dhclient(8) System Manager's Manual dhclient(8)
NAME
dhclient - Dynamic Host Configuration Protocol Client
SYNOPSIS
dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [ -P... ] ] [ -D LL|LLT ] [ -p port-num‐
ber ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r | -x ] [ -lf lease-file ] [ -pf pid-file ] [ --no-pid ] [ -cf
config-file ] [ -sf script-file ] [ -s server-addr ] [ -g relay ] [ -n ] [ -nw ] [ -w ] [ -nc ] [ -B ] [ -I
dhcp-client-identifier ] [ -H host-name ] [ -F fqdn.fqdn ] [ -V vendor-class-identifier ] [ -R request-option-
list ] [ -timeout timeout ] [ -v ] [ --version ] [ if0 [ ...ifN ] ]
DESCRIPTION
The Internet Systems Consortium DHCP Client, dhclient, provides a means for configuring one or more network
interfaces using the Dynamic Host Configuration Protocol, BOOTP protocol, or if these protocols fail, by stat‐
ically assigning an address.
OPERATION
The DHCP protocol allows a host to contact a central server which maintains a list of IP addresses which may
be assigned on one or more subnets. A DHCP client may request an address from this pool, and then use it on a
temporary basis for communication on network. The DHCP protocol also provides a mechanism whereby a client
can learn important details about the network to which it is attached, such as the location of a default
router, the location of a name server, and so on.
There are two versions of the DHCP protocol DHCPv4 and DHCPv6. At startup the client may be started for one
or the other via the -4 or -6 options.
On startup, dhclient reads the dhclient.conf for configuration instructions. It then gets a list of all the
network interfaces that are configured in the current system. For each interface, it attempts to configure
the interface using the DHCP protocol.
In order to keep track of leases across system reboots and server restarts, dhclient keeps a list of leases it
has been assigned in the dhclient.leases file. On startup, after reading the dhclient.conf file, dhclient
reads the dhclient.leases file to refresh its memory about what leases it has been assigned.
When a new lease is acquired, it is appended to the end of the dhclient.leases file. In order to prevent the
file from becoming arbitrarily large, from time to time dhclient creates a new dhclient.leases file from its
in-core lease database. The old version of the dhclient.leases file is retained under the name
dhclient.leases~ until the next time dhclient rewrites the database.
Old leases are kept around in case the DHCP server is unavailable when dhclient is first invoked (generally
during the initial system boot process). In that event, old leases from the dhclient.leases file which have
not yet expired are tested, and if they are determined to be valid, they are used until either they expire or
the DHCP server becomes available.
A mobile host which may sometimes need to access a network on which no DHCP server exists may be preloaded
with a lease for a fixed address on that network. When all attempts to contact a DHCP server have failed,
dhclient will try to validate the static lease, and if it succeeds, will use that lease until it is restarted.
A mobile host may also travel to some networks on which DHCP is not available but BOOTP is. In that case, it
may be advantageous to arrange with the network administrator for an entry on the BOOTP database, so that the
host can boot quickly on that network rather than cycling through the list of old leases.
COMMAND LINE
The names of the network interfaces that dhclient should attempt to configure may be specified on the command
line. If no interface names are specified on the command line dhclient will normally identify all network
interfaces, eliminating non-broadcast interfaces if possible, and attempt to configure each interface.
-6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration param‐
eters. It cannot be combined with -4. The -S -T -P -N and -D arguments provide more control over
aspects of the DHCPv6 processing. Note: it is not recommended to mix queries of different types
together or even to share the lease file between them.
-1 Try to get a lease once. On failure exit with code 2. In DHCPv6 this sets the maximum duration of the
initial exchange to timeout (from dhclient.conf with a default of sixty seconds).
-d Force dhclient to run as a foreground process. Normally the DHCP client will run in the foreground
until is has configured an interface at which time it will revert to running in the background. This
option is useful when running the client under a debugger, or when running it out of inittab on System
V systems. This implies -v.
-nw Become a daemon immediately (nowait) rather than waiting until an an IP address has been acquired.
-q Be quiet at startup, this is the default.
-v Enable verbose log messages.
-w Continue running even if no broadcast interfaces were found. Normally DHCP client will exit if it
isn't able to identify any network interfaces to configure. On laptop computers and other computers
with hot-swappable I/O buses, it is possible that a broadcast interface may be added after system
startup. This flag can be used to cause the client not to exit when it doesn't find any such inter‐
faces. The omshell(1) program can then be used to notify the client when a network interface has been
added or removed, so that the client can attempt to configure an IP address on that interface.
-nc Do not drop capabilities.
Normally, if dhclient was compiled with libcap-ng support, dhclient drops most capabilities immediately
upon startup. While more secure, this greatly restricts the additional actions that hooks in dhclient-
script (8) can take. (For example, any daemons that dhclient-script (8) starts or restarts will
inherit the restricted capabilities as well, which may interfere with their correct operation.) Thus,
the -nc option can be used to prevent dhclient from dropping capabilities.
The -nc option is ignored if dhclient was not compiled with libcap-ng support.
-B Set the BOOTP broadcast flag in request packets so servers will always broadcast replies.
-I <dhcp-client-identifier>
Specify the dhcp-client-identifier option to send to the DHCP server.
-H <host-name>
Specify the host-name option to send to the DHCP server. The host-name string only contains the
client's hostname prefix, to which the server will append the ddns-domainname or domain-name options,
if any, to derive the fully qualified domain name of the client. The -H option cannot be used with the
-F option.
-F <fqdn.fqdn>
Specify the fqdn.fqdn option to send to the DHCP server. This option cannot be used with the -H
subnet-mask, broadcast-address, time-offset, routers,
domain-search, domain-name, domain-name-servers, host-name,
nis-domain, nis-servers, ntp-servers, interface-mtu
-R option does not append options to the default request, it overrides the default request list. Keep
this in mind if you want to request an additional option besides the default request list. You will
have to specify all option names for the -R parameter.
-timeout <timeout>
Specify the time after which dhclient will decide that no DHCP servers can be contacted when no
responses have been received.
-n Do not configure any interfaces. This is most likely to be useful in combination with the -w flag.
-e VAR=value
Define additional environment variables for the environment where dhclient-script executes. You may
specify multiple -e options on the command line.
-r Release the current lease and stop the running DHCP client as previously recorded in the PID file.
When shutdown via this method dhclient-script will be executed with the specific reason for calling the
script set. The client normally doesn't release the current lease as this is not required by the DHCP
protocol but some cable ISPs require their clients to notify the server if they wish to release an
assigned IP address.
-x Stop the running DHCP client without releasing the current lease. Kills existing dhclient process as
previously recorded in the PID file. When shutdown via this method dhclient-script will be executed
with the specific reason for calling the script set.
-p port-number
The UDP port number on which the DHCP client should listen and transmit. If unspecified, dhclient uses
the default port of 68. This is mostly useful for debugging purposes. If a different port is speci‐
fied on which the client should listen and transmit, the client will also use a different destination
port - one less than the specified port.
-s server-addr
Specify the server IP address or fully qualified domain name to use as a destination for DHCP protocol
messages before dhclient has acquired an IP address. Normally, dhclient transmits these messages to
255.255.255.255 (the IP limited broadcast address). Overriding this is mostly useful for debugging
purposes. This feature is not supported in DHCPv6 (-6) mode.
-g relay
Set the giaddr field of all packets to the relay IP address simulating a relay agent. This is for
testing pruposes only and should not be expected to work in any consistent or useful way.
--version
Print version number and exit.
Options available for DHCPv6 mode:
requesting an address), or it creates an identifier based on the link-layer address plus a timestamp
(DUID-LLT) if it is running in stateful mode (without -S, requesting an address). -D overrides this
default, with a value of either LL or LLT.
-N Restore normal address query for IPv6. This implies -6. It is used to restore normal operation after
using -T or -P.
Modifying default file locations: The following options can be used to modify the locations a client uses for
it's files. They can be particularly useful if, for example, /var/lib/dhclient or /var/run have not been
mounted when the DHCP client is started.
-cf config-file
Path to the client configuration file. If unspecified, the default /etc/dhcp/dhclient.conf is used.
See dhclient.conf(5) for a description of this file.
-lf lease-file
Path to the lease database file. If unspecified, the default /var/lib/dhclient/dhclient.leases is
used. See dhclient.leases(5) for a description of this file.
-pf pid-file
Path to the process ID file. If unspecified, the default /var/run/dhclient.pid is used.
--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 attempt to kill any existing client processes even if invoked with
-r or -x.
-sf script-file
Path to the network configuration script invoked by dhclient when it gets a lease. If unspecified, the
default /usr/sbin/dhclient-script is used. See dhclient-script(8) for a description of this file.
CONFIGURATION
The syntax of the dhclient.conf(5) file is discussed separately.
OMAPI
The DHCP client provides some ability to control it while it is running, without stopping it. This capability
is provided using OMAPI, an API for manipulating remote objects. OMAPI clients connect to the client using
TCP/IP, authenticate, and can then examine the client'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 auto‐
matically. Dhcpctl and OMAPI are documented in dhcpctl(3) and omapi(3). Most things you'd want to do with
the client can be done directly using the omshell(1) command, rather than having to write a special program.
THE CONTROL OBJECT
The control object allows you to shut the client down, releasing all leases that it holds and deleting any DNS
records it may have added. It also allows you to pause the client - this unconfigures any interfaces the
client is using. You can then restart it, which causes it to reconfigure those interfaces. You would nor‐
mally pause the client prior to going into hibernation or sleep on a laptop computer. You would then resume
it after the power comes back. This allows PC cards to be shut down while the computer is hibernating or
sleeping, and then reinitialized to their previous state once the computer comes out of hibernation or sleep.
The dhclient.leases database.
PATH_DHCLIENT_PID
The dhclient PID file.
PATH_DHCLIENT_SCRIPT
The dhclient-script file.
FILES
/usr/sbin/dhclient-script, /etc/dhcp/dhclient.conf, /var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
/var/lib/dhclient/dhclient.leases~.
SEE ALSO
dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5), dhclient.leases(5), dhcp-eval(5).
AUTHOR
dhclient(8) has been written for Internet Systems Consortium by Ted Lemon in cooperation with Vixie Enter‐
prises. To learn more about Internet Systems Consortium, see https://www.isc.org To learn more about Vixie
Enterprises, see http://www.vix.com.
This client was substantially modified and enhanced by Elliot Poger for use on Linux while he was working on
the MosquitoNet project at Stanford.
The current version owes much to Elliot's Linux enhancements, but was substantially reorganized and partially
rewritten by Ted Lemon so as to use the same networking framework that the Internet Systems Consortium DHCP
server uses. Much system-specific configuration code was moved into a shell script so that as support for
more operating systems is added, it will not be necessary to port and maintain system-specific configuration
code to these operating systems - instead, the shell script can invoke the native tools to accomplish the same
purpose.
dhclient(8)
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