Messages in this thread | | | From | Alexander Clouter <> | Subject | Re: strange fragmented udp packets | Date | Tue, 4 Jan 2011 08:50:54 +0000 |
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Thomas Fjellstrom <thomas@fjellstrom.ca> wrote: > >> I can't explain the packets getting fragmented (or I couldn't, see below). >> And only have a guess as to why my VOIP provider isn't seeing them, I've >> heard that some firewalls explicitly block fragmented UDP as its often >> used in DoS attacks, and for little else. >> >> Here's at least what tcpdump has to say.. >> >> packet coming in: >> 23:38:26.136021 IP x.x.x.x.5060 > x.x.x.x.5060: SIP, length: 582 >> >> packet going out: >> 23:38:18.136085 IP x.x.x.x.5060 > x.x.x.x.5060: SIP, length: 544 >> 23:38:18.136169 IP x.x.x.x > x.x.x.x: udp >> >> ... >> >> After writing all that I've realized that the problem is the mtu on the nic >> attached to my cable modem, it was set to 576 for some odd reason. Changing >> the mtu to 1500 fixes my problem, but what is causing it to auto select an >> mtu of 576 anyhow? Could my modem be telling it that 576 is better for it? > > Anyone have a hint? I've checked with my sip provider and they don't have > iptables set up, so nothing should be getting in the way of these packets. For > some reason they just aren't reaching the SIP server. > pmtu?
Play around with the handy utility tracepath (and tracepath6) to <phun>discover</phun> more!
Cheers
-- Alexander Clouter .sigmonster says: May you have many handsome and obedient sons.
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