puku:~# tunctl -t tun42
Set 'tun42' persistent and owned by uid 0
puku:~# ip link set mtu 25000 dev tun42
puku:~# echo $?
0
puku:~# uname -a
Linux puku 2.6.25-rc3-cw3 #6 SMP Wed Feb 27 14:36:36 PST 2008 x86_64 GNU/Linux