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I eventually sometimes watch most initial tcp connection attempts like this:

tcpdump -nn -Z somepcapuser not src host (12x.x5.109.xxx or 62.75.160.xxx ) and not (port 9001 or 443 or 8080 ) and tcp[tcpflags]&(tcp-syn) !=0 and not tcp[tcpflags]& (tcp-ack) !=0 or icmp

this works pretty well to quickly identify oddness so far. However, I now have a question on the following output:

03:53:52.227884 IP 203.81.166.20.53786 > 62.75.160.xxx.80: S 846930886:846930886(0) win 61690 "<"mss 1460,nop,nop,sackOK,opt-178:f04700000000,nop,wscale 4">"

I wonder what the marked portion means, haven't seen that before.

Thanks for help wlaus

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This is TCP options. tcpdump can understand it. I'm not sure, but look like timestamp:

f04700 - current timestamp
000000 - previous timestamp

178 is probably offset from the start TCP options

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  • Hm, you might be right, however newer tcpdump reports it as: "Flags [S], seq 846930886, win 61690, options [mss 1460,nop,nop,sackOK,Unknown Option 178f04700000000,nop,wscale 4], length 0" and wireshark says "broken TCP" (ack field set, while ack bit is not), will provide some full capture later this day. – wlaus Mar 2 '11 at 5:05
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Maybe try feeding tcpdump capture to Wireshark? It nicely spells out meaning of packet content.

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  • Hi Pawel, see my other answer below or have a look at the dump above. – wlaus Mar 2 '11 at 22:21
  • @wlaus Hi, wlaus, I've fed your capture to Wireshark and it said "unknown options". :( – Paweł Brodacki Mar 10 '11 at 17:54
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Have seen this as well on several IDS's lately - 10000's of these directed at ports 80 & 443 from diverse sources - Myanmar, India, China... Definitely generated by the same software - always SYN, SEQ & ACK fields always identical. Weird. New exploit?

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There is now an offical sans diary entry:

http://isc.sans.edu/diary/Call+for+Packets+-+Unassigned+TCP+Options/10504

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