dhcpd - Dynamic Host Configuration Protocol Server
dhcpd [ -p port ] [ -f ] [ -d ] [ -q ]
[ -t | -T ] [ -4 | -6 ] [ -4o6 port
] [ -s server ] [ -cf config-file ] [ -lf
lease-file ] [ -pf pid-file ] [ --no-pid ] [
-user user ] [ -group group ] [ -chroot
dir ] [ -tf trace-output-file ] [ -play
trace-playback-file ] [ if0 [ ...ifN ] ]
dhcpd --version
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 functionality, with certain restrictions.
The DHCP protocol allows a host which is unknown to the network administrator 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 available 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 permitted 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
possible to dynamically assign addresses to BOOTP clients, but some
administrative 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 RUNDIR/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.
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 interfaces which are up, eliminating
non-broadcast interfaces if possible, and listen for DHCP broadcasts on each
interface.
- -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.
- -4o6 port
- Participate in the DHCPv4 over DHCPv6 protocol specified by RFC 7341. This
associates a DHCPv4 and a DHCPv6 server to allow the v4 server to receive
v4 requests that were encapsulated in a v6 packet. Communication between
the two servers is done on a pair of UDP sockets bound to ::1 port
and port + 1. Both servers must be launched using the same
port argument.
- -p port
- The UDP port number on which dhcpd should listen. If unspecified
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 daemon
in the background. This is useful when running dhcpd under a
debugger, or when running it out of inittab on System V systems.
- -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 configuration file automatically before
installing it.
- -T
- Test the lease file. The server tests the lease file for correct syntax,
but will not attempt to perform any network operations. In addition to
reading the lease file it will also write the leases to a temporary lease
file. The current lease file will not be modified and the temporary lease
file will be removed upon completion of the test. This can be used to test
a new lease file automatically before installing it.
- -user user
- Setuid to user after completing privileged operations, such as creating
sockets that listen on privileged ports. This also causes the lease file
to be owned by user. This option is only available if the code was
compiled with the PARANOIA patch (./configure --enable-paranoia).
- -group group
- Setgid to group after completing privileged operations, such as creating
sockets that listen on privileged ports. This also causes the lease file
to use group. This option is only available if the code was compiled with
the PARANOIA patch (./configure --enable-paranoia).
- -chroot dir
- Chroot to directory. This may occur before or after reading the
configuration files depending on whether the code was compiled with the
EARLY_CHROOT option enabled (./configure --enable-early-chroot). This
option is only available if the code was compiled with the PARANOIA patch
(./configure --enable-paranoia).
- -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 existing 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.
Modifying default file locations: The following options can
be used to modify the locations dhcpd uses for its files. Because of
the importance 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.
During operations the server may use multiple UDP and TCP ports to provide
different functions. Which ports are opened depends on both the way you
compiled your code and the configuration you supply. The following should
provide you an idea of what ports may be in use.
Normally a DHCPv4 server will open a raw UDP socket to receive and
send most DHCPv4 packets. It also opens a fallback UDP socket for use in
sending unicast packets. Normally these will both use the well known port
number for BOOTPS.
For each DHCPv4 failover peer you list in the configuration file
there will be a TCP socket listening for connections on the ports specified
in the configuration file. When the peer connects there will be another
socket for the established connection. For the established connection the
side (primary or secondary) opening the connection will use a random
port.
For DHCPv6 the server opens a UDP socket on the well known
dhcpv6-server port.
The server opens an icmp socket for doing ping requests to check
if addresses are in use.
If you have included an omapi-port statement in your configuration
file then the server will open a TCP socket on that port to listen for OMPAI
connections. When something connects another port will be used for the
established connection.
When DDNS is enabled at compile time (see includes/site.h) the
server will open both a v4 and a v6 UDP socket on random ports, unless DDNS
updates are globally disabled by setting ddns-update-style to none in the
configuration file.
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 reference
information.
dhcpd needs to know the subnet numbers and netmasks of all subnets for which it
will be providing service. In addition, in order to dynamically 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.
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
infrequently, 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 packaging 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 environment 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.
Each BOOTP client must be explicitly declared in the dhcpd.conf file. A very
basic client declaration will specify the client network interface'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";
}
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 something
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 "example.com";
}
A more 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 configuration
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.
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
understood 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
retransmission logic.
cltt time examine
The time of the last transaction with the client on this
lease.
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
processed.
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.
Named groups can be created, destroyed, looked up, examined and modified. If a
group declaration is created or deleted using OMAPI, that information will be
recorded in the dhcpd.leases file. It is permissible 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 declaration
references this group is processed.
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 resynchronize 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 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 unacknowledged at any given time, in this failover relationship.
mclt integer examine
Indicates the maximum client lead time in this failover
relationship.
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 service 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 communication 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.
ETCDIR/dhcpd.conf, DBDIR/dhcpd.leases, RUNDIR/dhcpd.pid,
DBDIR/dhcpd.leases~.
dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
dhcpd(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/.