SYN,URG combination considered INVALID by tcp conntrack
kaber at trash.net
Wed Sep 6 20:20:58 CEST 2006
Please break your lines at 72 characters ..
Martijn Posthuma wrote:
> We have been experiencing problems with tcp connection establishment when (for some reason) the URG flag and pointer were set in the SYN segment. The problem did not affect all machines that were contacted but a subset. All affected machines were using netfilter-conntrack and firewalling. Inspection of the conntrack code (on a 2.6 linux kernel source) reveiled that the above mentioned combination of flags is not considered valid. The code I am referring to is in nf_conntrack_proto_tcp.c version 2.2, lines 768 to 795:
Not sure if I understand you correctly, did the machines running
connection tracking emit these packets?
> As you can see the SYN,URG combination is not considered valid. I can not find any justification for this in RFC793. I do realize that it is a rare combination of flags, but legal none the less. (In the RFC there is even an explicit mention of the fact that a SYN segment can contain data, so an URG flag is perfectly feasible in that case).
> Does someone know of a good reason that this flag combination is excluded?
No, I guess it should be considered valid. Out of interest, did the
SYN packets actually carry data?
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