iptables dropping lots of packets

Hello friends i want to block the packets which has payload only ff in hex format but when i try to following command

iptables --append FORWARD -i p1p1 -p UDP   -d 192.168.1.2  --match string  --algo kmp --hex-string '|ff|'  -m length --length 10 -j DROP

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it drops all packets which include ff like 1213121ff000000    

how should i let it drop exact payload match
FireBallITAsked:
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Gerwin Jansen, EE MVETopic Advisor Commented:
You may want to specify the full string to match like:

--hex-string '|ff 00 00 00 00 00 00 00|'

(or some other place of your ff)
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nociSoftware EngineerCommented:
You did request to drop all UDP packets that contain 'ff' somewhere in the payload.

you probably meant -hex-string '| ff ff ff ff ff ff ff ff ff ff |'   for 10times ff

which would match any packet containing at least 10 times ff...
but it would miss a packet of size 6 containing only 6 time ff.
You can also make other cases like: checking for packet with length X and X occurences...
(as these are exact match patterns, you need to specify whole paterns)
 --hex-string '| ff |' --length 1
 --hex-string '| ff  ff |' --length 2
 --hex-string '| ff  ff ff |' --length 3
 --hex-string '| ff ff ff ff |' --length 4
 --hex-string '| ff ff ff ff ff|' --length 5
 --hex-string '| ff ff ff ff ff ff|' --length 6
--hex-string '| ff ff ff ff ff ff ff |' --length 7
...
which becomes rather inefficient.
Also better to first check for length and then content if this exact matching is needed.
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FireBallITAuthor Commented:
that is what i exactly looking for
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