To change the MTU of an interface on GNU/Linux, you just need to use ifconfig command to do so, like this for example

    sudo ifconfig eth0 mtu 1492

To change it permanently on Debian, put it in the /etc/network/interfaces file, where almost all network parameters are found. To do this, just add a line mtu to the definition of your interface and save the file.

    sudo gedit /etc/network/interfaces

Example

<file>
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
</file>

Daiup Users

For dialup users: the Maximum Transmission Unit (MTU) value can be changed within the file

/etc/ppp/options


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Source: http://www.ubuntugeek.com/how-to-change-mtu-maximum-transmission-unit-of-network-interface-in-ubuntu-linux.html