Catching interface state changes with Network Manager

Sometimes it is required that you can react on state changes of your network interfaces. Maybe you want to get notified by mail or you want to execute a script that does something in case an event is trapped. As usual I will be using CentOS 7 for this little demonstration but as most distributions use systemd and NetworkManager nowadays this should work more or less the same on Debian based distributions.

As this will be a short post anyway: The way to do it is to use the dispatcher scripts coming with Network Manager. All you need to do for getting notified on down or up events is to create a script in /etc/NetworkManager/dispatcher.d/:

root@:/etc/NetworkManager/dispatcher.d/ [] pwd
/etc/NetworkManager/dispatcher.d
root@:/etc/NetworkManager/dispatcher.d/ [] cat 30-network 
#!/bin/bash

INTERFACE=$1
ACTION=$2

echo $INTERFACE >> /var/log/tmp.log
echo $ACTION >> /var/log/tmp.log

The script will get two parameters passed in by default: The name of the interface and the action that just happened. What you do with these is totally up to you. The only point you need to take care of (which is also mentioned in the documentation): “Each script should be a regular executable file, owned by root. Furthermore, it must not be writable by group or other, and not setuid.”

root@:/etc/NetworkManager/dispatcher.d/ [] ls -la
total 16
drwxr-xr-x. 5 root root  131 Aug  6 17:02 .
drwxr-xr-x. 7 root root  134 Jul 20 10:18 ..
-rwxr-xr-x. 1 root root  175 Jan  2  2018 00-netreport
-rwxr-xr-x. 1 root root 1123 May 15 15:03 11-dhclient
-rwxr-xr-x. 1 root root  985 Sep 15  2017 20-chrony
-rwxr-xr-x. 1 root root  108 Aug  6 17:02 30-network
drwxr-xr-x. 2 root root    6 Jun 27 16:39 no-wait.d
drwxr-xr-x. 2 root root    6 Jun 27 16:39 pre-down.d
drwxr-xr-x. 2 root root    6 Jun 27 16:39 pre-up.d

As soon as you have your script in place it will get kicked off when the interface state is changing:

root@:/etc/NetworkManager/dispatcher.d/ [] systemctl restart network
root@:/etc/NetworkManager/dispatcher.d/ [] cat /var/log/tmp.log

connectivity-change
enp0s3
down
enp0s8
down
enp0s3
up

connectivity-change
enp0s8
up

Hope that helps …