Ubuntu Manpage: dhcpcanon-script - DHCP client network configuration script

NAME

       dhcpcanon-script - DHCP client network configuration script

DESCRIPTION

       The  DHCP  client network configuration script is invoked from time to time by dhcpcanon(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.

       This  script  is  not  meant  to be customized by the end user.  If local customizations are needed, they
       should be possible using the enter and exit hooks provided (see HOOKS for details).    These  hooks  will
       allow the user to override the default behaviour of the client in creating a /etc/resolv.conf file.

       No standard client script exists for some operating systems, even though the actual client may work, so a
       pioneering user may well need to create a new script or modify an existing one.

HOOKS

When it starts, the client script first defines a shell function, make_resolv_conf , which is later used
to create the /etc/resolv.conf file. To override the default behaviour, redefine this function in the
enter hook script.

On after defining the make_resolv_conf function, the client script checks for the presence of an
executable ETCDIR/dhcpcanon-enter-hooks script, and if present, it invokes the script inline, using the
Bourne shell ´.´ command. The entire environment documented under OPERATION is available to this
script, which may modify the environment if needed to change the behaviour of the script. If an error
occurs during the execution of the script, it can set the exit_status variable to a nonzero value, and
CLIENTBINDIR/dhcpcanon-script will exit with that error code immediately after the client script exits.

After all processing has completed, CLIENTBINDIR/dhcpcanon-script checks for the presence of an
executable ETCDIR/dhcpcanon-exit-hooks script, which if present is invoked using the ´.´ command. The
exit status of dhcpcanon-script will be passed to dhcpcanon-exit-hooks in the exit_status shell variable,
and will always be zero if the script succeeded at the task for which it was invoked. The rest of the
environment as described previously for dhcpcanon-enter-hooks is also present. The ETCDIR/dhcpcanon-
exit-hooks script can modify the valid of exit_status to change the exit status of dhcpcanon-script.

OPERATION

       When dhcpcanon needs to invoke the client configuration script, it defines a  set  of  variables  in  the
       environment, and then invokes CLIENTBINDIR/dhcpcanon-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, BOUND, RENEW, REBIND, REBOOT, EXPIRE, FAIL, STOP, RELEASE, NBI and TIMEOUT.

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. For clients which use the BSD socket library, this means configuring
the interface with an IP address of 0.0.0.0 and a broadcast address of 255.255.255.255. For other
clients, it may be possible to simply configure the interface up without actually giving it an IP address
at all. The interface name is passed in $interface, and the media type in $medium.

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, 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 $new_subnet_mask.
Options from a non-default universe will have the universe name prepended to the option name, for example
$new_dhcp6_server_id. The options that the client explicitly requested via a PRL or ORO option are
passed with the same option name as above but prepended with requested_ and with a value of 1, for
example requested_subnet_mask=1. No such variable is defined for options not requested by the client or
options that don't require a request option, such as the ip address (*_ip_address) or expiration time
(*_expire).

Before actually configuring the address, dhcpcanon-script should somehow ARP for it and exit with a
nonzero status if it receives a reply. In this case, the client will send a DHCPDECLINE message to the
server and acquire a different address. This may also be done in the RENEW, REBIND, or REBOOT states,
but is not required, and indeed may not be desirable.

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_,  and $requested_ 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.

STOP

       The dhcpcanon has been informed to shut down  gracefully,  the  dhcpcanon-script  should  unconfigure  or
       shutdown the interface as appropriate.

RELEASE

       The dhcpcanon has been executed using the -r flag, indicating that the administrator wishes it to release
       its lease(s).  dhcpcanon-script should unconfigure or shutdown the interface.

NBI

       No-Broadcast-Interfaces...dhcpcanon was unable to find any interfaces upon which it  believed  it  should
       commence DHCP.  What dhcpcanon-script should do in this situation is entirely up to the implementor.

TIMEOUT

       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.

FILES

       Each  operating  system  should generally have its own script file, although the script files for similar
       operating systems may be similar or even identical.   The  script  files  included  in  Internet  Systems
       Consortium  DHCP distribution appear in the distribution tree under client/scripts, and bear the names of
       the operating systems on which they are intended to work.

BUGS

       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 dhcpcanon-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 shouldn't cause any real problems, but it could be confusing.

AUTHOR

       dhcpcanon-script(8) To learn more about Internet Systems Consortium, see https://www.isc.org.

                                                                                             dhcpcanon-script(8)