typo3bloke.net

It appears that one major German ISP has troubles with his routers and traffic goes through backup connection. But that connection has problems as well: one of its routers badly configured. It does has MTU value set to 1492 and router before it does not accept ICMP messages that advise him to resend messages. Therefore anything longer than 1492 bytes get lost completely. Due to this fact I lost huge amount of time. Finally I remembered that I have this issue half a year ago and quickly tested connection usng ping:

ping -D -s 1500 bugs.typo3.org

This did not go through, told me that do-not-fragment is set but packet has to be split. WIth 1492 it passed through.

I configured my router and AirPort on MacBook to use 1492 as packet size. Pretty stupid to do it because of one misconfigured router but what other choice do I have?

Mac users can configure AirPort to use 1492 as MTU using this from terminal:

sudo ifconfig en1 mtu 1492

This has to be done on every startup or wake up. Built-in Ethernet has setting in System Preferences for MTU but AirPort does not. Interstingly Apple says to use Leopard help on MTU and AirPort issues but there is nothing in help about it.

Links in the end of this article tell more about MTU issues and improperly configured routers.