Re: Possible conntrack/kernel bug - not catching certain ICMP packets

[Ed W <lists@wildgooses.com>]

Hi, any chance I could get some of the netfilter gurus to have a look at
this?

Essentially we have a reproducible situation where conntrack is
(apparently) relating a packet to a connection, but not incrementing the
connection counters?

Thanks for any help figuring out a fix?

Ed W


On 15/07/2011 12:45, Ed W wrote:
> 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
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