Linux dhclient command

Updated: 04/26/2017 by Computer Hope

About dhclient

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.

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.

On startup, dhclient reads the file 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.

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. 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.

dhclient syntax

dhclient [ -4 | -6 ] [ -S ] [ -N [ -N... ] ] [ -T [ -T... ] ] [ -P [ -P... ] ]
         [ -p port ] [ -d ] [ -e VAR=value ] [ -q ] [ -1 ] [ -r | -x ]
         [ -lf lease-file ] [ -pf pid-file ] [ -cf config-file ]
         [ -sf script-file ] [ -s server ] [ -g relay ] [ -n ] [ -nc ] [ -nw ]
         [ -w ] [ -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 ] ]

Options

-4

Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters (default).

-6

Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parameters. The functionality of DHCPv6 mode may be modified with the -S, -T, and -N options.

-S

Perform an information-only request over DHCPv6 to get stateless configuration parameters. It is not recommended to combine this option with the -N, -P, or -T options or to share lease files between different modes of operation. Only valid with the -6 option.

-N

Perform a normal (IA_NA) address query over DHCPv6. It is not recommended to combine this option with the -P, -S, or -T options or to share lease files between different modes of operation. Only valid with the -6 option.

-T

Perform a temporary (IA_TA) address query over DHCPv6 (disables normal address query). It is not recommended to combine this option with the -N, -P, or -S options or to share lease files between different modes of operation. Only valid with the -6 option.

-P

Enable IPv6 prefix delegation (disables normal address query). It is not not recommended to combine this option with the -N, -S, or -T options or to share lease files between different modes of operation. Only valid with the -6 option.

-p <port number>

The UDP port number the DHCP client should listen and transmit on. If unspecified, dhclient uses the default port 68. This option is mostly useful for debugging purposes. If a different port is specified for the client to listen and transmit on, the client will also use a different destination port: one less than the specified port.

-d

Force dhclient to run as a foreground process. This is useful when running the client under a debugger, or when running it out of inittab on System V systems.

-e VAR=value

Define additional environment variables for the environment where dhclient-script executes. You may specify multiple -e options on the command line. For example: -e IF_METRIC=1

-q

Suppress all terminal and log output except error messages.

-1

Try once to get a lease. One failure, exit with code 2.

-r

Tell dhclient to release the current lease it has from the server. This is not required by the DHCP protocol, but some ISPs require their clients to notify the server if they wish to release an assigned IP address.

-lf <lease-file>

Path to the lease database file. If unspecified, the default /var/lib/dhclient/dhclient.leases is used.

-pf <pid-file>

Path to the process ID file. If unspecified, the default /var/run/dhclient.pid is used.

-cf <config-file>

Path to the client configuration file. If unspecified, the default /etc/dhcp/dhclient.conf is used.

-sf <script-file>

Path to the network configuration script invoked by dhclient when it gets a lease. If unspecified, the default /sbin/dhclient-script is used.

-s <server>

Specify the server IP address or fully qualified domain name to transmit DHCP protocol messages to. Normally, dhclient transmits these messages to 255.255.255.255 (the IP limited broadcast address). Overriding this is mostly useful for debugging purposes.

-g <relay>

Only for debugging. Set the giaddr field of all packets the client sends to the IP address specified. This should not be expected to work in any consistent or useful way.

-n

Do not configure any interfaces. Most useful combined with the -w option.

-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 can take. (For example, any daemons that dhclient-script 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.

-nw

Become a daemon process immediately (nowait) rather than waiting until an IP address has been acquired.

-w

Keep running even if no network interfaces are found. The omshell program can be used to notify the client when a network interface has been added or removed so it can attempt to configure an IP address on that interface.

-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 option. The fqdn.fqdn option must specify the complete domain name of the client host, which the server may use for dynamic DNS updates.

-V <vendor-class-identifier>

Specify the vendor-class-identifier option to send to the DHCP server.

-R <option>[,<option>...]

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-name, domain-name-servers, host-name, nis-domain, nis-servers, ntp-servers, interface-mtu

The -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.

-v

Enable verbose log messages.

If the client is killed by a signal (for example at shutdown or reboot), it will not execute the dhclient-script at exit. However, if you shut the client down gracefully with -r or -x it will execute dhclient-script at shutdown with the specific reason for calling the script set in the environment table.

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 automatically. Most things you'd want to do with the client can be done directly using the omshell 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.

dhclient examples

dhclient eth0

Renews the dynamically-assigned IP address of a primary ethernet device.

ifconfig — View or modify the configuration of network interfaces.