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)