HOWTO Change the MTU of an interface on Debian

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 [1]. 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

[1] see DBTS #294044, and DBTS #333519, and DBTS #309205 for more information

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