dhcpd - Dynamic Host Configuration Protocol Server
[ -p port ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -cf config-file ]
[ -lf lease-file ] [ -tf trace-output-file ]
trace-playback-
file ] [ if0 [ ...ifN ] ]
DESCRIPTION
Consortium
Server, dhcpd, implements the
Dynamic Host Configuration Protocol (DHCP) and the
Protocol (BOOTP).
DHCP allows hosts on a TCP/IP network to request and
be assigned IP addresses, and also to discover
information
network to which they are attached.
BOOTP provides similar functional-
ity, with certain restrictions.
CO***IBUTIONS
This software is free software.
At various times its
development
been underwritten by various organizations, including the ISC and Vixie
Enterprises.
The development of 3.0 has been funded almost entirely by
Nominum, Inc.
this point development is being shepherded by Ted Lemon, and hosted
by the ISC, but the future of this project depends on you.
If you have
features you want, please consider implementing them.
DHCP protocol allows a host which is unknown to the network admin-
istrator to be automatically assigned a new IP address out of a pool of
addresses for its network.
In order for this to work, the network
administrator allocates address pools in each subnet
into the dhcpd.conf(5) file.
On startup, dhcpd reads the dhcpd.conf file and stores a list of avail-
able addresses on each subnet in memory.
protocol, dhcpd allocates an address for it.
Each client is assigned a lease, which expires after an amount of
administrator
Before leases
expire, the clients to which leases are assigned are expected to
use the addresses.
Once a lease has
expired, the client to which that lease was assigned is no longer
mitted to use the leased IP address.
leases across system reboots and server
restarts, dhcpd
dhcpd.leases(5)
lease to a host, it
records the lease in this file and makes sure that the contents of
This ensures that even in the event of a
system crash, dhcpd will not forget about a lease that it has assigned.
dhcpd.conf
dhcpd.leases file to refresh its memory about
New leases are appended to the end of the dhcpd.leases file.
to prevent the file from becoming arbitrarily large, from time to
creates a new dhcpd.leases file from its in-core lease database.
Once this file has been written
dhcpd.leases~,
and the new file is renamed dhcpd.leases.
If the sys-
tem crashes in the middle of this process, whichever dhcpd.leases
remains will contain all the lease information, so there is no need for
a special crash recovery process.
BOOTP support is also provided by this server.
Unlike DHCP, the
recovering dynamically-
assigned addresses once they are no longer needed.
It is still possi-
ble to dynamically assign addresses to BOOTP clients, but some adminis-
trative process for reclaiming addresses
leases are granted to BOOTP clients in perpetuity, although the network
administrator may set an earlier cutoff date or a shorter lease
for BOOTP leases if that makes sense.
may also be served in the old standard way, which is to
simply provide a declaration in the
dhcpd.conf
client, permanently assigning an address to each client.
dhcpd.conf
dhcpd must be
restarted.
To restart dhcpd, send a SIGTERM (signal 15) to
/var/run/dhcpd.pid,
and then re-invoke dhcpd.
Because the DHCP server database is
lightweight
not automatically restart itself when it sees a
change to the dhcpd.conf file.
Note: We get a lot of complaints about this.
We realize that it would
be nice if one could send a SIGHUP to the server and have it reload the
This is not technically impossible, but it would require
of work, our resources are extremely limited, and they can
be better spent elsewhere.
So please don’t complain about this on the
mailing list unless you’re prepared to fund a project to implement this
feature, or prepared to do it yourself.
COMMAND LINE
The names of the network interfaces on which dhcpd
broadcasts
be specified on the command line.
This should be done
on systems where dhcpd is unable to identify non-broadcast
interfaces,
but should not be required on other systems.
If no interface names are
specified on the command line dhcpd will identify
are up, eliminating non-broadcast interfaces if possible,
and listen for DHCP broadcasts on each interface.
If dhcpd should listen on a port other than the standard (port 67), the
should be followed by the udp port number on
which dhcpd should listen.
This is mostly useful
To run dhcpd as a foreground process, rather than allowing it to run as
a daemon in the background, the -f flag should be specified.
running dhcpd under a debugger, or when running it out of
inittab on System V systems.
To have dhcpd log to the standard
descriptor,
This can be useful for debugging, and also at sites where a com-
plete log of all dhcp activity must be kept but syslogd is not reliable
Normally, dhcpd will log all output
using the syslog(3) function with the log facility set to LOG_DAEMON.
Dhcpd can be made to use an alternate configuration file with
alternate lease file with the -lf flag.
Because of the
importance of using the same lease database at all times
production,
should be used only for testing
lease files or database files in a non-production environment.
When starting dhcpd up from a system startup script (e.g., /etc/rc), it
out the entire copyright message on
To avoid printing this message, the -q flag
The DHCP server reads two files on startup: a configuration file, and a
lease database.
If the -t flag is specified, the server
test the configuration file for correct syntax, but will not attempt to
perform any network operations.
This can be used to test
configuration file automatically before installing it.
can be used to test the lease database file in a similar
The -tf and -play options allow you to specify a file
the server and all the transactions it pro-
cesses are either logged or played back from.
submitting bug reports - if you are getting a core dump every so often,
you can start the server with the -tf option and then, when the
the trace file will contain all the transactions that led
up to it dumping core, so that the problem can be easily debugged
-play option must be specified with an alternate lease file, using
the -lf switch, so that the DHCP server doesn’t wipe out your
file with its test data.
The DHCP server will refuse to operate
in playback mode unless you specify an alternate lease file.
CONFIGURATION
The syntax of the dhcpd.conf(5) file is
separately.
section should be used as an overview of the configuration process, and
the dhcpd.conf(5) documentation should be consulted for detailed refer-
ence information.
needs to know the subnet numbers and netmasks of all subnets for
which it will be providing service.
In addition, in order to
addresses,
must be assigned one or more ranges of
addresses on each subnet which it can in turn assign to client hosts as
Thus, a very simple configuration providing DHCP support
might look like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.250;
Multiple address ranges may be specified like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.107;
range 239.252.197.113 239.252.197.250;
If a subnet will only be provided with BOOTP
assignment, the range clause can be left out entirely, but the
subnet statement must appear.
Lease Lengths
DHCP leases can be assigned almost any
What lease length makes sense for any given subnet, or for
any given installation, will vary depending on the kinds of hosts being
For example, in an office environment where systems are added from time
to time and removed from time
relatively
might make sense to allow lease times of a month of more.
In a final test environment on a manufacturing floor, it may make
assign a maximum lease length of 30 minutes - enough time to
go through a simple test procedure on a network appliance before
aging it up for delivery.
to specify two lease lengths: the default length that
will be assigned if a client
particular
length, and a maximum lease length.
These are specified as clauses to
the subnet command:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.107;
default-lease-time 600;
max-lease-time 7200;
This particular subnet declaration specifies a default
and a maximum lease time of 7200 seconds
(two hours).
Other common values would be
(one week) and
Each subnet need not have the same lease--in the case of an office envi-
ronment and a manufacturing environment served by the same DHCP server,
to have widely disparate values for default and
maximum lease times on each subnet.
BOOTP Support
Each BOOTP client must be explicitly declared in the
dhcpd.conf
very basic client declaration will specify the client network inter-
face’s hardware address and the IP address to assign
to be able to load a boot file from the server,
that file’s name must be specified.
A simple bootp client declaration
might look like this:
host haagen {
hardware ethernet 08:00:2b:4c:59:23;
fixed-address 239.252.197.9;
filename "/tftpboot/haagen.boot";
Extensions)
provide a mechanism
whereby the server can provide the client with information about how to
network interface (e.g., subnet mask), and also how the
client can access various network services (e.g., DNS, IP routers,
be specified on a per-subnet basis, and, for BOOTP
clients, also on a per-client basis.
In the event that a BOOTP client
declaration
are also specified in its subnet
declaration, the options
declaration
precedence.
A reasonably complete DHCP configuration might look some-
thing like this:
subnet 239.252.197.0 netmask 255.255.255.0 {
range 239.252.197.10 239.252.197.250;
default-lease-time 600 max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 239.252.197.255;
option routers 239.252.197.1;
option domain-name-servers 239.252.197.2, 239.252.197.3;
option domain-name "isc.org";
A bootp host on that subnet that needs to be in a different domain
use a different name server might be declared as follows:
host haagen {
hardware ethernet 08:00:2b:4c:59:23;
fixed-address 239.252.197.9;
filename "/tftpboot/haagen.boot";
option domain-name-servers 192.5.5.1;
option domain-name "";
complete description of the dhcpd.conf file syntax is provided
in dhcpd.conf(5).
The DHCP server provides the capability to modify some of its
ration while it is running, without stopping it, modifying its database
files, and restarting it.
This capability is currently provided
- an API for manipulating remote objects.
OMAPI clients connect
to the server using TCP/IP, authenticate,
server’s current status and make changes to it.
implementing the underlying OMAPI protocol directly, user
programs should use the dhcpctl API or OMAPI
some of the housekeeping chores that OMAPI does
not do automatically.
Dhcpctl and OMAPI are documented in
dhcpctl(3)
and omapi(3).
objects, which can then be examined and modified.
DHCP server exports the following objects: lease, host,
failover-state
has a number of methods that are provided:
lookup, create, and destroy.
In addition, it is possible to
attributes
objects, and in some cases to modify
those attributes.
THE LEASE OBJECT
Leases can’t currently be created or destroyed, but they can be
up to examine and modify their state.
Leases have the following attributes:
state integer lookup, examine
2 = active
3 = expired
4 = released
5 = abandoned
7 = backup
8 = reserved
ip-address data lookup, examine
The IP address of the lease.
dhcp-client-identifier data lookup, examine, update
identifier
that the client used when it acquired the
Not all clients send client identifiers, so
client-hostname data examine, update
The value the client sent in the host-name option.
host handle examine
the host declaration associated with this lease, if any.
subnet handle examine
the subnet object associated with this lease (the subnet object is
not currently supported).
pool handle examine
the pool object associted with this lease (the pool object is
currently supported).
billing-class handle examine
to the class to which this lease is currently billed,
if any (the class object is not currently supported).
hardware-address data examine, update
the hardware address (chaddr) field sent by
acquired its lease.
hardware-type integer examine, update
the type of the network interface that the client reported when it
acquired its lease.
ends time examine
the time when the lease’s current state ends, as understood by the
tstp time examine
the time when the lease’s current state ends, as understood by the
tsfp time examine
the time when the lease’s current state ends, as understood by the
(if there is no failover peer, this value is unde-
cltt time examine
The time of the last transaction with the client on this lease.
THE HOST OBJECT
Hosts can be created, destroyed, looked up, examined and modified.
host declaration is created or deleted using OMAPI, that information
will be recorded in the
dhcpd.leases
permissible
delete host declarations that are declared in the dhcpd.conf file.
Hosts have the following attributes:
name data lookup, examine, modify
the name of the host declaration.
This name must be unique among
all host declarations.
group handle examine, modify
the named group associated with the host declaration, if there
hardware-address data lookup, examine, modify
link-layer
address that will be used to match the client, if
Only valid if hardware-type is also present.
hardware-type integer lookup, examine, modify
the type of the network interface that will be used to
Only valid if hardware-address is also present.
dhcp-client-identifier data lookup, examine, modify
the dhcp-client-identifier option that will be used to
client, if any.
ip-address data examine, modify
for a DHCP client that
matches this host declaration.
the client if it is valid for the network segment to
which the client is connected.
statements data modify
a list of statements in the format of
dhcpd.conf
be executed whenever a message from the client is being pro-
known integer examine, modify
if nonzero, indicates that a client matching this host declaration
in pool permit lists.
If zero, the
client will not be treated as known.
THE GROUP OBJECT
Named groups can be created, destroyed, looked up, examined
a group declaration is created or deleted using OMAPI, that
information will be recorded in the dhcpd.leases file.
It is permissi-
group declarations that are declared in the dhcpd.conf
Named groups currently can only be associated with hosts - this
set of statements to be efficiently attached to more than one host
declaration.
Groups have the following attributes:
the name of the group.
All groups that are
must have names, and the names must be unique among all groups.
statements data
statements
in the format of the dhcpd.conf file that
will be executed whenever a message from a client whose host
laration references this group is processed.
THE CO***OL OBJECT
control object allows you to shut the server down.
If the server
is doing failover with another peer, it will make
transition
state and notify its peer, so that the peer can go
into partner down, and then record the "recover"
file so that when the server is restarted, it will automatically resyn-
chronize with its peer.
On shutdown the server will also attempt to cleanly shut down all OMAPI
connections.
connections do not go down cleanly after five
seconds, they are shut down pre-emptively.
It can take as much
seconds from the beginning of the shutdown process to the time that the
server actually exits.
To shut the server down, open its control
attribute to 2.
THE FAILOVER-STATE OBJECT
failover-state
is the object that tracks the state of the
failover protocol as it is being managed for
The failover object has the following attributes (please see dhcpd.conf
(5) for explanations about what these attributes mean):
name data examine
Indicates the name of the failover peer relationship, as described
in the server’s dhcpd.conf file.
partner-address data examine
Indicates the failover partner’s IP address.
local-address data examine
Indicates the IP address that is being used by the DHCP server for
this failover pair.
partner-port data examine
Indicates the TCP port on which the failover partner is
for failover protocol connections.
local-port data examine
TCP port on which the DHCP server is listening for
failover protocol connections for this failover pair.
max-outstanding-updates integer examine
Indicates the number of updates that can be outstanding and
knowledged at any given time, in this failover relationship.
mclt integer examine
the maximum client lead time in this failover relation-
load-balance-max-secs integer examine
Indicates the maximum value for the secs field in a client request
before load balancing is bypassed.
load-balance-hba data examine
load balancing hash bucket array for this failover
relationship.
local-state integer examine, modify
Indicates the present state of the DHCP server
relationship.
Possible values for state are:
- partner down
- communications interrupted
- resolution interrupted
- potential conflict
- recover done
- shutdown
10 - startup
11 - recover wait
is not a good idea to make changes to this state.
However, in the case that the failover
it can be useful to set the DHCP server’s failover state to
partner down.
At this point the DHCP server will take over
partner’s leases as soon as possible, and
will give out normal leases, not leases
restricted
you do put the DHCP server into the partner-down when
the other DHCP server is not in the partner-down state, but is not
reachable,
assignment
are possible, even
Once a server has been put into partner-down
partner must not be brought back online until communica-
tion is possible between the two servers.
partner-state integer examine
Indicates the present state of the failover partner.
local-stos integer examine
Indicates the time at which the DHCP server
state in this failover relationship.
partner-stos integer examine
failover partner entered its
present state.
hierarchy integer examine
Indicates whether the DHCP server is primary (0) or secondary
in this failover relationship.
last-packet-sent integer examine
at which the most recent failover packet was
sent by this DHCP server to its failover partner.
last-timestamp-received integer examine
Indicates the timestamp that was
recently received from the failover partner.
skew integer examine
skew between the failover partner’s clock and this
DHCP server’s clock
max-response-delay integer examine
Indicates the time in
the failover partner, the partner is assumed to be
out of communication.
cur-unacked-updates integer examine
Indicates the number of update messages that
from the failover partner but not yet processed.
/etc/dhcpd.conf,
/var/lib/dhcp/dhcpd.leases,
/var/run/dhcpd.pid,
/var/lib/dhcp/dhcpd.leases~.
dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
dhcpd(8) was originally written by Ted
Funding for this project was provided by Internet Systems
Consortium.
Version 3 of the DHCP server was funded by Nominum,
Information
Consortium
http://www.isc.org/.
Information
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