dhcpd(8)dhcpd(8)NAMEdhcpd - Dynamic Host Configuration Protocol Server
SYNOPSISdhcpd [ -p port ] [ -f ] [ -d ] [ -q ] [ -t | -T ] [ -4 | -6 ] [ -s
server ] [ -cf config-file ] [ -lf lease-file ] [ -pf pid-file ] [
--no-pid ] [ -tf trace-output-file ] [ -user user ] [ -group group ] [
-chroot dir ] [ -play trace-playback-file ] [ if0 [ ...ifN ] ]
dhcpd--version
DESCRIPTION
The Internet Systems Consortium DHCP Server, dhcpd, implements the
Dynamic Host Configuration Protocol (DHCP) and the Internet Bootstrap
Protocol (BOOTP). DHCP allows hosts on a TCP/IP network to request and
be assigned IP addresses, and also to discover information about the
network to which they are attached. BOOTP provides similar functional‐
ity, with certain restrictions.
OPERATION
The 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
IP addresses for its network. In order for this to work, the network
administrator allocates address pools in each subnet and enters them
into the dhcpd.conf(5) file.
There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At
startup the server may be started for one or the other via the -4 or
-6 arguments.
On startup, dhcpd reads the dhcpd.conf file and stores a list of avail‐
able addresses on each subnet in memory. When a client requests an
address using the DHCP protocol, dhcpd allocates an address for it.
Each client is assigned a lease, which expires after an amount of time
chosen by the administrator (by default, one day). Before leases
expire, the clients to which leases are assigned are expected to renew
them in order to continue to use the addresses. Once a lease has
expired, the client to which that lease was assigned is no longer per‐
mitted to use the leased IP address.
In order to keep track of leases across system reboots and server
restarts, dhcpd keeps a list of leases it has assigned in the
dhcpd.leases(5) file. Before dhcpd grants a lease to a host, it
records the lease in this file and makes sure that the contents of the
file are flushed to disk. This ensures that even in the event of a
system crash, dhcpd will not forget about a lease that it has assigned.
On startup, after reading the dhcpd.conf file, dhcpd reads the
dhcpd.leases file to refresh its memory about what leases have been
assigned.
New leases are appended to the end of the dhcpd.leases file. In order
to prevent the file from becoming arbitrarily large, from time to time
dhcpd creates a new dhcpd.leases file from its in-core lease database.
Once this file has been written to disk, the old file is renamed
dhcpd.leases~, and the new file is renamed dhcpd.leases. If the system
crashes in the middle of this process, whichever dhcpd.leases file
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 BOOTP
protocol does not provide a protocol for 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 is required. By default,
leases are granted to BOOTP clients in perpetuity, although the network
administrator may set an earlier cutoff date or a shorter lease length
for BOOTP leases if that makes sense.
BOOTP clients may also be served in the old standard way, which is to
simply provide a declaration in the dhcpd.conf file for each BOOTP
client, permanently assigning an address to each client.
Whenever changes are made to the dhcpd.conf file, dhcpd must be
restarted. To restart dhcpd, send a SIGTERM (signal 15) to the process
ID contained in /var/run/dhcpd.pid, and then re-invoke dhcpd. Because
the DHCP server database is not as lightweight as a BOOTP database,
dhcpd does 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
database. This is not technically impossible, but it would require a
great deal 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 should listen for
broadcasts may 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 all network inter‐
faces which are up, eliminating non-broadcast interfaces if possible,
and listen for DHCP broadcasts on each interface.
COMMAND LINE OPTIONS-4 Run as a DHCP server. This is the default and cannot be combined
with -6.
-6 Run as a DHCPv6 server. This cannot be combined with -4.
-p port
The udp port number on which dhcpd should listen. If unspeci‐
fied dhcpd uses the default port of 67. This is mostly useful
for debugging purposes.
-s address
Specify an address or host name to which dhcpd should send
replies rather than the broadcast address (255.255.255.255).
This option is only supported in IPv4.
-f Force dhcpd to run as a foreground process instead of as a dae‐
mon in the background. This is useful when running dhcpd under
a debugger, or when running it out of inittab on System V sys‐
tems.
-d Send log messages to the standard error descriptor. This can be
useful for debugging, and also at sites where a complete log of
all dhcp activity must be kept but syslogd is not reliable or
otherwise cannot be used. Normally, dhcpd will log all output
using the syslog(3) function with the log facility set to
LOG_DAEMON. Note that -d implies -f (the daemon will not fork
itself into the background).
-q Be quiet at startup. This suppresses the printing of the entire
copyright message during startup. This might be desirable when
starting dhcpd from a system startup script (e.g., /etc/rc).
-t Test the configuration file. The server tests the configuration
file for correct syntax, but will not attempt to perform any
network operations. This can be used to test a new configura‐
tion file automatically before installing it.
-T Test the lease file. The server tests the lease file for cor‐
rect syntax, but will not attempt to perform any network opera‐
tions. This can be used to test a new leaes file automatically
before installing it.
-tf tracefile
Specify a file into which the entire startup state of the server
and all the transactions it processes are logged. This can be
useful in 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 server dumps core, the trace file will
contain all the transactions that led up to it dumping core, so
that the problem can be easily debugged with -play.
-play playfile
Specify a file from which the entire startup state of the server
and all the transactions it processed are read. The -play
option must be specified with an alternate lease file, using the
-lf switch, so that the DHCP server doesn't wipe out your exist‐
ing lease file with its test data. The DHCP server will refuse
to operate in playback mode unless you specify an alternate
lease file.
--version
Print version number and exit.
-user user
Setuid to user after completing privileged operations, such as
creating sockets that listen on privileged ports.
-group group
Setgid to group after completing privileged operations, such as
creating sockets that listen on privileged ports.
-chroot dir
Chroot to directory after processing the command line arguments,
but before reading the configuration file.
Modifying default file locations: The following options can be used to
modify the locations dhcpd uses for it's files. Because of the impor‐
tance of using the same lease database at all times when running dhcpd
in production, these options should be used only for testing lease
files or database files in a non-production environment.
-cf config-file
Path to alternate configuration file.
-lf lease-file
Path to alternate lease file.
-pf pid-file
Path to alternate pid file.
--no-pid
Option to disable writing pid files. By default the program
will write a pid file. If the program is invoked with this
option it will not check for an existing server process.
CONFIGURATION
The syntax of the dhcpd.conf(5) file is discussed separately. This
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.
Subnetsdhcpd needs to know the subnet numbers and netmasks of all subnets for
which it will be providing service. In addition, in order to dynami‐
cally allocate addresses, it must be assigned one or more ranges of
addresses on each subnet which it can in turn assign to client hosts as
they boot. 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 service and no dynamic
address assignment, the range clause can be left out entirely, but the
subnet statement must appear.
Lease Lengths
DHCP leases can be assigned almost any length from zero seconds to
infinity. What lease length makes sense for any given subnet, or for
any given installation, will vary depending on the kinds of hosts being
served.
For example, in an office environment where systems are added from time
to time and removed from time to time, but move relatively infre‐
quently, it might make sense to allow lease times of a month or more.
In a final test environment on a manufacturing floor, it may make more
sense to assign a maximum lease length of 30 minutes - enough time to
go through a simple test procedure on a network appliance before pack‐
aging it up for delivery.
It is possible to specify two lease lengths: the default length that
will be assigned if a client doesn't ask for any particular lease
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 lease time of
600 seconds (ten minutes), and a maximum lease time of 7200 seconds
(two hours). Other common values would be 86400 (one day), 604800 (one
week) and 2592000 (30 days).
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,
it might make sense 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 file.
A very basic client declaration will specify the client network inter‐
face's hardware address and the IP address to assign to that client.
If the client needs 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";
}
Options
DHCP (and also BOOTP with Vendor Extensions) provide a mechanism
whereby the server can provide the client with information about how to
configure its network interface (e.g., subnet mask), and also how the
client can access various network services (e.g., DNS, IP routers, and
so on).
These options can 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 specifies options that are also specified in its subnet
declaration, the options specified in the client declaration take
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 and
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 "vix.com";
}
A more complete description of the dhcpd.conf file syntax is provided
in dhcpd.conf(5).
OMAPI
The DHCP server provides the capability to modify some of its configu‐
ration while it is running, without stopping it, modifying its database
files, and restarting it. This capability is currently provided using
OMAPI - an API for manipulating remote objects. OMAPI clients connect
to the server using TCP/IP, authenticate, and can then examine the
server's current status and make changes to it.
Rather than implementing the underlying OMAPI protocol directly, user
programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
wrapper that handles some of the housekeeping chores that OMAPI does
not do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)
and omapi(3).
OMAPI exports objects, which can then be examined and modified. The
DHCP server exports the following objects: lease, host, failover-state
and group. Each object has a number of methods that are provided:
lookup, create, and destroy. In addition, it is possible to look at
attributes that are stored on objects, and in some cases to modify
those attributes.
THE LEASE OBJECT
Leases can't currently be created or destroyed, but they can be looked
up to examine and modify their state.
Leases have the following attributes:
state integer lookup, examine
1 = free
2 = active
3 = expired
4 = released
5 = abandoned
6 = reset
7 = backup
8 = reserved
9 = bootp
ip-address data lookup, examine
The IP address of the lease.
dhcp-client-identifier data lookup, examine, update
The client identifier that the client used when it acquired the
lease. Not all clients send client identifiers, so this may be
empty.
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 associated with this lease (the pool object is not
currently supported).
billing-class handle examine
the handle 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 the client when it
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
client.
tstp time examine
the time when the lease's current state ends, as understood by the
server.
tsfp time examine
the adjusted time when the lease's current state ends, as under‐
stood by the failover peer (if there is no failover peer, this
value is undefined). Generally this value is only adjusted for
expired, released, or reset leases while the server is operating
in partner-down state, and otherwise is simply the value supplied
by the peer.
atsfp time examine
the actual tsfp value sent from the peer. This value is forgotten
when a lease binding state change is made, to facilitate retrans‐
mission logic.
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. If
a host declaration is created or deleted using OMAPI, that information
will be recorded in the dhcpd.leases file. It is permissible to 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 is
one.
hardware-address data lookup, examine, modify
the link-layer address that will be used to match the client, if
any. 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 match the
client, if any. 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 match the
client, if any.
ip-address data examine, modify
a fixed IP address which is reserved for a DHCP client that
matches this host declaration. The IP address will only be
assigned to 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 the dhcpd.conf file that
will be executed whenever a message from the client is being pro‐
cessed.
known integer examine, modify
if nonzero, indicates that a client matching this host declaration
will be treated as known 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 and modi‐
fied. If a group declaration is created or deleted using OMAPI, that
information will be recorded in the dhcpd.leases file. It is permissi‐
ble to delete group declarations that are declared in the dhcpd.conf
file.
Named groups currently can only be associated with hosts - this allows
one set of statements to be efficiently attached to more than one host
declaration.
Groups have the following attributes:
name data
the name of the group. All groups that are created using OMAPI
must have names, and the names must be unique among all groups.
statements data
a list of statements in the format of the dhcpd.conf file that
will be executed whenever a message from a client whose host dec‐
laration references this group is processed.
THE CONTROL OBJECT
The control object allows you to shut the server down. If the server
is doing failover with another peer, it will make a clean transition
into the shutdown state and notify its peer, so that the peer can go
into partner down, and then record the "recover" state in the lease
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. If these connections do not go down cleanly after five
seconds, they are shut down preemptively. It can take as much as 25
seconds from the beginning of the shutdown process to the time that the
server actually exits.
To shut the server down, open its control object and set the state
attribute to 2.
THE FAILOVER-STATE OBJECT
The failover-state object is the object that tracks the state of the
failover protocol as it is being managed for a given failover peer.
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 listening
for failover protocol connections.
local-port data examine
Indicates the 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 unac‐
knowledged at any given time, in this failover relationship.
mclt integer examine
Indicates the maximum client lead time in this failover relation‐
ship.
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
Indicates the load balancing hash bucket array for this failover
relationship.
local-state integer examine, modify
Indicates the present state of the DHCP server in this failover
relationship. Possible values for state are:
1 - startup
2 - normal
3 - communications interrupted
4 - partner down
5 - potential conflict
6 - recover
7 - paused
8 - shutdown
9 - recover done
10 - resolution interrupted
11 - conflict done
254 - recover wait
(Note that some of the above values have changed since DHCP
3.0.x.)
In general it is not a good idea to make changes to this state.
However, in the case that the failover partner is known to be
down, it can be useful to set the DHCP server's failover state to
partner down. At this point the DHCP server will take over ser‐
vice of the failover partner's leases as soon as possible, and
will give out normal leases, not leases that are restricted by
MCLT. If 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, IP address assignment conflicts are possible, even
likely. Once a server has been put into partner-down mode, its
failover 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 entered its present
state in this failover relationship.
partner-stos integer examine
Indicates the time at which the failover partner entered its
present state.
hierarchy integer examine
Indicates whether the DHCP server is primary (0) or secondary (1)
in this failover relationship.
last-packet-sent integer examine
Indicates the time 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 on the failover message most
recently received from the failover partner.
skew integer examine
Indicates the skew between the failover partner's clock and this
DHCP server's clock
max-response-delay integer examine
Indicates the time in seconds after which, if no message is
received from the failover partner, the partner is assumed to be
out of communication.
cur-unacked-updates integer examine
Indicates the number of update messages that have been received
from the failover partner but not yet processed.
FILES
/etc/dhcp/dhcpd.conf, /var/lib/dhcpd/dhcpd.leases, /var/run/dhcpd.pid,
/var/lib/dhcpd/dhcpd.leases~.
SEE ALSOdhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)AUTHORdhcpd(8) was originally written by Ted Lemon under a contract with
Vixie Labs. Funding for this project was provided by Internet Systems
Consortium. Version 3 of the DHCP server was funded by Nominum, Inc.
Information about Internet Systems Consortium is available at
https://www.isc.org/.
dhcpd(8)
