Arch Linux

wor

Member

From: France

Registered: 2010-02-21

Posts: 6

Troubleshooting IPv6 / MTU issues

Greetings !

I recently had issues connecting to https://app.zencoder.com using IPv6. My ethernet interface MTU was set to the default 1500.

I am not willingly using any 6to4 mechanism, my workstation has a public IPv6 address.

At that point, something like `curl -6 -I https://app.zencoder.com` would systematically time out, with `strace` showing repeated timeouts.
Tools like `ping6 app.zencoder.com` work as expected.

One helpful fellow from Zencoder support let me know that other customers solved this by tuning down their MTU, and indeed, I figured out that lowering my MTU to 1467 (or lower) allowed the connection to be properly established.

I am puzzled by the fact that `strace curl -6 -I https://app.zencoder.com` only shows a single write to the socket, of 340 bytes. The packet size is probably 8 bytes larget (4 for IPV6 header, 4 for TCP segment) if I'm not mistaken.

So could anyone help me understand why lowering my MTU from 1500 to 1467 changes anything to what happens to a 348 bytes packet.
( Edit: I went to Wireshark to get some last word on this. Wireshark says: 414 bytes on wire )

I tried playing with tracepath6 but I can't figure out anything useful from it's output - no pmtu lines outside the first line. Past the 17th hop it's all "no reply".

I suspect that someone somewhere is blocking ICMP packets, but I do not know how of an experiment to prove/disprove that.

Any hints / tips / pointers welcome

Thanks for reading.

Last edited by wor (2014-01-16 15:46:47)