Possible conntrack/kernel bug - not catching certain ICMP packets

[Ed W <lists@wildgooses.com>]

Hi, This is related to a previous thread, but more complete problem
statement below:

I notice that I can get an ICMP packet to bypass parts of conntrack
under the following conditions

- Send a UDP packet that triggers some kind of UDP reply
- Close the listening UDP socket before that reply arrives
- Kernel generates an ICMP unreachable response which does not appear to
be tracked (as expected) by conntrack

Tested with kernel 2.6.38.4 + iptables 1.4.11.1

Reproduce this easily like so:

    nslookup www.yahoo.co.uk 8.8.8.8 & sleep 0.001 && killall nslookup

    (where the sleep obviously needs to be smaller than your DNS RTT
lookup time. Obviously substitute nslookup/dig as appropriate...)

My results

# conntrack -E

    [NEW] udp      17 30 src=10.141.86.7 dst=8.8.8.8 sport=60721
dport=53 [UNREPLIED] src=8.8.8.8 dst=10.141.86.7 sport=53 dport=60721
 [UPDATE] udp      17 29 src=10.141.86.7 dst=8.8.8.8 sport=60721
dport=53 src=8.8.8.8 dst=10.141.86.7 sport=53 dport=60721
[DESTROY] udp      17 src=10.141.86.7 dst=8.8.8.8 sport=60721 dport=53
packets=1 bytes=66 src=8.8.8.8 dst=10.141.86.7 sport=53 dport=60721
packets=1 bytes=110

# tcpdump

11:26:35.072564 IP 10.141.86.7.60721 > 8.8.8.8.domain: 2+ PTR?
8.8.8.8.in-addr.arpa. (38)
11:26:35.351804 IP 8.8.8.8.domain > 10.141.86.7.60721: 2 1/0/0 PTR
google-public-dns-a.google.com. (82)
11:26:35.352110 IP 10.141.86.7 > 8.8.8.8: ICMP 10.141.86.7 udp port
60721 unreachable, length 118


# iptables -A OUTPUT -j LOGMARK
# iptables -A OUTPUT -j LOG

Jul 15 11:26:35 localhost kern.warn kernel: [ 6676.964396] iif=0
hook=OUTPUT nfmark=0x0 secmark=0x0 classify=0x0 ctdir=ORIGINAL
ct=0xcf3d5060 ctmark=0x0 ctstate=NEW ctstatus= lifetime=6346s
Jul 15 11:26:35 localhost kern.warn kernel: [ 6676.964396] IN= OUT=ppp1
SRC=10.141.86.7 DST=8.8.8.8 LEN=66 TOS=0x00 PREC=0x00 TTL=64 ID=19971 DF
PROTO=UDP SPT=60721 DPT=53 LEN=46
Jul 15 11:26:35 localhost kern.warn kernel: [ 6677.249312] iif=0
hook=OUTPUT nfmark=0x0 secmark=0x0 classify=0x0 ctdir=ORIGINAL
ct=0xcf3d5060 ctmark=0x0 ctstate=RELATED ctstatus=SEEN_REPLY,CONFIRMED
lifetime=4294937s
Jul 15 11:26:35 localhost kern.warn kernel: [ 6677.249426] IN= OUT=ppp1
SRC=10.141.86.7 DST=8.8.8.8 LEN=138 TOS=0x00 PREC=0xC0 TTL=64 ID=18412
PROTO=ICMP TYPE=3 CODE=3 [SRC=8.8.8.8 DST=10.141.86.7 LEN=110 TOS=0x00
PREC=0x00 TTL=46 ID=3897 PROTO=UDP SPT=53 DPT=60721 LEN=90


Notice that logmark seems to show that the ctstatus on the ICMP packet 
is SEEN_REPLY, but conntrack -E shows only packets=1?  tcpdump shows
that the ICMP packet did indeed go out

Could someone with more knowledge of conntrack please investigate further? 

Thanks

Ed W