Provided by: isc-dhcp-server_4.4.1-2.1ubuntu5.20.04.5_amd64 
NAME
dhcpd - Dynamic Host Configuration Protocol Server
SYNOPSIS
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
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
functionality, with certain restrictions.
OPERATION
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 /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
interfaces 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.
-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.
PORTS
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.
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 reference information.
Subnets
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.
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 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.
BOOTP Support
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";
}
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 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).
OMAPI
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.
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 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.
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 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.
THE GROUP OBJECT
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
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
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.
FILES
/etc/dhcp/dhcpd.conf, /var/lib/dhcp/dhcpd.leases, /var/run/dhcpd.pid,
/var/lib/dhcp/dhcpd.leases~.
SEE ALSO
dhclient(8), dhcrelay(8), dhcpd.conf(5), dhcpd.leases(5)
AUTHOR
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/.
dhcpd(8)