Set MTU for VPN connection managed by NetworkManager daemon

Every time I connected to a Microsoft PPTP VPN server from Debian/Ubuntu desktop, I experienced slow network and dropped connections. Here’s how I fixed it.

After some digging around I found the default MTU of 1500 is too high. There’s nowhere to alter this in the GUI so each time after connecting I entered:

sudo ifconfig ppp0 mtu 1390

1390 works for me, as the Windows server is using 1400 for RRAS.

All good but not very efficient; so how to make this setting permanent? I tried appending /etc/ppp/ipup with

ifconfig $1 mtu 1390

but this didn’t work for me as the connection is managed by NetworkManager.

After reading through man NetworkManager, I found there are a set of scripts that can be run to act on various events, so all that’s needed is a script to do the job:

Create script:

sudo nano /etc/NetworkManager/dispatcher.d/vpn-up

Add:

#!/bin/sh
#/etc/NetworkManager/dispatcher.d/vpn-up
if [ "$2" = "vpn-up" ]; then
  /sbin/ifconfig "$1" mtu 1390
fi

Set mode on the script:

sudo chmod 0755 /etc/NetworkManager/dispatcher.d/vpn-up

After connecting to the VPN, check MTU is set correctly by issuing command and look for MTU of VPN interface (ppp0 in my case):

ifconfig ppp

ppp0      Link encap:Point-to-Point Protocol  
          inet addr:10.0.0.104  P-t-P:10.0.0.105  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1390  Metric:1
          RX packets:1302 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1728 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3 
          RX bytes:442401 (442.4 KB)  TX bytes:191311 (191.3 KB)

Works for me on Kubuntu 12.10 x64