Maximum Transmission Unit(MTU), the largest physical packet size, measured in bytes, that a network can transmit. Any messages larger than the MTU are divided into smaller packets before being sent.By optimizing the MTU setting you can gain substantial network performance increases, especially when using dial-up modem connections.
Default MTU Size for Different Network Topology
| Network | MTU(Bytes) |
| 16 Mbit/Sec Token Ring | 17914 |
| 4 Mbits/Sec Token Ring | 4464 |
| FDDI | 4352 |
| Ethernet | 1500 |
| IEEE 802.3/802.2 | 1492 |
| X.25 | 576 |
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 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.
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
Warning: the following is mostly obsolete in Debian 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
More common Recommended Values
Dial-up Connections – 576 Bytes
PPPoE Broadband Connections – 1492 Bytes
Ethernet, DSL and Cable Broadband Connections – 1500 Bytes
pre-up /sbin/ifconfig $IFACE mtu 1492
You might like using post-up instead.
those who need to change mtu during debian install, activate a console (alt+f2, for example) and use the busybox’s ip utility, examples:
ip link show
ip link set eth0 mtu 1492
modify the parameters to your own needs… it works for debian lenny