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DHCLIENT(8) System Manager's Manual DHCLIENT(8)

dhclient — Dynamic Host Configuration Protocol (DHCP) client

dhclient [-dqu] [-c file] [-i options] [-L file] [-l file] interface

The Dynamic Host Configuration Protocol (DHCP) allows hosts on a TCP/IP network to configure one or more network interfaces based on information collected from a DHCP server. DHCP is often used, for example, by cable modem and DSL network providers to automate network configuration for their customers.

Information typically provided via DHCP includes address and subnet mask for the interface, default route, and domain name server.

To have OpenBSD configure an interface using DHCP (or its predecessor, BOOTP) the dhclient utility is used. dhclient is run on the command line with the name of the interface to be configured. dhclient can also be run at boot time from hostname.if(5), in which case netstart(8) reads the hostname files and runs dhclient for each interface that is to be configured via DHCP.

The options are as follows:

file
Specify an alternate location to /etc/dhclient.conf for the configuration file.
Forces dhclient to always run as a foreground process. By default, dhclient runs in the foreground until it has configured the interface, and then will revert to running in the background.
options
dhclient will ignore any values provided by leases for the options specified. This list will override any ignore statements in dhclient.conf(5). options must be a comma separated list of valid option names. Invalid option names will cause the entire list to be discarded.
file
Specify a file to write the option data to. This causes dhclient to write two pseudo-leases, “offered” and “effective”, to the specified file. The offered block will contain the lease offered by the DHCP server; the effective block will contain the modified lease used to configure the interface.
file
Specify an alternate location to /var/db/dhclient.leases.⟨IFNAME⟩ for the leases file.
Forces dhclient to be less verbose on startup.
Forces dhclient to reject leases with unknown options in them. The default behaviour is to accept such lease offers.

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 the 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 /etc/dhclient.conf for configuration instructions. It then attempts to configure the network interface interface with DHCP. The special value “egress” may be used instead of a network interface name. In this case dhclient will look for the network interface currently in the interface group “egress” and configure it with DHCP. If there is more than one network interface in the egress group dhclient will exit with an error.

When configuring the interface, dhclient attempts to remove any existing addresses, gateway routes that use the interface, and non-permanent arp(8) entries. Conversely, if the interface is later manipulated to add or delete addresses then dhclient will automatically exit. It thus automatically exits whenever a new dhclient is run on the same interface.

Once the interface is configured, dhclient constructs a resolv.conf(5) file. It does this only when one or both of the options domain-name and domain-name-servers is present (note that these options may be offered by the DHCP server but suppressed by dhclient.conf(5)). If a resolv.conf is constructed, dhclient appends any contents of the resolv.conf.tail(5) file, which are read once at start up. The constructed resolv.conf is copied into /etc/resolv.conf whenever the default route goes out the interface dhclient is running on. dhclient monitors the system for changes to the default route and re-checks whether it should write its resolv.conf when possible changes are detected.

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 /var/db/dhclient.leases.⟨IFNAME⟩ file. IFNAME represents the network interface of the DHCP client (e.g. em0), one for each interface. On startup, after reading the dhclient.conf(5) file, dhclient reads the leases file to refresh its memory about what leases it has been assigned.

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.⟨IFNAME⟩ 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, it 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 requires at least one /dev/bpf* file for each broadcast network interface. See bpf(4) for more information.

While running, dhclient reacts to a few different signals:

On receiving HUP dhclient will restart itself, reading dhclient.conf(5) and obtaining a new lease.
On receiving INT dhclient will exit after attempting to remove any routes, interface addresses or temporary files it created.
On receiving QUIT dhclient will dump core and exit without attempting to remove any routes, interface addresses or temporary files it created.
On receiving TERM dhclient will exit without attempting to remove any routes, interface addresses or temporary files it created.
On receiving either USR1 or USR2, dhclient will exit after attempting to remove any routes, interface addresses or temporary files it created.

/etc/dhclient.conf
DHCP client configuration file
/etc/hostname.XXX
interface-specific configuration files
/var/db/dhclient.leases.⟨IFNAME⟩
database of acquired leases

bpf(4), dhclient.conf(5), dhclient.leases(5), hostname.if(5), dhcpd(8), dhcrelay(8)

R. Droms, Interoperation Between DHCP and BOOTP, RFC 1534, October 1993.

R. Droms, Dynamic Host Configuration Protocol, RFC 2131, March 1997.

S. Alexander and R. Droms, DHCP Options and BOOTP Vendor Extensions, RFC 2132, March 1997.

T. Lemon and S. Cheshire, Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4), RFC 3396, November 2002.

T. Lemon, S. Cheshire, and B. Volz, The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4, RFC 3442, December 2002.

dhclient was written by Ted Lemon <mellon@fugue.com> and Elliot Poger <elliot@poger.com>.

The current implementation was reworked by Henning Brauer <henning@openbsd.org>.

August 22, 2013 OpenBSD-5.5