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dhclient-script - DHCP client network configuration script



      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.
      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.
      dhclient.conf(5), dhclient.leases(5), dhclient(8), dhcpd(8), dhcrelay(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 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.


Based on BSD UNIX
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