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Manually configure MTU on Mac OSX (10.1.x - 10.2.x)


Article ID: HT004

This Information applies to:

  • AMX-64
  • X-Modem
  • Buffalo Airstation
  • Mac OSX


This document explains how to adjust the MTU (Maximum Transmission Unit) settings to optimise your ADSL Service on Mac OS 10.1.x and 10.2.x. This document does not cover 10.3.x.

BT Wholesale recommend an MTU setting of 1458 (bytes) and state that it is best suited to the performance of the ADSL broadband network.


Temporary Change

A temporary change can be made by typing a command in to the terminal.

The terminal utility can be found inside a folder called Utilities located in the Applications folder on your Hard Drive. Double click the icon to start the terminal.

A message similar to the following will be displayed.

Last login: Fri Dec 27 11:51:52 on console
Welcome to Darwin!
[Customer's Mac:~] myname%

The % symbol is a prompt where you can type commands. If your AMX-64 is connected directly via Ethernet type the following command exactly as shown and press return.

sudo ifconfig en0 mtu 1458

Note: This is en zero not O. If you are connecting over an Airport Card substitute en0 for en1.

You will be prompted for your password.

To check that it has worked type the following command and press return.


If it has worked correctly you should see information similar to the following showing en0 with an mtu value of 1458.

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
inet netmask 0xff000000
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
inet6 fe80::203:93ff:fea9:9726%en0 prefixlen 64 scopeid 0x4
inet netmask 0xffffff00 broadcast
ether 00:00:00:00:00:00
media: autoselect (10baseT/UTP <half-duplex>) status: active
supported media: none autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <half-duplex,hw-loopback> 10baseT/UTP <full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 100baseTX <half-duplex> 100baseTX <half-duplex,hw-loopback> 100baseTX <full-duplex> 100baseTX <full-duplex,hw-loopback>

Permanent change

When satisfied that the MTU adjustment has the desired affect the change can be made permanent. This involves editing a system file so be extra careful to make sure that you don't make any errors.

In the terminal window type the following command.

sudo pico /System/Library/StartupItems/Network/Network

A text editor called pico will open as shown below.

To make the permanent change our MTU command must be added to this file so that it is applied every time the computer is switched on. Use the directional arrows on the keyboard to move the cursor. Change the text as follows.


# Configure network interfaces and host name

. /etc/rc.common

StartService ()
    ConsoleMessage "Initializing network"

    # Configure interfaces
    ipconfig waitall > /dev/null 2>&1
    ifconfig lo0 netmask up
    ifconfig en0 mtu 1458
    # See if the network is up now

When the changes are complete press Control X to exit pico. When prompted to save press Y for yes. A prompt for file name to write will then be shown, accept the default by pressing return.

Changes are complete.