So to follow up with the previous post about MTU and MSS, I wanted to show you how to set the MTU value on your computer. On most Unix-like boxes it is a fairly straight forward process to change your MTU. On Windows it gets a little trickier with registry changes and such things that can brick your computer.
For working with the MTU under windows you can look at the assorted tutorials on the Microsoft site, or you can get a tool like DrTCP to do it for you. You can use it to adjust your MaxMTU setting.
With OS X it is a little easier. Just pull up your system preferences, select Network under “Internet & Network”. Next select “Built-in Ethernet” in the “Show” drop down and then select the Ethernet tab.
Change the “Configure” dropdown to “Manually (Advanced)” and you will be able to change your MTU.
You can also do it from the command line in OS X and other Unix-ish OSes with ifconfig. The first thing you need to do is identify the interface you want to change. On OS X the ethernet interface is en0. On a linux system it may be something like eth0. To get information about the various interfaces on your system you can type…
Once you know which interface you want to adjust, you supply your interface and mtu value to ifconfig like this
ifconfig en0 mtu 1460
In this case it would change the MTU of the interface en0 to 1460.
This should give you another tool to troubleshoot network issues. If you find that you are having some o the odd behavior mentioned in the previous article, try dropping your MTU to a low value like 512 and see if it works. If this fixes things, try to ratchet up the value as high as you can to see what the real MSS of the network path is.