Solving the MacOS/Airport/Linksys/PPPoE MTU problem

September 4, 2004

DHCP is your friend.

If you Google for “Airport MTU”, you will find a number of entries describing a problem where the combination of DSL (usually Verizon, which uses PPPoE) and Airport requires users to reset their MTU again, and again, and again. It’s very annoying.

My hardware configuration is a DSL modem connected to a Linksys BEFSX41 connected to an Airport base station (don’t ask why I have two routers, it just happened — the BEFSX41 IS a very capable beast), and the Airport had a manually assigned address behind the Linksys router. The Airport handed out wireless addresses dynamically. Anyone using wireless had to constantly reset their MTU to a smaller value (for some reason, 1464 is what worked) because there is no way in MacOS to permanently set the Airport MTU.

I found a solution. Use DHCP. I changed the Airport to get its address dynamically, and the problems vanished. I set my wireless MTU back to the default (1500), started browsing, read mail, sent mail, and it all worked.

However, at least for now, I am having a hard time accessing printers on the external-to-Airport network. The firewall subnet is 192.168.1.*, the Airport subnet is 10.0.1.*, and the Airport is doing NAT for everything behind it. I think I will eventually figure this out, but for now it is suboptimal. Maybe I could cook something up with an SSH tunnel.

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