name identifying the host.  If a hostname option is not specified for the
       host, hostname is used.

       Host declarations are matched to actual DHCP or BOOTP clients by matching the  dhcp-client-iden‐
       tifier  or  pxe-client-id  options  specified in the host declaration to the one supplied by the
       client, or, if the host declaration or the client does not provide a  dhcp-client-identifier  or
       pxe-client-id options, by matching the hardware parameter in the host declaration to the network
       hardware address supplied by the client.  BOOTP clients do not normally provide  a  dhcp-client-
       identifier,  so  the hardware address must be used for all clients that may boot using the BOOTP
       protocol.

       DHCPv6 servers can use the host-identifier option parameter in the host declaration, and specify
       any option with a fixed value to identify hosts.

       Please  be aware that only the dhcp-client-identifier and pxe-client-id options and the hardware
       address can be used to match a host declaration, or the  host-identifier  option  parameter  for
       DHCPv6  servers.   For  example,  it  is not possible to match a host declaration to a host-name
       option.  This is because the host-name option cannot be guaranteed to be unique  for  any  given
       client,  whereas  both the hardware address and dhcp-client-identifier option are at least theo‐
       retically guaranteed to be unique to a given client.

       The group statement

        group {
          [ parameters ]
          [ declarations ]
        }

       The group statement is used simply to apply one or more parameters to a group  of  declarations.
       It can be used to group hosts, shared networks, subnets, or even other groups.

REFERENCE: ALLOW AND DENY
       The  allow and deny statements can be used to control the response of the DHCP server to various
       sorts of requests.  The allow and deny keywords actually have different  meanings  depending  on
       the  context.   In a pool context, these keywords can be used to set up access lists for address
       allocation pools.  In other contexts, the keywords simply control general server  behavior  with
       respect  to  clients  based  on scope.  In a non-pool context, the ignore keyword can be used in
       place of the deny keyword to prevent logging of denied requests.

ALLOW DENY AND IGNORE IN SCOPE
       The following usages of allow and deny will work in any scope, although it  is  not  recommended
       that they be used in pool declarations.

       The unknown-clients keyword

        allow unknown-clients;
        deny unknown-clients;
        ignore unknown-clients;

       The unknown-clients flag is used to tell dhcpd whether or not to dynamically assign addresses to
       unknown clients.  Dynamic address assignment to unknown  clients  is  allowed  by  default.   An
       unknown client is simply a client that has no host declaration.

       The  use of this option is now deprecated.  If you are trying to restrict access on your network
       to known clients, you should use deny unknown-clients; inside of your address pool, as described
       under the heading ALLOW AND DENY WITHIN POOL DECLARATIONS.

       The bootp keyword

        allow bootp;
        deny bootp;
        ignore bootp;

       The  bootp flag is used to tell dhcpd whether or not to respond to bootp queries.  Bootp queries
       are allowed by default.

       The booting keyword

        allow booting;
        deny booting;
        ignore booting;

       The booting flag is used to tell dhcpd whether or not to respond to queries  from  a  particular
       client.  This keyword only has meaning when it appears in a host declaration.  By default, boot‐
       ing is allowed, but if it is disabled for a particular client, then that client will not be able
       to get an address from the DHCP server.

       The duplicates keyword

        allow duplicates;
        deny duplicates;

       Host  declarations can match client messages based on the DHCP Client Identifier option or based
       on the client's network hardware type and MAC address.  If the MAC address  is  used,  the  host
       declaration  will  match  any  client with that MAC address - even clients with different client
       identifiers.  This doesn't normally happen, but is possible when one computer has more than  one
       operating system installed on it - for example, Microsoft Windows and NetBSD or Linux.

       The  duplicates  flag  tells  the  DHCP  server that if a request is received from a client that
       matches the MAC address of a host declaration, any other leases matching that MAC address should
       be  discarded  by  the server, even if the UID is not the same.  This is a violation of the DHCP
       protocol, but can prevent clients whose client identifiers change regularly  from  holding  many
       leases at the same time.  By default, duplicates are allowed.

       The declines keyword

        allow declines;
        deny declines;
        ignore declines;

       The  DHCPDECLINE  message  is  used  by  DHCP  clients to indicate that the lease the server has
       offered is not valid.  When the server receives a DHCPDECLINE for a particular address, it  nor‐
       mally abandons that address, assuming that some unauthorized system is using it.  Unfortunately,
       a malicious or buggy client  can,  using  DHCPDECLINE  messages,  completely  exhaust  the  DHCP
       server's allocation pool.  The server will reclaim these leases, but while the client is running
       through the pool, it may cause serious thrashing in the DNS, and it will  also  cause  the  DHCP
       server to forget old DHCP client address allocations.

       The  declines flag tells the DHCP server whether or not to honor DHCPDECLINE messages.  If it is
       set to deny or ignore in a particular scope, the DHCP server will  not  respond  to  DHCPDECLINE
       messages.

       The client-updates keyword

        allow client-updates;
        deny client-updates;

       The  client-updates flag tells the DHCP server whether or not to honor the client's intention to
       do its own update of its A record.  This is only relevant when doing interim DNS  updates.   See
       the documentation under the heading THE INTERIM DNS UPDATE SCHEME for details.

       The leasequery keyword

        allow leasequery;
        deny leasequery;

       The  leasequery  flag tells the DHCP server whether or not to answer DHCPLEASEQUERY packets. The
       answer to a DHCPLEASEQUERY packet includes information about a specific lease, such as  when  it
       was issued and when it will expire. By default, the server will not respond to these packets.

ALLOW AND DENY WITHIN POOL DECLARATIONS
       The  uses of the allow and deny keywords shown in the previous section work pretty much the same
       way whether the client is sending a DHCPDISCOVER or a DHCPREQUEST message - an address  will  be
       allocated to the client (either the old address it's requesting, or a new address) and then that
       address will be tested to see if it's okay to let the client have it.  If the  client  requested
       it,  and it's not okay, the server will send a DHCPNAK message.  Otherwise, the server will sim‐
       ply not respond to the client.  If it is okay to give the address to the client, the server will
       send a DHCPACK message.

       The  primary motivation behind pool declarations is to have address allocation pools whose allo‐
       cation policies are different.  A client may be denied access to one pool, but allowed access to
       another  pool  on the same network segment.  In order for this to work, access control has to be
       done during address allocation, not after address allocation is done.

       When a DHCPREQUEST message is processed, address allocation simply consists of  looking  up  the
       address  the  client is requesting and seeing if it's still available for the client.  If it is,
       then the DHCP server checks both the address pool permit lists and the relevant  in-scope  allow
       and  deny  statements  to  see  if  it's okay to give the lease to the client.  In the case of a
       DHCPDISCOVER message, the allocation process is done as  described  previously  in  the  ADDRESS
       ALLOCATION section.

       When  declaring permit lists for address allocation pools, the following syntaxes are recognized
       following the allow or deny keywords:

        known-clients;

       If specified, this statement either allows or prevents allocation from this pool to  any  client
       that has a host declaration (i.e., is known).  A client is known if it has a host declaration in
       any scope, not just the current scope.

        unknown-clients;

       If specified, this statement either allows or prevents allocation from this pool to  any  client
       that has no host declaration (i.e., is not known).

        members of "class";

       If  specified,  this statement either allows or prevents allocation from this pool to any client
       that is a member of the named class.

        dynamic bootp clients;

       If specified, this statement either allows or prevents allocation from this pool  to  any  bootp
       client.

        authenticated clients;

       If  specified,  this statement either allows or prevents allocation from this pool to any client
       that has been authenticated using the DHCP authentication protocol.  This is not yet supported.

        unauthenticated clients;

       If specified, this statement either allows or prevents allocation from this pool to  any  client
       that  has  not  been authenticated using the DHCP authentication protocol.  This is not yet sup‐
       ported.

        all clients;

       If specified, this statement either allows or prevents allocation from this pool to all clients.
       This  can  be  used  when  you  want to write a pool declaration for some reason, but hold it in
       reserve, or when you want to renumber your network quickly, and thus want the  server  to  force
       all  clients  that  have been allocated addresses from this pool to obtain new addresses immedi‐
       ately when they next renew.

        after time;

       If specified, this statement either allows or prevents allocation from this pool after  a  given
       date.  This  can  be  used  when  you  want to move clients from one pool to another. The server
       adjusts the regular lease time so that the latest expiry time is at  the  given  time+min-lease-
       time.  A short min-lease-time enforces a step change, whereas a longer min-lease-time allows for
       a gradual change.  time is either second since epoch, or a UTC time string  e.g.   4  2007/08/24
       09:14:32 or a string with time zone offset in seconds e.g. 4 2007/08/24 11:14:32 -7200

REFERENCE: PARAMETERS
       The adaptive-lease-time-threshold statement

         adaptive-lease-time-threshold percentage;

         When  the  number  of  allocated leases within a pool rises above the percentage given in this
         statement, the DHCP server decreases the lease length for new clients within this pool to min-
         lease-time seconds. Clients renewing an already valid (long) leases get at least the remaining
         time from the current lease. Since the leases expire faster, the  server  may  either  recover
         more  quickly  or  avoid  pool  exhaustion entirely.  Once the number of allocated leases drop
         below the threshold, the server reverts back to normal lease  times.   Valid  percentages  are
         between 1 and 99.

       The always-broadcast statement

         always-broadcast flag;

         The  DHCP  and BOOTP protocols both require DHCP and BOOTP clients to set the broadcast bit in
         the flags field of the BOOTP message header.  Unfortunately, some DHCP and  BOOTP  clients  do
         not  do  this,  and therefore may not receive responses from the DHCP server.  The DHCP server
         can be made to always broadcast its responses to clients by setting this flag to ´on´ for  the
         relevant  scope; relevant scopes would be inside a conditional statement, as a parameter for a
         class, or as a parameter for a host declaration.  To avoid creating excess  broadcast  traffic
         on  your  network,  we recommend that you restrict the use of this option to as few clients as
         possible.  For example, the Microsoft DHCP client is known not to have this  problem,  as  are
         the OpenTransport and ISC DHCP clients.

       The always-reply-rfc1048 statement

         always-reply-rfc1048 flag;

         Some  BOOTP  clients  expect  RFC1048-style  responses, but do not follow RFC1048 when sending
         their requests.  You can tell that a client is having this problem if it is  not  getting  the
         options  you  have  configured  for  it  and  if  you see in the server log the message "(non-
         rfc1048)" printed with each BOOTREQUEST that is logged.

         If you want to send rfc1048 options to such a client, you  can  set  the  always-reply-rfc1048
         option  in  that  client's  host  declaration,  and  the  DHCP  server  will  respond  with an
         RFC-1048-style vendor options field.  This flag can be set in any scope, and will  affect  all
         clients covered by that scope.

       The authoritative statement

         authoritative;

         not authoritative;

         The  DHCP server will normally assume that the configuration information about a given network
         segment is not known to be correct and is not authoritative.  This is so that if a naive  user
         installs  a DHCP server not fully understanding how to configure it, it does not send spurious
         DHCPNAK messages to clients that have obtained addresses from a legitimate DHCP server on  the
         network.

         Network  administrators setting up authoritative DHCP servers for their networks should always
         write authoritative; at the top of their configuration file to indicate that the  DHCP  server
         should  send  DHCPNAK messages to misconfigured clients.  If this is not done, clients will be
         unable to get a correct IP address after changing subnets until their old lease  has  expired,
         which could take quite a long time.

         Usually,  writing  authoritative; at the top level of the file should be sufficient.  However,
         if a DHCP server is to be set up so that it is aware of some networks for which it is authori‐
         tative  and some networks for which it is not, it may be more appropriate to declare authority
         on a per-network-segment basis.

         Note that the most specific scope for which the concept of authority makes any  sense  is  the
         physical network segment - either a shared-network statement or a subnet statement that is not
         contained within a shared-network statement.  It is not meaningful to specify that the  server
         is  authoritative  for some subnets within a shared network, but not authoritative for others,
         nor is it meaningful to specify that the server is authoritative for  some  host  declarations
         and not others.

       The boot-unknown-clients statement

         boot-unknown-clients flag;

         If the boot-unknown-clients statement is present and has a value of false or off, then clients
         for which there is no host declaration will not be allowed to obtain IP  addresses.   If  this
         statement  is not present or has a value of true or on, then clients without host declarations
         will be allowed to obtain IP addresses, as long as those addresses are not restricted by allow
         and deny statements within their pool declarations.

       The db-time-format statement

         db-time-format [ default | local ] ;

         The DHCP server software outputs several timestamps when writing leases to persistent storage.
         This configuration parameter selects one of two output formats.  The default format prints the
         day,  date, and time in UTC, while the local format prints the system seconds-since-epoch, and
         helpfully provides the day and time in the system timezone in a comment.  The time formats are
         described in detail in the dhcpd.leases(5) manpage.

       The ddns-hostname statement

         ddns-hostname name;

         The  name  parameter should be the hostname that will be used in setting up the client's A and
         PTR records.  If no ddns-hostname is specified in scope, then the server will derive the host‐
         name automatically, using an algorithm that varies for each of the different update methods.

       The ddns-domainname statement

         ddns-domainname name;

         The name parameter should be the domain name that will be appended to the client's hostname to
         form a fully-qualified domain-name (FQDN).

       The ddns-rev-domainname statement

         ddns-rev-domainname name; The name parameter should be the domain name that will  be  appended
         to  the client's reversed IP address to produce a name for use in the client's PTR record.  By
         default, this is "in-addr.arpa.", but the default can be overridden here.

         The reversed IP address to which this domain name is appended is always the IP address of  the
         client,  in  dotted  quad  notation, reversed - for example, if the IP address assigned to the
         client is 10.17.92.74, then the reversed IP address is 74.92.17.10.  So a client with that  IP
         address would, by default, be given a PTR record of 10.17.92.74.in-addr.arpa.

       The ddns-update-style parameter

         ddns-update-style style;

         The  style  parameter must be one of ad-hoc, interim or none.  The ddns-update-style statement
         is only meaningful in the outer scope - it is evaluated  once  after  reading  the  dhcpd.conf
         file, rather than each time a client is assigned an IP address, so there is no way to use dif‐
         ferent DNS update styles for different clients. The default is none.

       The ddns-updates statement

          ddns-updates flag;

         The ddns-updates parameter controls whether or not the server will attempt to do a DNS  update
         when  a  lease  is  confirmed.  Set this to off if the server should not attempt to do updates
         within a certain scope.  The ddns-updates parameter is on by default.  To disable DNS  updates
         in  all  scopes, it is preferable to use the ddns-update-style statement, setting the style to
         none.

       The default-lease-time statement

         default-lease-time time;

         Time should be the length in seconds that will be assigned to a lease if the client requesting
         the  lease  does  not  ask  for  a specific expiration time.  This is used for both DHCPv4 and
         DHCPv6 leases (it is also known as the "valid lifetime" in DHCPv6).  The default is 43200 sec‐
         onds.

       The delayed-ack and max-ack-delay statements

         delayed-ack count; max-ack-delay microseconds;

         Count  should  be  an integer value from zero to 2^16-1, and defaults to 28.  The count repre‐
         sents how many DHCPv4 replies maximum will be queued pending transmission until after a  data‐
         base  commit event.  If this number is reached, a database commit event (commonly resulting in
         fsync() and representing a performance penalty) will be made, and the reply  packets  will  be
         transmitted in a batch afterwards.  This preserves the RFC2131 direction that "stable storage"
         be updated prior to replying to clients.  Should the DHCPv4 sockets "go dry" (select() returns
         immediately with no read sockets), the commit is made and any queued packets are transmitted.

         Similarly,  microseconds indicates how many microseconds are permitted to pass inbetween queu‐
         ing a packet pending an fsync, and performing the fsync.  Valid values range from 0 to 2^32-1,
         and defaults to 250,000 (1/4 of a second).

         Please note that as delayed-ack is currently experimental, the delayed-ack feature is not com‐
         piled in by default, but must be enabled at compile time with  ´./configure  --enable-delayed-
         ack´.

       The do-forward-updates statement

         do-forward-updates flag;

         The  do-forward-updates statement instructs the DHCP server as to whether it should attempt to
         update a DHCP client's A record when the client acquires or renews a  lease.   This  statement
         has no effect unless DNS updates are enabled and ddns-update-style is set to interim.  Forward
         updates are enabled by default.  If this statement is used to  disable  forward  updates,  the
         DHCP  server will never attempt to update the client's A record, and will only ever attempt to
         update the client's PTR record if the client supplies an FQDN that should be placed in the PTR
         record  using  the  fqdn  option.   If forward updates are enabled, the DHCP server will still
         honor the setting of the client-updates flag.

       The dynamic-bootp-lease-cutoff statement

         dynamic-bootp-lease-cutoff date;

         The dynamic-bootp-lease-cutoff statement sets the ending time for all leases assigned  dynami‐
         cally  to  BOOTP  clients.   Because BOOTP clients do not have any way of renewing leases, and
         don't know that their leases could expire, by default dhcpd assigns  infinite  leases  to  all
         BOOTP  clients.   However,  it  may make sense in some situations to set a cutoff date for all
         BOOTP leases - for example, the end of a school term, or the time at night when a facility  is
         closed and all machines are required to be powered off.

         Date should be the date on which all assigned BOOTP leases will end.  The date is specified in
         the form:

                                             W YYYY/MM/DD HH:MM:SS

         W is the day of the week expressed as a number from zero (Sunday) to six (Saturday).  YYYY  is
         the  year,  including the century.  MM is the month expressed as a number from 1 to 12.  DD is
         the day of the month, counting from 1.  HH is the hour, from zero to 23.  MM is the minute and
         SS is the second.  The time is always in Coordinated Universal Time (UTC), not local time.

       The dynamic-bootp-lease-length statement

         dynamic-bootp-lease-length length;

         The  dynamic-bootp-lease-length  statement  is  used  to  set the length of leases dynamically
         assigned to BOOTP clients.  At some sites, it may be possible to assume that  a  lease  is  no
         longer  in  use  if  its holder has not used BOOTP or DHCP to get its address within a certain
         time period.  The period is specified in length as a number of seconds.  If a  client  reboots
         using  BOOTP  during  the  timeout  period,  the lease duration is reset to length, so a BOOTP
         client that boots frequently enough will never lose its lease.  Needless to say, this  parame‐
         ter should be adjusted with extreme caution.

       The filename statement

         filename "filename";

         The filename statement can be used to specify the name of the initial boot file which is to be
         loaded by a client.  The filename should be a filename recognizable to whatever file  transfer
         protocol the client can be expected to use to load the file.

       The fixed-address declaration

         fixed-address address [, address ... ];

         The  fixed-address  declaration  is used to assign one or more fixed IP addresses to a client.
         It should only appear in a host declaration.  If more than one address is supplied, then  when
         the  client boots, it will be assigned the address that corresponds to the network on which it
         is booting.  If none of the addresses in the fixed-address statement are valid for the network
         to  which  the client is connected, that client will not match the host declaration containing
         that fixed-address declaration.  Each address  in  the  fixed-address  declaration  should  be
         either an IP address or a domain name that resolves to one or more IP addresses.

       The fixed-address6 declaration

         fixed-address6 ip6-address ;

         The  fixed-address6  declaration  is  used  to  assign a fixed IPv6 addresses to a client.  It
         should only appear in a host declaration.

       The get-lease-hostnames statement

         get-lease-hostnames flag;

         The get-lease-hostnames statement is used to tell dhcpd whether or not to look up  the  domain
         name  corresponding  to  the IP address of each address in the lease pool and use that address
         for the DHCP hostname option.  If flag is true, then this lookup is done for all addresses  in
         the current scope.  By default, or if flag is false, no lookups are done.

       The hardware statement

         hardware hardware-type hardware-address;

         In  order  for  a BOOTP client to be recognized, its network hardware address must be declared
         using a hardware clause in the host statement.  hardware-type must be the name of  a  physical
         hardware  interface  type.   Currently, only the ethernet and token-ring types are recognized,
         although support for a fddi hardware type (and others) would also be desirable.  The hardware-
         address should be a set of hexadecimal octets (numbers from 0 through ff) separated by colons.
         The hardware statement may also be used for DHCP clients.

       The host-identifier option statement

         host-identifier option option-name option-data;

         This identifies a DHCPv6 client in a host statement.  option-name is any option,  and  option-
         data is the value for the option that the client will send. The option-data must be a constant
         value.

       The infinite-is-reserved statement

         infinite-is-reserved flag;

         ISC DHCP now supports ´reserved´ leases.  See the section on RESERVED LEASES below.   If  this
         flag  is on, the server will automatically reserve leases allocated to clients which requested
         an infinite (0xffffffff) lease-time.

         The default is off.

       The lease-file-name statement

         lease-file-name name;

         Name  should  be  the  name  of  the  DHCP  server's  lease  file.   By   default,   this   is
         /var/lib/dhcpd/dhcpd.leases.   This statement must appear in the outer scope of the configura‐
         tion file - if it appears in some other scope, it will have no effect.  Furthermore, it has no
         effect if overridden by the -lf flag or the PATH_DHCPD_DB environment variable.

       The limit-addrs-per-ia statement

         limit-addrs-per-ia number;

         By  default,  the  DHCPv6  server  will limit clients to one IAADDR per IA option, meaning one
         address.  If you wish to permit clients to hang onto multiple addresses at a time, configure a
         larger number here.

         Note  that there is no present method to configure the server to forcibly configure the client
         with one IP address per each subnet on a shared network.  This is left to future work.

       The dhcpv6-lease-file-name statement

         dhcpv6-lease-file-name name;

         Name is the name of the lease file to use if and only if the server is running in DHCPv6 mode.
         By  default, this is /var/lib/dhcpd/dhcpd6.leases.  This statement, like lease-file-name, must
         appear in the outer scope of the configuration file.  It has no effect if  overridden  by  the
         -lf  flag or the PATH_DHCPD6_DB environment variable.  If dhcpv6-lease-file-name is not speci‐
         fied, but lease-file-name is, the latter value will be used.

       The local-port statement

         local-port port;

         This statement causes the DHCP server to listen for DHCP requests on the UDP port specified in
         port, rather than on port 67.

       The local-address statement

         local-address address;

         This  statement  causes  the  DHCP  server  to  listen for DHCP requests sent to the specified
         address, rather than requests sent to all addresses.  Since  serving  directly  attached  DHCP
         clients implies that the server must respond to requests sent to the all-ones IP address, this
         option cannot be used if clients are on directly attached networks...it is only  realistically
         useful  for  a  server  whose  only  clients  are reached via unicasts, such as via DHCP relay
         agents.

         Note:  This statement is only effective if the  server  was  compiled  using  the  USE_SOCKETS
         #define  statement,  which  is  default  on  a  small number of operating systems, and must be
         explicitly chosen at compile-time for all others.  You can be sure if your server is  compiled
         with USE_SOCKETS if you see lines of this format at startup:

          Listening on Socket/eth0

         Note  also  that  since  this bind()s all DHCP sockets to the specified address, that only one
         address may be supported in a daemon at a given time.

       The log-facility statement

         log-facility facility;

         This statement causes the DHCP server to do all of its logging on the specified  log  facility
         once  the dhcpd.conf file has been read.  By default the DHCP server logs to the daemon facil‐
         ity.  Possible log facilities include auth, authpriv, cron,  daemon,  ftp,  kern,  lpr,  mail,
         mark,  news,  ntp,  security, syslog, user, uucp, and local0 through local7.  Not all of these
         facilities are available on all systems, and there may be other facilities available on  other
         systems.

         In  addition  to setting this value, you may need to modify your syslog.conf file to configure
         logging of the DHCP server.  For example, you might add a line like this:

              local7.debug /var/log/dhcpd.log

         The syntax of the syslog.conf file may be different on some operating systems  -  consult  the
         syslog.conf  manual page to be sure.  To get syslog to start logging to the new file, you must
         first create the file with correct ownership and permissions (usually, the same owner and per‐
         missions of your /var/log/messages or /usr/adm/messages file should be fine) and send a SIGHUP
         to syslogd.  Some systems support log rollover using a shell script or program called  newsys‐
         log  or logrotate, and you may be able to configure this as well so that your log file doesn't
         grow uncontrollably.

         Because the log-facility setting is controlled by the dhcpd.conf file,  log  messages  printed
         while parsing the dhcpd.conf file or before parsing it are logged to the default log facility.
         To prevent this, see the README file included with this distribution, which describes  how  to
         change  the  default  log  facility.   When this parameter is used, the DHCP server prints its
         startup message a second time after parsing the configuration file, so that the log will be as
         complete as possible.

       The max-lease-time statement

         max-lease-time time;

         Time  should  be  the  maximum  length  in  seconds  that will be assigned to a lease.  If not
         defined, the default maximum lease time is 86400.  The only exception to this is that  Dynamic
         BOOTP lease lengths, which are not specified by the client, are not limited by this maximum.

       The min-lease-time statement

         min-lease-time time;

         Time should be the minimum length in seconds that will be assigned to a lease.  The default is
         the minimum of 300 seconds or max-lease-time.

       The min-secs statement

         min-secs seconds;

         Seconds should be the minimum number of seconds since a client began trying to acquire  a  new
         lease  before  the DHCP server will respond to its request.  The number of seconds is based on
         what the client reports, and the maximum value that the client  can  report  is  255  seconds.
         Generally,  setting  this to one will result in the DHCP server not responding to the client's
         first request, but always responding to its second request.

         This can be used to set up a secondary DHCP server which never offers an address to  a  client
         until the primary server has been given a chance to do so.  If the primary server is down, the
         client will bind to the secondary server, but otherwise clients should always bind to the pri‐
         mary.   Note  that this does not, by itself, permit a primary server and a secondary server to
         share a pool of dynamically-allocatable addresses.

       The next-server statement

         next-server server-name;

         The next-server statement is used to specify the host address of the  server  from  which  the
         initial  boot  file (specified in the filename statement) is to be loaded.  Server-name should
         be a numeric IP address or a domain name.  If no next-server  statement  applies  to  a  given
         client, the address 0.0.0.0 is used.

       The omapi-port statement

         omapi-port port;

         The  omapi-port statement causes the DHCP server to listen for OMAPI connections on the speci‐
         fied port.  This statement is required to enable the OMAPI protocol, which is used to  examine
         and modify the state of the DHCP server as it is running.

       The one-lease-per-client statement

         one-lease-per-client flag;

         If  this  flag  is  enabled, whenever a client sends a DHCPREQUEST for a particular lease, the
         server will automatically free any other leases the client holds.  This presumes that when the
         client  sends  a  DHCPREQUEST,  it  has forgotten any lease not mentioned in the DHCPREQUEST -
         i.e., the client has only a single network interface and it  does  not  remember  leases  it's
         holding  on  networks to which it is not currently attached.  Neither of these assumptions are
         guaranteed or provable, so we urge caution in the use of this statement.

       The pid-file-name statement

         pid-file-name name;

         Name should be the name of the DHCP server's process ID file.  This is the file in  which  the
         DHCP   server's   process  ID  is  stored  when  the  server  starts.   By  default,  this  is
         /var/run/dhcpd.pid.  Like the lease-file-name statement, this statement  must  appear  in  the
         outer  scope of the configuration file.  It has no effect if overridden by the -pf flag or the
         PATH_DHCPD_PID environment variable.

         The dhcpv6-pid-file-name statement

            dhcpv6-pid-file-name name;

            Name is the name of the pid file to use if and only if the  server  is  running  in  DHCPv6
            mode.   By default, this is /var/lib/dhcpd/dhcpd6.pid.  This statement, like pid-file-name,
            must appear in the outer scope of the configuration file.  It has no effect  if  overridden
            by  the  -pf  flag or the PATH_DHCPD6_PID environment variable.  If dhcpv6-pid-file-name is
            not specified, but pid-file-name is, the latter value will be used.

         The ping-check statement

            ping-check flag;

            When the DHCP server is considering dynamically allocating an IP address to  a  client,  it
            first  sends  an  ICMP Echo request (a ping) to the address being assigned.  It waits for a
            second, and if no ICMP Echo response has been heard, it assigns the address.  If a response
            is heard, the lease is abandoned, and the server does not respond to the client.

            This  ping  check  introduces a default one-second delay in responding to DHCPDISCOVER mes‐
            sages, which can be a problem for some clients.  The default delay of  one  second  may  be
            configured using the ping-timeout parameter.  The ping-check configuration parameter can be
            used to control checking - if its value is false, no ping check is done.

         The ping-timeout statement

            ping-timeout seconds;

            If the DHCP server determined it should send an ICMP echo  request  (a  ping)  because  the
            ping-check  statement  is  true,  ping-timeout allows you to configure how many seconds the
            DHCP server should wait for an ICMP Echo response to be heard, if no ICMP Echo response has
            been  received before the timeout expires, it assigns the address.  If a response is heard,
            the lease is abandoned, and the server does not respond to the client.  If no value is set,
            ping-timeout defaults to 1 second.

         The preferred-lifetime statement

            preferred-lifetime seconds;

            IPv6  addresses  have  ´valid´ and ´preferred´ lifetimes.  The valid lifetime determines at
            what point at lease might be said to have expired, and is no longer useable.   A  preferred
            lifetime  is  an  advisory  condition to help applications move off of the address and onto
            currently valid addresses (should there still be any open TCP sockets or similar).

            The preferred lifetime defaults to the renew+rebind timers, or 3/4 the default  lease  time
            if none were specified.

         The remote-port statement

            remote-port port;

            This  statement  causes the DHCP server to transmit DHCP responses to DHCP clients upon the
            UDP port specified in port, rather than on port 68.  In the event that the UDP response  is
            transmitted  to a DHCP Relay, the server generally uses the local-port configuration value.
            Should the DHCP Relay happen to be addressed as 127.0.0.1, however, the DHCP Server  trans‐
            mits  its  response  to the remote-port configuration value.  This is generally only useful
            for testing purposes, and this configuration value should generally not be used.

         The server-identifier statement

            server-identifier hostname;

            The server-identifier statement can be used to define the value that is sent  in  the  DHCP
            Server  Identifier option for a given scope.  The value specified must be an IP address for
            the DHCP server, and must be reachable by all clients served by a particular scope.

            The use of the server-identifier statement is not recommended - the only reason to  use  it
            is  to force a value other than the default value to be sent on occasions where the default
            value would be incorrect.  The default value is the first IP address  associated  with  the
            physical network interface on which the request arrived.

            The  usual  case  where the server-identifier statement needs to be sent is when a physical
            interface has more than one IP address, and the one being sent by default isn't appropriate
            for  some or all clients served by that interface.  Another common case is when an alias is
            defined for the purpose of having a consistent IP address for the DHCP server,  and  it  is
            desired that the clients use this IP address when contacting the server.

            Supplying  a value for the dhcp-server-identifier option is equivalent to using the server-
            identifier statement.

         The server-duid statement

            server-duid LLT [ hardware-type timestamp hardware-address ] ;

            server-duid EN enterprise-number enterprise-identifier ;

            server-duid LL [ hardware-type hardware-address ] ;

            The server-duid statement configures the server DUID. You may pick either LLT  (link  local
            address plus time), EN (enterprise), or LL (link local).

            If  you  choose  LLT  or LL, you may specify the exact contents of the DUID.  Otherwise the
            server will generate a DUID of the specified type.

            If you choose EN, you must include the enterprise number and the enterprise-identifier.

            The default server-duid type is LLT.

         The server-name statement

            server-name name ;

            The server-name statement can be used to inform the client of the name of the  server  from
            which it is booting.  Name should be the name that will be provided to the client.

         The site-option-space statement

            site-option-space name ;

            The  site-option-space statement can be used to determine from what option space site-local
            options will be taken.  This can be used in much the same way  as  the  vendor-option-space
            statement.   Site-local  options  in DHCP are those options whose numeric codes are greater
            than 224.  These options are intended for site-specific uses, but are  frequently  used  by
            vendors of embedded hardware that contains DHCP clients.  Because site-specific options are
            allocated on an ad hoc basis, it is quite possible that one vendor's DHCP client might  use
            the  same option code that another vendor's client uses, for different purposes.  The site-
            option-space option can be used to assign a different set of site-specific options for each
            such vendor, using conditional evaluation (see dhcp-eval (5) for details).

         The stash-agent-options statement

            stash-agent-options flag;

            If the stash-agent-options parameter is true for a given client, the server will record the
            relay agent information options sent during the client's initial DHCPREQUEST  message  when
            the  client  was  in the SELECTING state and behave as if those options are included in all
            subsequent DHCPREQUEST messages sent in the RENEWING state.  This works  around  a  problem
            with  relay agent information options, which is that they usually not appear in DHCPREQUEST
            messages sent by the client in the  RENEWING  state,  because  such  messages  are  unicast
            directly to the server and not sent through a relay agent.

         The update-conflict-detection statement

            update-conflict-detection flag;

            If  the update-conflict-detection parameter is true, the server will perform standard DHCID
            multiple-client, one-name conflict detection.  If the parameter has  been  set  false,  the
            server  will  skip this check and instead simply tear down any previous bindings to install
            the new binding without question.  The default is true.

         The update-optimization statement

            update-optimization flag;

            If the update-optimization parameter is false for a given client, the server will attempt a
            DNS update for that client each time the client renews its lease, rather than only attempt‐
            ing an update when it appears to be necessary.  This will allow the DNS to heal from  data‐
            base  inconsistencies  more  easily, but the cost is that the DHCP server must do many more
            DNS updates.  We recommend leaving this option enabled, which is the default.  This  option
            only affects the behavior of the interim DNS update scheme, and has no effect on the ad-hoc
            DNS update scheme.  If this parameter is not specified, or is true, the  DHCP  server  will
            only  update when the client information changes, the client gets a different lease, or the
            client's lease expires.

         The update-static-leases statement

            update-static-leases flag;

            The update-static-leases flag, if enabled, causes the DHCP server to  do  DNS  updates  for
            clients  even  if  those  clients are being assigned their IP address using a fixed-address
            statement - that is, the client is being given a static assignment.   This  can  only  work
            with  the  interim DNS update scheme.  It is not recommended because the DHCP server has no
            way to tell that the update has been done, and therefore will not delete the record when it
            is  not  in  use.  Also, the server must attempt the update each time the client renews its
            lease, which could have a significant performance impact in environments that  place  heavy
            demands on the DHCP server.

         The use-host-decl-names statement

            use-host-decl-names flag;

            If the use-host-decl-names parameter is true in a given scope, then for every host declara‐
            tion within that scope, the name provided for the host declaration will be supplied to  the
            client as its hostname.  So, for example,

                group {
                  use-host-decl-names on;

                  host joe {
                    hardware ethernet 08:00:2b:4c:29:32;
                    fixed-address joe.fugue.com;
                  }
                }

            is equivalent to

                  host joe {
                    hardware ethernet 08:00:2b:4c:29:32;
                    fixed-address joe.fugue.com;
                    option host-name "joe";
                  }

            An  option  host-name statement within a host declaration will override the use of the name
            in the host declaration.

            It should be noted here that most DHCP clients completely ignore the host-name option  sent
            by the DHCP server, and there is no way to configure them not to do this.  So you generally
            have a choice of either not having any hostname to  client  IP  address  mapping  that  the
            client  will  recognize,  or doing DNS updates.  It is beyond the scope of this document to
            describe how to make this determination.

         The use-lease-addr-for-default-route statement

            use-lease-addr-for-default-route flag;

            If the use-lease-addr-for-default-route parameter is true in a given scope, then instead of
            sending  the  value  specified  in  the routers option (or sending no value at all), the IP
            address of the lease being assigned is sent to the client.  This  supposedly  causes  Win95
            machines to ARP for all IP addresses, which can be helpful if your router is configured for
            proxy ARP.  The use of this feature is not recommended, because it won't work for many DHCP
            clients.

         The vendor-option-space statement

            vendor-option-space string;

            The  vendor-option-space  parameter  determines  from  what option space vendor options are
            taken.  The use of this configuration parameter is illustrated in the dhcp-options(5)  man‐
            ual page, in the VENDOR ENCAPSULATED OPTIONS section.

SETTING PARAMETER VALUES USING EXPRESSIONS
       Sometimes  it's  helpful  to  be  able to set the value of a DHCP server parameter based on some
       value that the client has sent.  To do this, you  can  use  expression  evaluation.   The  dhcp-
       eval(5)  manual  page describes how to write expressions.  To assign the result of an evaluation
       to an option, define the option as follows:

         my-parameter = expression ;

       For example:

         ddns-hostname = binary-to-ascii (16, 8, "-",
                                          substring (hardware, 1, 6));

RESERVED LEASES
       It's often useful to allocate a single address to a single client,  in  approximate  perpetuity.
       Host  statements with fixed-address clauses exist to a certain extent to serve this purpose, but
       because host statements are intended to approximate ´static configuration´, they suffer from not
       being  referenced  in  a  littany  of  other Server Services, such as dynamic DNS, failover, ´on
       events´ and so forth.

       If a standard dynamic lease, as from any range statement, is marked ´reserved´, then the  server
       will only allocate this lease to the client it is identified by (be that by client identifier or
       hardware address).

       In practice, this means that the lease follows the normal state engine, enters ACTIVE state when
       the  client  is bound to it, expires, or is released, and any events or services that would nor‐
       mally be supplied during these events are processed normally, as with any other  dynamic  lease.
       The  only  difference  is that failover servers treat reserved leases as special when they enter
       the FREE or BACKUP states - each server applies the lease into the state it may allocate from  -
       and  the  leases  are not placed on the queue for allocation to other clients.  Instead they may
       only be ´found´ by client identity.  The result is that the lease is only offered to the return‐
       ing client.

       Care should probably be taken to ensure that the client only has one lease within a given subnet
       that it is identified by.

       Leases may be set ´reserved´ either through OMAPI, or through the ´infinite-is-reserved´ config‐
       uration option (if this is applicable to your environment and mixture of clients).

       It should also be noted that leases marked ´reserved´ are effectively treated the same as leases
       marked ´bootp´.

REFERENCE: OPTION STATEMENTS
       DHCP option statements are documented in the dhcp-options(5) manual page.

REFERENCE: EXPRESSIONS
       Expressions used in DHCP option statements and elsewhere are documented in the dhcp-eval(5) man‐
       ual page.

SEE ALSO
       dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.

AUTHOR
       dhcpd.conf(5)  was  written  by  Ted  Lemon  under a contract with Vixie Labs.  Funding for this
       project was provided by Internet Systems Consortium.  Information about Internet Systems Consor‐
       tium can be found at https://www.isc.org.



                                                                                          dhcpd.conf(5)

Konfigurationsbeispiel

Ein rudimentäres Konfigurationsbeispiel finden wir in der Datei /usr/share/doc/dhcp-4.2.5/dhcpd.conf.example, welches die Installation des RPM_Paketes dhcp mitbrachte.

 # less /usr/share/doc/dhcp-4.2.5/dhcpd.conf.example
# dhcpd.conf
#
# Sample configuration file for ISC dhcpd
#

# option definitions common to all supported networks...
option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;

default-lease-time 600;
max-lease-time 7200;

# Use this to enble / disable dynamic dns updates globally.
#ddns-update-style none;

# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
#authoritative;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;

# No service will be given on this subnet, but declaring it helps the 
# DHCP server to understand the network topology.

subnet 10.152.187.0 netmask 255.255.255.0 {
}

# This is a very basic subnet declaration.

subnet 10.254.239.0 netmask 255.255.255.224 {
  range 10.254.239.10 10.254.239.20;
  option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
}

# This declaration allows BOOTP clients to get dynamic addresses,
# which we don't really recommend.

subnet 10.254.239.32 netmask 255.255.255.224 {
  range dynamic-bootp 10.254.239.40 10.254.239.60;
  option broadcast-address 10.254.239.31;
  option routers rtr-239-32-1.example.org;
}

# A slightly different configuration for an internal subnet.
subnet 10.5.5.0 netmask 255.255.255.224 {
  range 10.5.5.26 10.5.5.30;
  option domain-name-servers ns1.internal.example.org;
  option domain-name "internal.example.org";
  option routers 10.5.5.1;
  option broadcast-address 10.5.5.31;
  default-lease-time 600;
  max-lease-time 7200;
}

# Hosts which require special configuration options can be listed in
# host statements.   If no address is specified, the address will be
# allocated dynamically (if possible), but the host-specific information
# will still come from the host declaration.

host passacaglia {
  hardware ethernet 0:0:c0:5d:bd:95;
  filename "vmunix.passacaglia";
  server-name "toccata.fugue.com";
}

# Fixed IP addresses can also be specified for hosts.   These addresses
# should not also be listed as being available for dynamic assignment.
# Hosts for which fixed IP addresses have been specified can boot using
# BOOTP or DHCP.   Hosts for which no fixed address is specified can only
# be booted with DHCP, unless there is an address range on the subnet
# to which a BOOTP client is connected which has the dynamic-bootp flag
# set.
host fantasia {
  hardware ethernet 08:00:07:26:c0:a5;
  fixed-address fantasia.fugue.com;
}

# You can declare a class of clients and then do address allocation
# based on that.   The example below shows a case where all clients
# in a certain class get addresses on the 10.17.224/24 subnet, and all
# other clients get addresses on the 10.0.29/24 subnet.

class "foo" {
  match if substring (option vendor-class-identifier, 0, 4) = "SUNW";
}

shared-network 224-29 {
  subnet 10.17.224.0 netmask 255.255.255.0 {
    option routers rtr-224.example.org;
  }
  subnet 10.0.29.0 netmask 255.255.255.0 {
    option routers rtr-29.example.org;
  }
  pool {
    allow members of "foo";
    range 10.17.224.10 10.17.224.250;
  }
  pool {
    deny members of "foo";
    range 10.0.29.10 10.0.29.230;
  }
}

Damit der DHCP-Server richtig funktioniert, muss dessen Konfigurationsdatei noch angepasst werden. Diese hierarchisch aufgebaute config-Datei dhcpd.conf liegt unter /etc. Zuerst werden allgemein gültige globale Parameter festgelegt, welche für alle Klienten im Netz gelten, z.B. Domainname und Gateway-Adresse.

Als nächstes folgen in dieser Hierarchie die Subnetze. Hier dürfen ebenfalls Parameter für die Klienten festgelegt werden, die sich in diesem Subnetz befinden. So könnte z.B. jedes Subnetz einen eigenen Default-Router besitzen. Die Parameter innerhalb des Subnetzes überschreiben die global definierten Parameter! Bei solchen Subnetzen ist es wichtig, dass der DHCP-Server nur auf Anfragen aus dem Subnetz antwortet, welche in der dhcpd.conf definiert wurden. Wurde ein Subnetz nicht beschrieben, so werden Anfrage einfach ignoriert.

Die nächste Hierarchiestufe ist der Pool, dieser wird innerhalb eines Subnetzes angelegt. In so einem Pool können auch Bereiche angelegt werden, so können z.B. auch mehrere Pools in einem Subnetz existieren.

Als letzte Stufe in der Hierarchie gibt es die Host-Stufe. In dieser Stufe können einzelne Rechner konfiguriert werden, wenn z.B. diese immer die gleiche IP bekommen sollen. Diese festen IP-Adressen widerspricht zwar der Grundüberlegung von DHCP, aber manchmal kann dies wünschenswert sein, wenn z.B. Zugriffsregeln auf andere Hosts oder Server (Paketfilter oder TCP-Wrapper Regeln) für eine bestimmte IP festgelegt wurden.

Das „group“ Statement hilft dabei, neben der Unterteilung nach Subnetzen, Konfigurationsblöcke mit gleichen Parametern zusammenzufassen. So muss nicht für jeden Host die gesamte Palette wiederholt werden.

Vergebene Adressen werden in der Datei /var/lib/dhcpd/dhcpd.leases gespeichert.

Anschließend wird die Konfigurationsdatei unter /etc/dhcpd.conf entsprechend den eigenen Anforderungen angelegt.

 # vim /etc/dhcp/dhcpd.conf
/etc/dhcp/dhcpd.conf
subnet 10.0.10.0 netmask 255.255.255.192 {

        option routers                  10.0.10.1;
        option subnet-mask              255.255.255.192;

        option nis-domain               "nausch.org";
        option domain-name              "nausch.org";
	option domain-search		"dmz.nausch.org", "intra.nausch.org", "nausch.org";
	option domain-name-servers      10.0.10.1;

        option time-offset              -18000; # Eastern Standard Time
        option ntp-servers              10.0.10.1;
        option log-servers               10.0.10.1;

        range dynamic-bootp 10.0.10.50 10.0.10.62;
        default-lease-time 21600;
        max-lease-time 43200;

        }

  host pml010010
  { hardware ethernet 00:22:68:5C:A4:8A;
    fixed-address 10.0.10.10;
  }

Bevor wir nun unseren DHCP-Server das erste mal starten, überprüfen wir unsere Konfiguration mit:

 # service dhcpd configtest
 Syntax: OK

Den ersten Start unseres DHCP-Server nehmen wir wie folgt vor.

 # service dhcpd start
 dhcpd starten:                                             [  OK  ]

Im syslog wird der erfolgreiche Start entsprechend quittiert:

Oct  6 15:36:23 vml000020 dhcpd: Internet Systems Consortium DHCP Server 4.1.1-P1
Oct  6 15:36:23 vml000020 dhcpd: Copyright 2004-2010 Internet Systems Consortium.
Oct  6 15:36:23 vml000020 dhcpd: All rights reserved.
Oct  6 15:36:23 vml000020 dhcpd: For info, please visit https://www.isc.org/software/dhcp/
Oct  6 15:36:23 vml000020 dhcpd: Not searching LDAP since ldap-server, ldap-port and ldap-base-dn were not specified in the config file
Oct  6 15:36:23 vml000020 dhcpd: Wrote 0 deleted host decls to leases file.
Oct  6 15:36:23 vml000020 dhcpd: Wrote 0 new dynamic host decls to leases file.
Oct  6 15:36:23 vml000020 dhcpd: Wrote 2 leases to leases file.
Oct  6 15:36:23 vml000020 dhcpd: Listening on LPF/eth0/52:54:00:c0:d5:ae/10.0.10.0/26
Oct  6 15:36:23 vml000020 dhcpd: Sending on   LPF/eth0/52:54:00:c0:d5:ae/10.0.10.0/26
Oct  6 15:36:23 vml000020 dhcpd: Sending on   Socket/fallback/fallback-net

Beim Starten eines Klienten, in unserem Faller der Host proton mit der MAC-Adresse 00:22:68:5c:a4:8a, wird das Aushandeln der IP-Adresse vermerkt:

Oct  4 21:36:14 vml000020 dhcpd: DHCPDISCOVER from 00:22:68:5c:a4:8a via eth0
Oct  4 21:36:14 vml000020 dhcpd: DHCPOFFER on 10.0.10.10 to 00:22:68:5c:a4:8a via eth0
Oct  4 21:36:14 vml000020 dhcpd: DHCPREQUEST for 10.0.10.10 (10.0.10.1) from 00:22:68:5c:a4:8a via eth0
Oct  4 21:36:14 vml000020 dhcpd: DHCPACK on 10.0.10.10 to 00:22:68:5c:a4:8a via eth0

Damit nun unser DHCP-server beim Booten automatisch gestartet wird, nehmen wir noch folgende Konfigurationsschritte vor.

 # chkconfig dhcpd on

Anschließend überprüfen wir noch unsere Änderung:

 # chkconfig --list | grep dhcpd
 dhcpd          	0:off	1:off	2:on	3:on	4:on	5:on	6:off
 dhcpd6         	0:off	1:off	2:off	3:off	4:off	5:off	6:off
 

Mit nachfolgendem Befehl, kann die sogenannte Lease-Datei eingesehene werden, die standardmäßig im den Verzeichnis /var/lib/dhcpd/, inklusive der zugehörigen Backup-Datei, welche am Namensende mit einem '~'-Zeichen gekennzeichnet ist, also:

  • /var/lib/dhcpd/dhcpd.leases
  • /var/lib/dhcpd/dhcpd.leases~
 # cat /var/lib/dhcpd/dhcpd.leases
 
/var/lib/dhcpd/dhcpd.leases
# The format of this file is documented in the dhcpd.leases(5) manual page.
# This lease file was written by isc-dhcp-4.1.1-P1

lease 10.0.10.50 {
  starts 6 2011/08/20 20:34:41;
  ends 0 2011/08/21 02:34:41;
  tstp 0 2011/08/21 02:34:41;
  cltt 6 2011/08/20 20:34:41;
  binding state free;
  hardware ethernet 00:11:22:33:44:55;
}
lease 10.0.10.51 {
  starts 5 2011/08/26 03:01:55;
  ends 5 2011/08/26 09:01:55;
  tstp 5 2011/08/26 09:01:55;
  cltt 5 2011/08/26 03:01:55;
  binding state free;
  hardware ethernet f0:11:22:33:44:55;
}
server-duid "\000\001\000\001\025\342\3146RT\000\300\325\256";

Überwachung der ARP Tabelle mit arpwatch

Mit dem Netzwerktool arpwatch können wir die MAC-Adressen mit den zugehörigen IP-Adressen im lokalen Netzwerk überwachen. Es setzt die Netzwerkkarte in den Promiscuous Mode und überwacht die ARP-Pakete, welche das lokale Netz passieren. arpwatch ist jedoch kein IDS6)-Programm, sondern ein Hilfsprogramm, das sich auf eine spezielle Art von Angriffen spezialisiert hat. So ist arpwatch nur in der Lage, Angriffe auf das ARP-Protokoll zu erkennen (z. B. ARP-Spoofing). Ausserdem erkennt es neu im Netz auftauchende Rechner.

arpwatch wird bei Bedarf einben Alarm auslösen, wenn einer der folgenden Zustände bei der Überwachung der ARP-IP Tabelle auftritt.

Meldung Beschreibung
New Activity Das Adress Paar wird sechs Monate oder mehr wieder benutzt
New Station Diese MAC Adresse wurde zum ersten Mal beobachtet
Changed MAC Address Die MAC Adresse hat sich geändert
Flip Flop Die MAC Adresse hat sich zu einer schon mal verwendeten MAC Adresse geändert

Tritt mindestens eines dieser Ereignisse auf, kann der BOfH7) unmittelbar via eMail gewarnt und informiert werden. Ferner werden im Syslog folgende Ereignisse protokolliert:

Meldung Beschreibung
MAC broadcast Die MAC Adresse des Rechners ist die Broadcast Adresse oder besteht nur aus Nullen
ip broadcast Die IP-Adresse des Hosts ist eine Broadcast Adresse
Bogon Die IP-Adresse des Hosts gehört nicht in dieses Subnetz
MAC mismatch Die Quell MAC Adresse entspricht nicht der MAC Adresse innerhalb des ARP Pakets
reused old MAC address Die MAC Adresse hat sich auf eine MAC Adresse geändert, die entweder der drittletzten oder noch älteren MAC Adresse entspricht

Mit diesem Überwachungs-Programm können wir nun sehr schnell doppelte MAC- und IP-Adressen, oder weitere Netzwerkfehlkonfigurationen wie auch Angriffe von aktiven Sniffern erkennen.

In der Datei /var/arpwatch/arp.dat legt arpwatch die Adresspaare ab, die es bisher im Netzwerk beobachtet hat.

0:4:13:2a:b:6b  192.168.100.174   1254422914      snom300-3
0:4:13:40:3:35  192.168.100.170   1254422890      snom820
0:4:13:23:3f:b5 192.168.100.171   1254422865      snom360
0:4:13:25:c:90  192.168.100.173   1254422441      snom300-2
0:9:45:40:f2:b3 192.168.100.176   1254422688      ST-100

Die Installation unter CentOS geht am einfachsten via yum.

 # yum install arpwatch -y

Was uns das Programmpaket mitbringt, erkunden wir mit der Option iql beim Programm rpm.

 # rpm -qil arpwatch
Name        : arpwatch                     Relocations: (not relocatable)
Version     : 2.1a15                            Vendor: CentOS
Release     : 14.el6                        Build Date: Mon 23 Aug 2010 09:18:20 PM CEST
Install Date: Sat 20 Aug 2011 10:26:47 PM CEST      Build Host: c6b3.bsys.dev.centos.org
Group       : Applications/System           Source RPM: arpwatch-2.1a15-14.el6.src.rpm
Size        : 462477                           License: BSD with advertising
Signature   : RSA/8, Sun 03 Jul 2011 06:02:34 AM CEST, Key ID 0946fca2c105b9de
Packager    : CentOS BuildSystem <http://bugs.centos.org>
URL         : http://ee.lbl.gov/
Summary     : Network monitoring tools for tracking IP addresses on a network
Description :
The arpwatch package contains arpwatch and arpsnmp.  Arpwatch and
arpsnmp are both network monitoring tools.  Both utilities monitor
Ethernet or FDDI network traffic and build databases of Ethernet/IP
address pairs, and can report certain changes via email.

Install the arpwatch package if you need networking monitoring devices
which will automatically keep track of the IP addresses on your
network.
/etc/rc.d/init.d/arpwatch
/etc/sysconfig/arpwatch
/usr/sbin/arp2ethers
/usr/sbin/arpsnmp
/usr/sbin/arpwatch
/usr/sbin/massagevendor
/usr/share/doc/arpwatch-2.1a15
/usr/share/doc/arpwatch-2.1a15/CHANGES
/usr/share/doc/arpwatch-2.1a15/README
/usr/share/doc/arpwatch-2.1a15/arpfetch
/usr/share/man/man8/arp2ethers.8.gz
/usr/share/man/man8/arpsnmp.8.gz
/usr/share/man/man8/arpwatch.8.gz
/usr/share/man/man8/massagevendor.8.gz
/var/lib/arpwatch
/var/lib/arpwatch/arp.dat
/var/lib/arpwatch/ethercodes.dat

Viel gibt es nicht zu Konfigurieren, lediglich in der Datrei /etc/sysconfig/arpwatch, können wir angeben unter welchen User arpwatch laufen, wer die eMail bekommen und wer als Absender benutzt werden soll.

 # vim /etc/sysconfig/arpwatch
/etc/sysconfig/arpwatch
# -u <username> : defines with what user id arpwatch should run
# -e <email>    : the <email> where to send the reports
# -s <from>     : the <from>-address 
#OPTIONS="-u arpwatch -e root -s 'root (Arpwatch)'"
# Django : 2011-08-20 arpwatch individualisiert
OPTIONS="-u arpwatch -e root -s 'arpwatch (Arpwatch-Daemon on vml000020)'"

Den arpwatch daemon starten wir einfach mit dem Aufruf:

 # service arpwatch start
 arpwatch starten:                                          [  OK  ]

Der erfolgreiche Programmstart wird uns im syslog dokumentiert:

Oct  1 20:33:37 nss kernel: device eth0 entered promiscuous mode
Oct  1 20:33:37 vml000020 arpwatch: listening on eth0
Oct  1 20:33:39 vml000020 arpwatch: new station 10.0.10.10 0:4:13:2a:4b:6b

Damit nun unser arpwatch-Daemon beim Booten automatisch gestartet wird, nehmen wir noch folgende Konfigurationsschritte vor.

 # chkconfig arpwatch on

Anschließend überprüfen wir noch unsere Änderung:

 # chkconfig --list | grep arpwatch
 arpwatch       	0:off	1:off	2:on	3:on	4:on	5:on	6:off

Beschafft sich nun ein Klient von unserem DHCP-Server eine Adresse, so wird nunmehr diese Aktion an den Sys-Admin per eMail gemeldet:

Datum  :  	Sat, 20 Aug 2011 22:37:12 +0200 (CEST) [20.08.2011 22:37:12 CEST]
Von    :  	"Arpwatch-Daemon @ vml000020" <arpwatch@nausch.org>arpwatch@nausch.org 
An     :  	root@nausch.orgroot@nausch.org zu meinem Adressbuch hinzufügen
Betreff:  	new station

            hostname: proton.intra.nausch.org
          ip address: 10.0.10.51
    ethernet address: f0:11:22:33:44:55
     ethernet vendor: <unknown>
           timestamp: Saturday, August 20, 2011 22:37:12 +0200 

Links


1)
Dynamic Host Configuration Protocol
2)
Bootstrap Protocol
3)
Preboot eXecution Environment
4)
Unreliable Datagram Protocol
5)
Media Access Control
6)
Intrusion Detection System, ein Computer-Programm zur Erkennung von Angriffen auf Computernetzwerke
7)
Bastard Operator From Hell
Diese Website verwendet Cookies. Durch die Nutzung der Website stimmen Sie dem Speichern von Cookies auf Ihrem Computer zu. Außerdem bestätigen Sie, dass Sie unsere Datenschutzbestimmungen gelesen und verstanden haben. Wenn Sie nicht einverstanden sind, verlassen Sie die Website.Weitere Information
  • centos/dhcp_c7.txt
  • Zuletzt geändert: 28.12.2022 08:40.
  • von django