DHCP client network configuration script
dhclient-script
QNX Neutrino
None.
The DHCP client network configuration script is invoked from time to time by dhclient. 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 behavior 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. In general, customizations specific to a particular computer should be done in the /etc/dhclient.conf file. If you find that you can't make such a customization without customizing /etc/dhclient.conf or using the enter and exit hooks, please submit a bug report.
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 behavior, redefine this function in the enter hook script.
After defining the make_resolv_conf function, the client script checks for the presence of an executable /etc/dhclient-enter-hooks script, and if present, it invokes the script inline, using the Bourne shell dot (.) command. The entire environment documented under Operations is available to this script, which may modify the environment if needed to change the behavior 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 /sbin/dhclient-script will exit with that error code immediately after the client script exits.
After all processing has completed, /sbin/dhclient-script checks for the presence of an executable /etc/dhclient-exit-hooks script, which if present is invoked using the dot command. The exit status of dhclient-script is passed to dhclient-exit-hooks in the exit_status shell variable, and is always zero if the script succeeded at the task for which it was invoked. The rest of the environment as described previously for dhclient-enter-hooks is also present. The /etc/dhclient-exit-hooks script can modify the value of exit_status to change the exit status of dhclient-script.
Operations
When dhclient needs to invoke the client configuration script, it defines a set of variables in the environment, and then invokes /sbin/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:
If an IP alias has been declared in dhclient.conf, its address will be passed in $alias_ip_address, and that IP alias should be deleted from the interface, along with any routes to it.
Before actually configuring the address, dhclient-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 isn't 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 $new_. Care should be taken that the alias IP address not be used if it's identical to the bound IP address ($new_ip_address), since the other alias parameters may be incorrect in this case.
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.
The dhclient-script depends on the following libraries and binaries:
It uses the following configuration files:
dhclient-script has been written for Internet Systems Consortium by Ted Lemon in cooperation with Vixie Enterprises. To learn more about Internet Systems Consortium, see http://www.isc.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 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.