This article was first written in September 2005 for the BeezNest technical website (http://glasnost.beeznest.org/articles/290).
To change the MTU of an interface on GNU/Linux, you just need to tell ifconfig to do so, like this for example:
/sbin/ifconfig eth0 mtu 1492
To change it permanently on Debian, put it in the /etc/network/interfaces, where almost all network parameters are found. To do this, just add a line mtu to the definition of your interface. Example:
iface eth0 inet static address 192.168.0.1 network 192.168.0.0 gateway 192.168.0.254 netmask 255.255.255.0 mtu 1492
There is an exception, though.
Warning: the following is mostly obsolete in Sid and Etch
It seems that the dhcp clients are not configured by default to do the same for dynamically-assigned configurations . So, you need to use a tweak to achieve the same. We’re going to use the pre-up feature of /etc/network/interfaces like this:
iface eth0 inet dhcp hostname "mymachine" name LAN Interface pre-up /sbin/ifconfig $IFACE mtu 1492