dhclient-script man page on FreeBSD

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DHCLIENT-SCRIPT(8)	  BSD 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

     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 inter‐
	       face 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

     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 inter‐
	       face 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

	       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 “$new_”.  Care should
	       be taken that the alias IP address not be used if it is identi‐
	       cal to the bound IP address ($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

     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 relin‐
	       quished, 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 dynami‐
     cally modify the enviornment at appropriate times during the DHCP negoti‐
     ations.  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 con‐
     nected.  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 ⟨mellon@fugue.com⟩ in cooperation with
     Vixie Enterprises.

     The OpenBSD implementation of dhclient-script was written by Kenneth R.
     Westerback ⟨krw@openbsd.org⟩.

     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 repeat‐
     edly 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.

BSD			       September 6, 2010			   BSD

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