It is August 2015. Unfortunately it is still not possible to manually specify the MTU to be used on non-Ethernet NetworkManager connections (eg. VPN connections but also PPP).
Not that is has not been requested years ago. Choose one of the many bug reports:
- #584200: support additional options (using custom mtu, mss) (from 2009)
- #603575: MTU calculation is wrong and may cause vpn connection to hang (from 2009)
- #652517: MTU/MRU for PPP cannot be changed in nm-connection-editor (from 2011)
- #658018: No option to change MTU for mobile broadband in NetworkManager (from 2011)
- #741907: Unable to set base-mtu when connecting via openconnect (from 2014)
- #751830: Manual and auto MTU/MRU handling (from 2015)
Safe for the fact that they’re partially redundant reports, just imagine what would have happened if the devs had simply provided a MTU field for every connection type, empty by default but allowing to explicitly configure one. But no, that would have confused users *rolleyes*
Enough ranting, for VPN connections the workaround remains. Create a script /etc/NetworkManager/dispatcher.d/vpn-up:
#!/bin/sh if [ "$2" = "vpn-up" ]; then /sbin/ifconfig "$1" mtu 1392 fi
Still thinking that it’s sufficient to backup /home before your next distribution upgrade/reinstall? Good luck…