dhclient



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


NAME

     dhclient - Dynamic Host Configuration Protocol (DHCP) client


SYNOPSIS

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


DESCRIPTION

     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:

     -c file
             Specify an alternate location to /etc/dhclient.conf for the
             configuration file.

     -d      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.

     -i 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.

     -L 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.

     -l file
             Specify an alternate location to /var/db/dhclient.leases.<IFNAME>
             for the leases file.

     -q      Forces dhclient to be less verbose on startup.

     -u      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.


SIGNALS

     While running, dhclient reacts to a few different signals:

     HUP            On receiving HUP dhclient will restart itself, reading
                    dhclient.conf(5) and obtaining a new lease.

     INT            On receiving INT dhclient will exit after attempting to
                    remove any routes, interface addresses or temporary files
                    it created.

     QUIT           On receiving QUIT dhclient will dump core and exit without
                    attempting to remove any routes, interface addresses or
                    temporary files it created.

     TERM           On receiving TERM dhclient will exit without attempting to
                    remove any routes, interface addresses or temporary files
                    it created.

     USR1, USR2     On receiving either USR1 or USR2, dhclient will exit after
                    attempting to remove any routes, interface addresses or
                    temporary files it created.


FILES

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


SEE ALSO

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


STANDARDS

     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.


AUTHORS

     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>.

OpenBSD 5.4                      July 16, 2013                     OpenBSD 5.4

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