When I started to use OS X 10.3 panther I was very pleased to find that the MTU setting of the Ethernet interface can be changed using Network preferences. As I'm using a PPPoE connection through a NAT router, I need to set the MTU to 1492 instead of the default 1500 to get my internet connection fully operational. For those of you who did not find the setting yet, it's in the Ethernet tab of the Built-in Ethernet configuration - set Configure to "Manually (Advanced)" and enter any MTU value you like!
However, Apple forgot to implement this setting for the Airport interface. The solution is basically the same as for previous OS X versions. To change the setting manually, enter the following line in the terminal (assuming en1 is your Airport interface):
% sudo ifconfig en1 mtu 1492To fix the Airport MTU permanently, I added a line to /System -> Library -> StartupItems -> Network -> Network. At the end of the StartService() function, I added ifconfig en1 mtu 1492. This seems to work, even after waking the computer from sleep. Yet I'm not sure if this is the best way to do this - any suggestions?
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031116054614609