dhclient



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


NAME

     dhclient - Dynamic Host Configuration Protocol (DHCP) client


SYNOPSIS

     dhclient [-d | -q] [-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      Do not daemonize.  If this option is specified, dhclient will run
             in the foreground and log to stderr.

     -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.  "offered" will be the lease offered by the DHCP
             server; "effective" will be the modified lease bound to 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.

     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.  dhclient 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 are 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.


SIGNALS

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


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

     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 6.2                   September 22, 2017                   OpenBSD 6.2

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