dhclient(8) System Manager's Manual dhclient(8)
dhclient - Dynamic Host Configuration Protocol Client
dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [ -P... ] ] [ -D LL|LLT ] [ -p port-number ] [ -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 ] ]
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 statically assigning an address.
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
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 arbitrar-
ily 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 suc-
ceeds, 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.
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.
It is also possible to specify interfaces by name in the dhclient.conf file. If interfaces are specified in this way, then the client will
only configure interfaces that are either specified in the configuration file or on the command line, and will ignore all other interfaces.
The client normally prints no output during its startup sequence. It can be made to emit verbose messages displaying the startup sequence
events until it has acquired an address by supplying the -v command line argument. In either case, the client logs messages using the sys-
-4 Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters. This is the default and cannot be combined with -6.
-6 Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parameters. 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 debug-
ger, 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 net-
work 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
interfaces. 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.
Specify the dhcp-client-identifier option to send to the DHCP server.
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.
Specify the fqdn.fqdn option to send to the DHCP server. This option cannot be used with the -H option. The fqdn.fqdn option must
specify the complete domain name of the client host, which the server may use for dynamic DNS updates.
Specify the vendor-class-identifier option to send to the DHCP server.
Specify the list of options the client is to request from the server. The option list must be a single string consisting of option
names separated by at least one command and optional space characters. The default option list is:
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.
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.
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 cur-
rent 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.
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 specified on which the client should listen and transmit, the
client will also use a different destination port - one less than the specified port.
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). Over-
riding this is mostly useful for debugging purposes. This feature is not supported in DHCPv6 (-6) mode.
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.
Print version number and exit.
Options available for DHCPv6 mode:
-S Use Information-request to get only stateless configuration parameters (i.e., without address). This implies -6. It also doesn't
rewrite the lease database.
-T Ask for IPv6 temporary addresses, one set per -T flag. This implies -6 and also disables the normal address query. See -N to
-P Enable IPv6 prefix delegation. This implies -6 and also disables the normal address query. See -N to restore it. Note only one
requested interface is allowed.
-D LL or LLT
Override the default when selecting the type of DUID to use. By default, DHCPv6 dhclient creates an identifier based on the link-
layer address (DUID-LL) if it is running in stateless mode (with -S, not 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 par-
ticularly useful if, for example, /var/lib/dhclient or /var/run have not been mounted when the DHCP client is started.
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.
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.
Path to the process ID file. If unspecified, the default /var/run/dhclient.pid is used.
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.
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.
The syntax of the dhclient.conf(5) file is discussed separately.
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 automatically. 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 normally 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 control object has one attribute - the state attribute. To shut the client down, set its state attribute to 2. It will automatically
do a DHCPRELEASE. To pause it, set its state attribute to 3. To resume it, set its state attribute to 4.
The following environment variables may be defined to override the builtin defaults for file locations. Note that use of the related com-
mand-line options will ignore the corresponding environment variable settings.
The dhclient.conf configuration file.
The dhclient.leases database.
The dhclient PID file.
The dhclient-script file.
/usr/sbin/dhclient-script, /etc/dhcp/dhclient.conf, /var/lib/dhclient/dhclient.leases, /var/run/dhclient.pid,
dhcpd(8), dhcrelay(8), dhclient-script(8), dhclient.conf(5), dhclient.leases(5), dhcp-eval(5).
dhclient(8) has been written for Internet Systems Consortium by Ted Lemon in cooperation with Vixie Enterprises. 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
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-spe-
cific configuration code to these operating systems - instead, the shell script can invoke the native tools to accomplish the same purpose.