10.8 Triggering network configuration

We have seen how interfaces can be configured or reconfigured. This needs to be done at appropriate times.

Traditionally the network was configured during the boot sequence via the /etc/rcS.d/S40networking initscript and was rarely reconfigured. Services that depended on networking were started later in the boot sequence. On shutdown or reboot the initscripts were run in the opposite order.

Currently, however, there is a trend in GNU and Linux toward supporting hardware and circumstances that change dynamically. First support was added for hot swappable PCMCIA cards; more recently the hotplug mechanism has been added so that many more peripherals can be swapped in and out while the computer is running. This includes networking hardware. Note that services that depend on hardware that is hot swapped must only be started after the hardware is inserted and must be stopped when the hardware is removed. This means that such services must be removed from the control of the System V init system and put under the control of ifupdown instead.

For example, suppose service foo controlled by initscript /etc/init.d/foo depends on dynamically reconfigured network interface eth0.

• First remove foo from the control of the init system. If you are using the sysv-rc init system then do the following. [61]

       # rm /etc/rc[2345].d/S??foo
  

• Then put foo under the control of ifupdown by adding up and down options to the eth0 stanza in /etc/network/interfaces which contain calls to the foo initscript:

       iface eth0 inet dhcp
               up /etc/init.d/foo start
               down /etc/init.d/foo stop