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I would just like some heads up on this as I really don't understand -why- this is the case.

Further down is the TCPDUMP output (tcpdump -s0 -XXnni eth0 tcp port 80) of the 'HTTP' response a webserver gives when a normal GET request is made to it. What I am trying to do is use Linux u32 tc filter to match on the contents of the TCP ack packet, looking for the string 'HTTP/1.[01] 200' in the data payload of the TCP ack packet (in otherwords, looking for a typical 'HTTP/1.0 200 OK' response or, 'HTTP/1.1 200 OK' response.

Here's a snippet of the tc filter command - this will probably help put things in context:

tc filter add dev eth0 parent ffff: protocol ip u32 \
  match ip protocol 6 0xff \
  match ip sport 80 0xffff \
  match u8 0x10 0xff at 33 \
  match u32 0x48545450 0xffffffff at 52 \
  match u32 0x2f312e31 0xfffffffe at 56 \
  match u32 0x20323030 0xffffffff at 60 \
<do something>

The last 3 'match u32' lines will match 'HTTP/1.0 200' or 'HTTP/1.1 200' while the u8 one matches the TCP ack flag, and the others match source port 80 protocol TCP.

My query is - why, on two different Linux boxes, do I have to change the numbers 52, 56 and 60 to 40, 44 and 48 ? (subtract 12 from the offset). It seams on my Slackware Linux box at home, I have to use 52, 56 and 60, where as on a RedHat/CentOS server, I have to use 40, 44 and 48.

The reason for this is simple; compare these two TCPDUMPs from each server:

Slackware: 0x0000:  0040 63c9 c3a0 0018 7d05 dd11 0800 4500  .@c.....}.....E.
           0x0010:  05be d41c 0000 3606 9ea2 4266 0963 c0a8  ......6...Bf.c..
           0x0020:  000a 0050 a278 e948 dcdb fa41 ac84 8010  ...P.x.H...A....
           0x0030:  0059 3cb2 0000 0101 080a 9380 9172 0008  .Y<..........r..
           0x0040:  3bea 4854 5450 2f31 2e31 2032 3030 204f  ;.HTTP/1.1.200.O

RedHat or: 0x0000:  0016 3e32 3fcf 0010 dbff 2050 0800 4500  ..>2?......P..E.
CentOs:    0x0010:  0554 cf64 0000 3706 e08b 4266 0969 0a64  .T.d..7...Bf.i.d
           0x0020:  7881 0050 b316 c917 2062 b4a8 cff4 5018  x..P.....b....P.
           0x0030:  005a a17f 0000 4854 5450 2f31 2e31 2034  .Z....HTTP/1.1.4
           0x0040:  3034 204e 6f74 2046 6f75 6e64 0d0a 436f  04.Not.Found..Co

As you can see, the offset point where the 'HTTP' part starts (the packet data payload) is different in both cases. Why is this? And what would cause it?

Thanks in advance to anyone who can explain this mystery to me.

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up vote 0 down vote accepted

Found the answer. Having TCP timestamps ON introduces 12 bytes extra header information, resulting in the offset difference.

You can toggle them on/off in Linux by doing this:

echo 0 > /proc/sys/net/ipv4/tcp_timestamps
echo 1 > /proc/sys/net/ipv4/tcp_timestamps

Also see if they are on/off by doing:

cat /proc/sys/net/ipv4/tcp_timestamps
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