For systems that are only accessed remotely via SSH, such a scenario can be painful. What is needed is a daemon that watches the link status of the Ethernet jack and reconfigures the network (or sends out another DHCP request) when it detects a cable is plugged in (or the power to the router is restored).

ifplugd does exactly that:

ifplugd is a Linux daemon which will automatically configure your ethernet device when a cable is plugged in and automatically unconfigure it if the cable is pulled.

On a Debian system, installing and configuring ifplugd is relatively simple using apt-get install ifplugd. Once its been installed, it needs to be configured by editing /etc/default/ifplugd. The most basic configuration is to simply set INTERFACES="auto" and HOTPLUG_INTERFACES="all". This configuration tells ifplugd to watch all network interfaces for a new link status and automatically reconfigure them using the Debian network configuration defined in /etc/network/interfaces.

I recently needed to automate the install and configuration of ifplugd on many remote Linux systems, so I wrote this simple script.

Download: install-ifplugd.tar.gz

#!/bin/sh

#########################################
# Author: Raam Dev
#
# This script installs ifplugd and configures
# it to automatically attempt to restore any
# lost connections.
#
# Must be run as root!
#########################################

# Check if we're running this as root
if [ $EUID -ne 0 ]; then
   echo "This script must be run as root" 1>&2
   exit 1
fi

# Files used when configuring ifplugd
OUTFILE=/tmp/outfile.$$
CONFIG_FILE=/etc/default/ifplugd

# Update package list and install ifplugd, assuming yes to any questions asked
# (to insure the script runs without requiring manual intervention)
apt-get update --assume-yes ; apt-get install --assume-yes ifplugd

# Configure ifplugd to watch all interfaces and automatically attempt configuration
sed 's/INTERFACES=\"\"/INTERFACES=\"auto\"/g' < $CONFIG_FILE > $OUTFILE
mv $OUTFILE $CONFIG_FILE

sed 's/HOTPLUG_INTERFACES=\"auto\"/HOTPLUG_INTERFACES=\"all\"/g' < $CONFIG_FILE > $OUTFILE
mv $OUTFILE $CONFIG_FILE

If you’re interested in doing more with ifplugd, check out this article.

When using a static IP configuration on Linux, you normally add the DNS servers to the /etc/resolv.conf. However, if you try to add a DNS server to /etc/resolv.conf under a DHCP configuration, you’ll notice that your static entry disappears as soon as the DHCP client runs (usually on boot). To prevent this, you need to tell the DHCP client to prepend the static DNS server(s) to /etc/resolv.conf before adding the ones provided from the DHCP server (if any).

The configuration file you’ll need to edit is the same on both Debian and Ubuntu, however depending on your setup the location of the file may vary. Here are the two common places I’ve found the file:

Debian: /etc/dhclient.conf
Ubuntu: /etc/dhcp3/dhclient.conf

Open the file in your favorite editor and add one of two lines at the top, separating multiple DNS servers with a comma and ending the entry with a semi-colon:

If you simply want to add static DNS servers to be used in addition to the ones provided by DHCP, use a prepend entry:

prepend domain-name-servers 208.67.222.222, 208.67.220.220;

If you want to override the DNS servers provided by DHCP entirely and force the system to use the ones you provide, use the supersede entry:

supersede domain-name-servers 208.67.222.222, 208.67.220.220;

Before these static DNS servers will to be appended to your /etc/resolv.conf file, you’ll need to re-run the DHCP client. The easiest way to do this is by running /etc/init.d/networking restart (sudo required) or you can try running the dhclient command.

After re-running the DHCP client, check your /etc/resolv.conf file to confirm the static DNS servers have been added.