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using tc's match to find ACK packets

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tnt
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Joined: 27 Feb 2004
Posts: 1222

Posted: Sat Sep 15, 2012 12:55 pm Post subject: using tc's match to find ACK packets

in LATRC we are advised to use the following to find ACK packets:

Code:

tc filter add dev ppp14 parent 1:0 protocol ip prio 10 u32 \
match ip protocol 6 0xff \
match u8 0x10 0xff at nexthdr+13 \
match u16 0x0000 0xffc0 at 2 \
flowid 1:3

where part of the code

Code:

match u8 0x10 0xff at nexthdr+13

should check if ACK bit is set.
but, as mask given to that match is 0xff, it returns "true" just if ONLY ACK is set.
is this right, or should this filter have mask 0x10 to match if, besides other bits, also ACK is set?

Code:

match u8 0x10 0x10 at nexthdr+13

http://lartc.org/howto/lartc.adv-filter.html
http://www.compago.it/images/stories/TCP_Header.png

thx.
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