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Tcpdump output -- need interpretation

Posted on 2004-10-20
Last Modified: 2007-12-19
Hello All,

I need to understand what is going on in this tcpdump output. Its from a simple http request from a vpn'd client to an FC2 box:

19:49:33.516582 IP > S 1753009426:1753009426(0) win 65535 <mss 1260,nop,nop,sackOK>
19:49:33.552420 IP > S 2686314030:2686314030(0) ack 1753009427 win 5840 <mss 1460,nop,nop,sackOK>
19:49:36.487145 IP > S 1753009426:1753009426(0) win 65535 <mss 1260,nop,nop,sackOK>
19:49:36.487191 IP > S 2686314030:2686314030(0) ack 1753009427 win 5840 <mss 1460,nop,nop,sackOK>
19:49:37.115512 IP > S 2686314030:2686314030(0) ack 1753009427 win 5840 <mss 1460,nop,nop,sackOK>
19:49:42.495742 IP > S 1753009426:1753009426(0) win 65535 <mss 1260,nop,nop,sackOK>
19:49:42.495785 IP > S 2686314030:2686314030(0) ack 1753009427 win 5840 <mss 1460,nop,nop,sackOK>
19:49:43.121442 IP > S 2686314030:2686314030(0) ack 1753009427 win 5840 <mss 1460,nop,nop,sackOK>

The client is unable to complete http session (timesout), I have tried this using same vpn client to another Linux box (RH9)in the same internal net, but the session is completed and the tcpdump out looks normal. Any ideas?

Question by:GR999
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Expert Comment

ID: 12360847
i think this log is ok. check the errors in the NIC with ifconfig

Expert Comment

ID: 12361403
On which box did you take this tcpdump? I would say on the FC2?

What is happening is that the connection does not come up completely, you see a syn and a syn/ack. This sort of "half" connected problem normally indicates a routing problem.

Author Comment

ID: 12363929
Do you guys know of anything on the internet to help decipher some of the output? I have no idea what mss, nop and sack (syn/ack?) is. I probably won't go into detail on the network as this is probably wrong forum for that, was just wondering on how to understand more of this ouput.

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Author Comment

ID: 12363953
Sorry, yes this was taken on the FC2 box. Another note is the FC2 is actually NAT'd to a different public ip off the router all others are running PAT off the router ip. I'm thinking that my be the issue but not sure why.


Accepted Solution

de2Zotjes earned 125 total points
ID: 12367303
If you are looking for deciphering of the actual letters: rtfm for tcpdump.

If you want to understand more of what this bit of output is telling us:

first of all the same pattern of lines repeats 3 times, with increasing intervals (the very first field is capturetime). That is the first clue that the connection is not established because the tcp protocol will retry sending packets with a "exponential backoff"

Second all the packets that we see here have the syn flag set (the capital S in the output, if no flags then that spot would have a . ) The syn flag is what is used to set up a connection between to machines.
If you are looking for exact details on how tcp does that I can suggest some good books :) Basically there are 3 packets needed to setup a connection:
syn ->
syn/ack <-
ack ->

You see that the last does not have the syn flag set, this means you can easily spot the point where actual data exchange between machines starts: when the syn flag is no longer set! Since in your dump all packets have the syn flag it is clear that the connection is never established.

Next step is just experience: this pattern of packets is consistent with a routing problem. Or in other words: packets can travel in one direction but not the other, whether that is caused by a faulty routing entry or by a malfunctioning nat rule remains to be seen...

Author Comment

ID: 12374445
Ok gotcha. I'm pretty sure it would not be a routing issue as I can ping back and forth:

05:27:33.353643 IP > icmp 40: echo request seq 256
05:27:33.354417 IP > icmp 40: echo reply seq 256
05:27:34.362542 IP > icmp 40: echo request seq 512
05:27:34.362591 IP > icmp 40: echo reply seq 512
05:27:35.363961 IP > icmp 40: echo request seq 768
05:27:35.364010 IP > icmp 40: echo reply seq 768
05:27:36.365449 IP > icmp 40: echo request seq 1024
05:27:36.365498 IP > icmp 40: echo reply seq 1024

Well either way I appreciate the help clearing up the original tcpdump question.


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