dhclient

Dynamic Host Configuration Protocol client

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 ] [ -nw ] [ -w ] [ -v ] [ --version ]
          [ if0 [ ...ifN ] ]

Options:

-1
Cause dhclient to try once to get a lease. If it fails, dhclient exits with exit code 2. In DHCPv6, the -1 flag sets the maximum duration of the initial exchange to timeout (from dhclient.conf, default 60 seconds).
-4
Use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters.
-6
Use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parameters. If you specify this option, the names of the default files include a "6":
IPv4 name IPv6 name
dhclient.conf dhclient6.conf
dhclient.leases dhclient6.leases
dhclient.pid dhclient6.pid
-cf config-file
The name of the file to read configuration information from. The default is /etc/dhclient.conf for DHCPv4, and /etc/dhclient6.conf for DHCPv6.
-d
Force dhclient to always run as a foreground process.
-e VAR=value
Define extra environment variables and their values.
-g relay
For testing purposes, set the giaddr field of all packets that the client sends to the given IP address.
-lf lease-file
The name of the lease database to use. The default is /var/db/dhclient.leases for DHCPv4, and /var/db/dhclient6.leases for DHCPv6.
-m
(BlackBerry 10 OS extension) Write the resolver configuration to memory (using confstr) rather than to /etc/resolv.conf.
--no-resolve
(BlackBerry 10 OS extension) Don't install the resolver configuration at all.
-N [-N... ]
Restore the normal address query (which is disabled if you use temporary addresses (-T or prefix delegation (-P).
-n
Don't attempt to configure any interfaces. This is most likely to be useful in combination with the -w flag.
-nw
Make dhclient become a daemon immediately, rather than waiting until it has acquired an IP address.
-P [-P... ]
Enable the IPv6 prefix delegation.
-p port
The UDP port that the DHCP client should listen and transmit on; the default is port 68. This is mostly useful for debugging purposes.
-pf pid-file
The name of the file in which dhclient stores its process ID. The default is /var/run/dhclient.pid for DHCPv4, and /var/run/dhclient6.pid for DHCPv6.
-q
Be quiet (although dhclient is quiet by default).
-r
Release the current lease. Once the lease has been released, the client exits.
-S
Use Information-request to get only (i.e., without address) stateless configuration parameters.
-s server
The IP address or domain name to which dhclient should transmit any protocol messages that it sends before acquiring an IP address. The default is 255.255.255.255, the IP limited broadcast address. This is mostly for debugging purposes, and isn't supported by DHCPv6.
-sf script-file
The name of the script file that configures the network interface. The default is /sbin/dhclient-script.
-T [-T... ]
Ask for IPv6 temporary addresses, one set per -T flag.
-v
Make dhclient emit verbose messages displaying the startup sequence events until it has acquired an address.
-w
Make the DHCP client not 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's possible that a broadcast interface may be added after system startup.
-x
Tell any currently running client to exit gracefully without releasing leases first.
--version
Display the version number for dhclient, and then exit.
if0 [ ...ifN ]
The names of the network interfaces that dhclient should attempt to configure. If you don't specify any interface names on the command line, dhclient identifies all network interfaces, eliminating non-broadcast interfaces if possible, and attempts to configure each interface.

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 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.

If given the -4 command-line argument (default), dhclient will use the DHCPv4 protocol to obtain an IPv4 address and configuration parameters.

If given the -6 command-line argument, dhclient will use the DHCPv6 protocol to obtain whatever IPv6 addresses are available along with configuration parameters. But with -S, it uses Information-request to get only (i.e., without address) stateless configuration parameters.

The default DHCPv6 behavior is modified too with -T, which asks for IPv6 temporary addresses, one set per -T flag. -P enables the IPv6 prefix delegation. As temporary addresses or prefix delegation disables the normal address query, -N restores it. Note it is not recommended to mix queries of different types together, or even to share the lease file between them.

If given the --version command-line argument, dhclient will display its version number and exit.

On startup, dhclient reads the dhclient.conf file 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's 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're 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 isn't 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.

It's 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.

If the DHCP client should listen and transmit on a port other than the standard (port 68), the -p flag may used. It should be followed by the UDP port number that dhclient should use. This is mostly useful for debugging purposes. If a different port is specified for the client to listen on and transmit on, the client will also use a different destination port — one less than the specified port.

The DHCP client normally transmits any protocol messages it sends before acquiring an IP address to 255.255.255.255, the IP limited broadcast address. For debugging purposes, it may be useful to have the server transmit these messages to some other address. This can be specified with the -s flag, followed by the IP address or domain name of the destination. This feature is not supported by DHCPv6.

For testing purposes, the giaddr field of all packets that the client sends can be set using the -g flag, followed by the IP address to send. This is only useful for testing, and should not be expected to work in any consistent or useful way.

The DHCP client will normally run in the foreground until it has configured an interface, and then will revert to running in the background. To force dhclient to always run as a foreground process, the -d flag should be specified. This is useful when running the client under a debugger, or when running it out of inittab on System V systems.

The dhclient daemon creates its own environment when executing the dhclient-script to do the grunt work of interface configuration. To define extra environment variables and their values, use the -e flag, followed by the environment variable name and value assignment, just as you'd assign a variable in a shell. For example, -e IF_METRIC=1.

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 syslog facility. A -q command-line argument is provided for backwards compatibility, but since dhclient is quiet by default, it has no effect.

The client normally doesn't release the current lease as it is not required by the DHCP protocol. Some cable ISPs require their clients to notify the server if they wish to release an assigned IP address. The -r flag explicitly releases the current lease, and once the lease has been released, the client exits.

The -x flag tells any currently running client to exit gracefully without releasing leases first.

If the client is killed by a signal (for example at shutdown or reboot) it won't 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.

The -1 flag will cause dhclient to try once to get a lease. If it fails, dhclient exits with exit code 2. In DHCPv6, the -1 flag sets the maximum duration of the initial exchange to timeout (from dhclient.conf, default 60 seconds).

The DHCP client normally gets its configuration information from /etc/dhclient.conf, its lease database from /var/db/dhclient.leases, stores its process ID in a file called /var/run/dhclient.pid, and configures the network interface using /sbin/dhclient-script. To specify different names and/or locations for these files, use the -cf, -lf, -pf, and -sf flags, respectively, followed by the name of the file. This can be particularly useful if, for example, /var/db or /var/run hasn't yet been mounted when the DHCP client is started.

The DHCP client normally exits 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's possible that a broadcast interface may be added after system startup. The -w flag can be used to cause the client not to exit when it doesn't find any such interfaces.

The DHCP client can be directed not to attempt to configure any interfaces using the -n flag. This is most likely to be useful in combination with the -w flag.

The client can also be instructed to become a daemon immediately, rather than waiting until it has acquired an IP address. This can be done by supplying the -nw flag.

Configuration

The syntax of the dhclient.conf 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 automatically.

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.

Files:

The dhclient depends on the following libraries and binaries:

  • libcrypto.so
  • libsocket.so
  • io-pkt-v4, io-pkt-v4-hc, or io-pkt-v6-hc (depending on whether you're using IPv4 or IPv6)
  • /sbin/dhclient-script (required; you can override it with -sf script-file on startup)

It uses the following configuration files:

/etc/dhclient6.conf
DHCP client configuration file for IPv6 (optional; defaults are used if this file isn't present; you can override it on startup).
/etc/dhclient.conf
DHCP client configuration file for IPv4.
dhclient6.leases
DHCP client lease database for IPv6 (optional; if not present, it will generate a new client ID on every startup; you can override it on startup).
dhclient.leases
DHCP client lease database for IPv4.
/sbin/dhclient-script
DHCP client network configuration script.
/var/run/dhclient.pid
Process ID of dhclient.

Contributing author:

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 http://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.

Last modified: 2013-12-21

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