Hack 79. Magically Empower Your Network Cable

Configure your Ethernet devices simply by plugging in or unplugging the cable.

This hack is actually a utility called ifplugd, a daemon that watches your Ethernet connection to see if it is live or disconnected. Plug the wire into the network, and ifplugd configures the interface. Unplug the wire, and ifplugd disables the interface. Plug the wire back in, and it reconfigures the interface, even if it needs to use DHCP to get an IP address. It's a perfect utility for laptops that frequently change their network connections, but it can come in handy for workstations, too.

The ifplugd utility simply checks your network interface(s) to see if they have a link beat, which indicates a live connection to a network. When a link beat appears, ifplugd configures the interface (eth0, for example) as being up and ready to use. When the link beat disappears (you disconnect the cable), ifplugd brings the interface down.

Most distributions package ifplugd in such a way that it uses the default method for bringing down the interface if there is no connection, and then it uses the default method for bringing up the interface when ifplugd detects a connection. In other words, on a Debian system with the interface eth0, it uses the default methods of ifdown eth0 and ifup eth0 for disconnect and reconnect, respectively. It simply obeys how you originally configured eth0 to work.

You don't usually need to use ifplugd if you are using a laptop with a PC Card/PCMCIA network adapter and static IP address, for two reasons. The PCMCIA driver generally configures the device automatically anyway, and ifplugd is rarely able to detect a link beat through a PCMCIA device.

ifplugd is known to have problems with some USB network adapters (wireless or otherwise), especially when the driver is available only from a third party and is not part of the default Linux kernel. This is a driver issue related to the kernel's ability to activate and deactivate the USB device. This cannot be fixed by new versions of ifplugd; it is something the kernel driver must handle. If you have not yet purchased a USB network adapter, research the latest kernel versions to find out which ones are best supported. If you already have your USB network adapter, report problems to the manufacturer. Some companies are surprisingly accommodating and eager to fix problems such as these.