MirOS Manual: dhclient-script(8)

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 ac-
               tual 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 ad-
               dress 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 of-
               fered to it be checked to see if somebody else is using it, by
               sending an ARP request for that address. It's 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.
               $new_ip_address and $interface 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 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). The /etc/resolv.conf file is not created
               if the /etc/resolv.conf.lock file exists. In that case, the
               file which would have been created as /etc/resolv.conf is writ-
               ten to /etc/resolv.conf.std instead. A default route should be
               set using $new_routers, and static routes may need to be set up
               using $new_static_routes.

               Note: since dhclient-script effectively overwrites
               /etc/resolv.conf, any information contained therein is lost. If
               options must be passed to the resolver, they may be contained
               in /etc/resolv.conf.tail, which is appended to the generated
               /etc/resolv.conf by dhclient-script. See resolv.conf.tail(5)
               for further information.

               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 oth-
               er DHCP options that are set for the alias (e.g., subnet mask)
               will be passed in variables named as described previously ex-
               cept 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 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 de-
               configured. 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.

     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 list-
     ed in $new_routers, as well as those listed in $new_static_routes, but
     current scripts do not do this.


     dhclient.conf(5), dhclient.leases(5), resolv.conf.tail(5), dhclient(8),
     dhcpd(8), dhcrelay(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>.

     The /etc/resolv.conf preservation code has been adapted into MirOS by
     Thorsten Glaser <tg@mirbsd.org>.


     If more than one interface is being used, there's 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 in-
     terface is being configured, /etc/resolv.conf will be repeatedly initial-
     ized to the values provided by one server, and then the other. Assuming
     the information provided by both servers is valid, this shouldn't cause
     any real problems, but it could be confusing.

MirOS BSD #10-current          January 1, 1997                               2

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