MTU

Since a week or so I have an ADSL connection by XS4ALL because I was so sick of having too much trouble with my cable connection. The installation went pretty fast and everything worked fine after (ab)using an extra DSL splitter as inline low-band filter for another telephone in the house. This is not really supported, but it works quite fine as described here.

There was one problem that I couldn't figure out though. Some sites, including my Internet banking, a TV Guide site and the service site of XS4ALL where unable to show me any reply to my HTTP requests. I could see with tcpdump that the three way handshake worked fine, but after that no data came back to me (some information has been removed to make it look better on the site):

:22.073702 kossu.41688 > website.http:
        S 4228908137:4228908137(0) win 5840
:22.106315 website.http > kossu.41688:
        S 2750130311:2750130311(0) ack 4228908138 win 5792 (DF)
:22.107974 kossu.41688 > website.http:
        . ack 1 win 5840
:22.109591 kossu.41688 > website.http:
        . 1:501(500) ack 1 win 5840
:22.110054 kossu.41688 > website.http:
        P 501:548(47) ack 1 win 5840
:22.339458 kossu.41688 > website.http:
        . 1:501(500) ack 1 win 5840
:22.799424 kossu.41688 > website.http:
        . 1:501(500) ack 1 win 5840


But then after a tip from Stefan Esser, which told me that it could be a problem with the MTA or something and other sites totally disabling ICMP to negotiate a new MTU, I found something interesting in the output of tcpdump:

:25.559806 fw > kossu: icmp:
        website unreachable - need to frag (mtu 500) [tos 0xc0]


I was wondering why the hell the MTU would be "500" as the default for Ethernet is 1500 iirc. Then I remembered that I changed this a couple of weeks before because of problems with my Chelloween connection and apparently I forgot to change it back. Issuing "ifconfig eth0 mtu 1500" made everything work perfect again so I can finally enjoy the full power ofmy DSL line.

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