Question: I'm starting a VPN connection using Network Manager.
Once the connection is established I have to change [MTU](http://en.wikipedia.org/wiki/Maximum_transmission_unit) in order it to work properly.
For example:

```
sudo ifconfig ppp0 mtu 777

```

It is very annoying to execute this command every time I open a VPN connection. Is there any idea to create a script that would execute this command automatically once the connection is established (ppp0 interface is up)?

Answer: 
Create a script in `/etc/network/if-up.d`, containing 

```
#!/bin/sh

if [ "$IFACE" = "ppp0" ]; then
    ifconfig ppp0 mtu 777
fi

```

and [make it executable](https://askubuntu.com/questions/122428/how-to-run-sh-file).
Here's a modernized (using `ip` instead of `ifconfig`) and generic (`openconnect` on the command-line uses `tun0`, Network Manager uses `vpn0` through the network applet; the numbers may vary if another network is already active) variant of @enzotib's excellent answer, tested on Ubuntu 20.04:

```
#!/bin/bash

if [[ "$IFACE" =~ ^(tun|vpn)[0-9] ]]; then
    ip link set "$IFACE" mtu 777
fi

```