Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
DHCLIENT-SCRIPT(8)      FreeBSD System Manager's Manual     DHCLIENT-SCRIPT(8)

     dhclient-script - DHCP client network configuration script

     The DHCP client network configuration script is invoked from time to time
     by dhclient(8).  This script is used by the DHCP client to set each
     interface's initial configuration prior to requesting an address, to test
     the address once it has been offered, and to set the interface's final
     configuration once a lease has been acquired.  If no lease is acquired,
     the script is used to test predefined leases, if any, and also called
     once if no valid lease can be identified.

     In general, customizations specific to a particular computer should be
     done in the /etc/dhclient.conf file.

     When dhclient(8) needs to invoke the client configuration script, it sets
     up a number of environment variables and runs dhclient-script.  In all
     cases, $reason is set to the name of the reason why the script has been
     invoked.  The following reasons are currently defined: MEDIUM, PREINIT,

     MEDIUM        The DHCP client is requesting that an interface's media
                   type be set.  The interface name is passed in $interface,
                   and the media type is passed in $medium.

     PREINIT       The DHCP client is requesting that an interface be
                   configured as required in order to send packets prior to
                   receiving an actual address.  This means configuring the
                   interface with an IP address of and a broadcast
                   address of  The interface name is passed
                   in $interface, and the media type in $medium.

                   If an IP alias has been declared in dhclient.conf(5), its
                   address will be passed in $alias_ip_address, and that IP
                   alias should be deleted from the interface, along with any
                   routes to it.

     ARPSEND       The DHCP client is requesting that an address that has been
                   offered to it be checked to see if somebody else is using
                   it, by sending an ARP request for that address.  It is not
                   clear how to implement this, so no examples exist yet.  The
                   IP address to check is passed in $new_ip_address, and the
                   interface name is passed in $interface.

     ARPCHECK      The DHCP client wants to know if a response to the ARP
                   request sent using ARPSEND has been received.  If one has,
                   the script should exit with a nonzero status, indicating
                   that the offered address has already been requested and
                   should be declined.  The $new_ip_address and $interface
                   variables are set as with ARPSEND.

     BOUND         The DHCP client has done an initial binding to a new
                   address.  The new IP address is passed in $new_ip_address,
                   and the interface name is passed in $interface.  The media
                   type is passed in $medium.  Any options acquired from the
                   server are passed using the option name described in
                   dhcp-options(5), except that dashes (`-') are replaced by
                   underscores (`_') in order to make valid shell variables,
                   and the variable names start with ``new_''.  So for
                   example, the new subnet mask would be passed in

                   When a binding has been completed, a lot of network
                   parameters are likely to need to be set up.  A new
                   /etc/resolv.conf needs to be created, using the values of
                   $new_domain_name and $new_domain_name_servers (which may
                   list more than one server, separated by spaces).  A default
                   route should be set using $new_routers, and static routes
                   may need to be set up using $new_static_routes.

                   If an IP alias has been declared, it must be set up here.
                   The alias IP address will be written as $alias_ip_address,
                   and other DHCP options that are set for the alias (e.g.,
                   subnet mask) will be passed in variables named as described
                   previously except starting with ``$alias_'' instead of
                   ``$new_''.  Care should be taken that the alias IP address
                   not be used if it is identical to the bound IP address
                   ($new_ip_address), since the other alias parameters may be
                   incorrect in this case.

     RENEW         When a binding has been renewed, the script is called as in
                   BOUND, except that in addition to all the variables
                   starting with ``$new_'', there is another set of variables
                   starting with ``$old_''.  Persistent settings that may have
                   changed need to be deleted - for example, if a local route
                   to the bound address is being configured, the old local
                   route should be deleted.  If the default route has changed,
                   the old default route should be deleted.  If the static
                   routes have changed, the old ones should be deleted.
                   Otherwise, processing can be done as with BOUND.

     REBIND        The DHCP client has rebound to a new DHCP server.  This can
                   be handled as with RENEW, except that if the IP address has
                   changed, the ARP table should be cleared.

     REBOOT        The DHCP client has successfully reacquired its old address
                   after a reboot.  This can be processed as with BOUND.

     EXPIRE        The DHCP client has failed to renew its lease or acquire a
                   new one, and the lease has expired.  The IP address must be
                   relinquished, and all related parameters should be deleted,
                   as in RENEW and REBIND.

     FAIL          The DHCP client has been unable to contact any DHCP
                   servers, and any leases that have been tested have not
                   proved to be valid.  The parameters from the last lease
                   tested should be deconfigured.  This can be handled in the
                   same way as EXPIRE.

     TIMEOUT       The DHCP client has been unable to contact any DHCP
                   servers.  However, an old lease has been identified, and
                   its parameters have been passed in as with BOUND.  The
                   client configuration script should test these parameters
                   and, if it has reason to believe they are valid, should
                   exit with a value of zero.  If not, it should exit with a
                   nonzero value.

     Before taking action according to $reason, dhclient-script will check for
     the existence of /etc/dhclient-enter-hooks.  If found, it will be sourced
     (see sh(1)).  After taking action according to $reason, dhclient-script
     will check for the existence of /etc/dhclient-exit-hooks.  If found, it
     will be sourced (see sh(1)).  These hooks scripts can be used to
     dynamically modify the enviornment at appropriate times during the DHCP
     negotiations.  For example, if the administrator wishes to disable alias
     IP numbers on the DHCP interface, they might want to put the following in

           [ ."$reason" = .PREINIT ] && ifconfig $interface

     The usual way to test a lease is to set up the network as with REBIND
     (since this may be called to test more than one lease) and then ping the
     first router defined in $routers.  If a response is received, the lease
     must be valid for the network to which the interface is currently
     connected.  It would be more complete to try to ping all of the routers
     listed in $new_routers, as well as those listed in $new_static_routes,
     but current scripts do not do this.

     sh(1), dhclient.conf(5), dhclient.leases(5), dhclient(8), dhcpd(8),

     The original version of dhclient-script was written for the Internet
     Software Consortium by Ted Lemon <> in cooperation with
     Vixie Enterprises.

     The OpenBSD implementation of dhclient-script was written by Kenneth R.
     Westerback <>.

     If more than one interface is being used, there is no obvious way to
     avoid clashes between server-supplied configuration parameters - for
     example, the stock dhclient-script rewrites /etc/resolv.conf.  If more
     than one interface is being configured, /etc/resolv.conf will be
     repeatedly initialized to the values provided by one server, and then the
     other.  Assuming the information provided by both servers is valid, this
     should not cause any real problems, but it could be confusing.

FreeBSD 11.0-PRERELEASE        September 6, 2010       FreeBSD 11.0-PRERELEASE


Want to link to this manual page? Use this URL:

home | help