asterisk users - Sep 2010 - Asterisk stops processing SIP UDP messages

Last week I had a couple of outages one machine, the problem was that
Asterisk suddly stopped responding to UDP SIP requests. tcpdump show
requests arriving on the machine, sip debug log in asterisk doesn't show
anything for the UDP peers, TCP functions just fine.

In all 3 cases the log is something like below, a +/- 10m gap
in any SIP/UDP related traffic in the logs, followed by a bunch of
"Really destroying SIP dialog" messages.

Versions of Asterisk affected where 1.6.2.9 and 1.6.2.13 on a
Debian/stable machine. There is no NAT involved and reloading/flushing
iptables has no effect, one of the first rules is to accept both
incoming tcp/udp traffic:
Chain INPUT (policy DROP)
fail2ban-asterisk-tcp tcp -- 0.0.0.0/0 0.0.0.0/0 multiport dports 5060,5061 
fail2ban-asterisk-udp udp -- 0.0.0.0/0 0.0.0.0/0 multiport dports 5060,5061 
fail2ban-ssh tcp -- 0.0.0.0/0 0.0.0.0/0 multiport dports 22 
ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpts:10000:20000 
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:5060 
ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:5060
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state ESTABLISHED

Sadly enough I'm not sure wether the RTP streams also stop functioning
(forgot to capture all traffic), but since a TCP peer tried to call us
(also a TCP peer) and failed to do so my guess is no UDP is working at
all.

I don't have enough information to make a decent bugreport so my
question is if anyone experienced something like this or how to
accumilate further information for a better bugreport?

=====================================================================
[2010-09-18 14:22:51] VERBOSE[15309] chan_sip.c:
<--- SIP read from UDP:109.235.33.10:1038 --->



<------------->
[2010-09-18 14:22:52] VERBOSE[15309] chan_sip.c: Really destroying SIP
dialog '194a9bd477ab104d236a1bcb778ff518 at 109.235.32.137' Method:
OPTIONS

[2010-09-18 14:34:12] VERBOSE[15309] chan_sip.c: Really destroying SIP
dialog '6f71dc6f11b708b733be1ce869353509 at 109.235.32.36' Method:
OPTIONS
[2010-09-18 14:34:12] VERBOSE[15309] chan_sip.c: Really destroying SIP
dialog '444376ba0f3d42746a9cd51761356eb8 at 109.235.32.36' Method:
OPTIONS
<<<<<DELETED 50 other Really destroying
messages>>>>>
[2010-09-18 14:34:12] VERBOSE[15309] chan_sip.c: Really destroying SIP
dialog '43e3a3b678b75cef1a3d362f75188c21 at 109.235.32.36' Method:
OPTIONS
[2010-09-18 14:34:12] VERBOSE[15309] chan_sip.c: Really destroying SIP
dialog '20d68c0d288ef7f6682d0c1f7b5cd2e1 at 109.235.32.36' Method:
OPTIONS
[2010-09-18 14:34:12] VERBOSE[15309] chan_sip.c: 
<--- SIP read from UDP:88.159.80.32:5060 --->
NOTIFY sip:voip.pocos.nl SIP/2.0
Via: SIP/2.0/UDP 10.201.0.120:5060;branch=z9hG4bK-6ca6c7ca
From: Fax <sip:fax01 at voip.pocos.nl>;tag=613dd9bfc3f8c5co0
To: <sip:voip.pocos.nl>
Call-ID: 1385478b-23141900 at 10.201.0.120
CSeq: 231 NOTIFY
Max-Forwards: 70
Event: keep-alive
User-Agent: Linksys/PAP2T-5.1.6(LS)
Content-Length: 0


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