SysAdmin's Journey

Use NetworkManager to Launch Scripts Based on Network Location

NetworkManager is fast becoming the de facto network provider in desktop Linux distributions. The reason it’s so popular is that it “does the right thing” 99% of the time. However, there’s not many examples out there that extend that functionality. NetworkManager provides hooks in which you can have scripts launch when network settings change. In today’s post, I will show you how to launch the Synergy client whenever you plug into your corporate network.

I’m lazy. I hate having to fire up my Synergy client on my laptop to connect to my desktop at work when I get to work everyday. Now, I could just fire up the Synergy client at boot, but when I’m at home if I have the VPN connected, Synergy will hook up and I don’t want it to.

I needed a way to fire a script that says “if I’m on this network, then fire up synergyc, otherwise do nothing”. Writing the script was pretty easy, but I was stumped on how to get to run not only at bootup, but whenever I change networks – I very often suspend my laptop at night at home and resume it at work the next morning.

It turns out that NetworkManager has a piece called NetworkManagerDispatcher that does all of this for us. Any script in /etc/NetworkManager/dispatcher.d will be called with two arguments, the name of the interface, and the status of the interface (up/down).

If a picture is worth a thousand words, scripts are worth a million, so let’s get to it.

First, a little background is needed. I know that I am on my corporate network if my eth0 interface has obtained an IP in the 10.0.0.0/8 subnet. Without further ado, I present to you /etc/NetworkManager/dispatcher.d/99smartsynergy.sh:

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#!/bin/sh
IF=$1
STATUS=$2
USER=justintime

wait_for_process() {
  PNAME=$1
  PID=`/usr/bin/pgrep $PNAME`
  while [ -z "$PID" ]; do
        sleep 3;
        PID=`/usr/bin/pgrep $PNAME`
  done
}

start_synergy() {
     wait_for_process nm-applet
     /bin/su $USER -c "/usr/bin/synergyc $1"
}

if [ "$IF" = "eth0" ] && [ "$STATUS" = "up" ]; then

        #LAN Subnet at work
        NETMASK="10.0.0.0/8"
        if [ -n "`/sbin/ip addr show $IF to $NETMASK`" ]; then
                ARGS="jentoo.bucklehq.com"
                start_synergy $ARGS
                exit $?
        fi

fi

The IF and STATUS variables are those fed in from NetworkManager. The USER variable is the user that I run synergyc as. You could add some intelligence here, but it was overkill for my situation.

The if at the bottom states that we are only concerned if the interface eth0 has changed it’s status to “up”. I then use the /sbin/ip command to determine if eth0 is within the 10.0.0.0/8 subnet. If so, then I call start_synergy, passing it my desktop’s hostname.

Within the start_synergy() function we call the wait_for_process function, passing it nm-applet. We need this function because if we try to run synergyc before I’ve logged in via GDM, it will exit (this happens on bootup). By calling wait_for_process, we create a way to make synergyc wait until after the nm-applet (NetworkManager Applet) has started. Finally, once nm-applet has been detected as running, the script executes synergyc, and exits.

TODO’s

  • I should probably create a function that kills the remaining synergyc processes when eth0 goes down.
  • If there’s already a synergyc running, we should just exit as the client will continue to try to reconnect.

Conclusion

The purpose of this article wasn’t to show you how to launch synergyc (although I think it’s really handy), it was to get the creative juices flowing. Have you already utilized NetworkManagerDispatcher for something? What would you like to have it do? Comment away!

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