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
DHCP-Adressvergabe
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.
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.
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.
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.
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.
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
Installation
Die Installation und Konfiguration des DHCP-Servers gestaltet sich relativ einfach.
Zuerst ist via yum der dhcp-Server zu installieren.
# yum install dhcp -y
RPM Inhalte
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
Konfiguration
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.
IPv4
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.
Konfigurationsdatei bearbeiten
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; }
DHCP-Konfiguration überprüfen
Bevor wir nun unseren DHCP-Server das erste mal starten, überprüfen wir unsere Konfiguration mit:
# service dhcpd configtest
Syntax: OK
DHCP-Server starten
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
automatisches Starten des Dienste beim Systemstart
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
lease-File des DHCP Servers
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
Installation
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
Konfiguration
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)'"
Programmstart
erster Programmstart
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
automatischer Programmstart
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
Status-eMail
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
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