Bring network interface down if physical connectivity is lost

Question

I have a headless Debian server with two network cards:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 01:02:1d:54:7c:01 brd ff:ff:ff:ff:ff:ff
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 01:02:1d:54:7e:01 brd ff:ff:ff:ff:ff:ff

If I disconnect the cable connected to eth1, then port will go down, but administratively it will stay up:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 01:02:1d:54:7c:01 brd ff:ff:ff:ff:ff:ff
2: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT qlen 1000
    link/ether 01:02:1d:54:7e:01 brd ff:ff:ff:ff:ff:ff

This means that routes associated with eth1 are still present in routing table. Is there a way to force eth1 administratively down once it's physically disconnected?

Answer 1

A quick and dirty solution would be to use ip monitor and a script that "ifupdowns" the interface.

The script would be:

#!/bin/sh

ip monitor link | while IFS=':' read num name status; do
    read extraline # iproute2 outputs physical address on a separate line.
    newstate=$(echo "$status"|awk -F' ' '{print $NF}')
    if [ "$newstate" = "UP" ]; then
        ifup "$name"
    elif [ "$newstate" = "DOWN" ]; then
        ifdown "$name"
    fi
done

The script doesn't check network interface status before attempting to change it, but that shouldn't be a problem.

To use it just place it in /opt/bin/linkmonitor.sh, make it executable with chmod +x /opt/bin/linkmonitor.sh and add it to /etc/rc.local with an ampersand at the end:

/opt/bin/linkmonitor.sh &

If you need a more robust solution you may be interested in a route failover solution.