iptables

iptables -I INPUT 1 -p tcp --dport 80 -m string --string "domain.com" --algo kmp -j DROP

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iptables -I INPUT 1 -p tcp --dport 80 -m string --string "cmd.exe" --algo bm -j DROP


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iptables -I INPUT 1 -p udp --dport 53 -m string --string "x99moyu" --algo kmp -j DROP
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string         This  modules  matches  a  given string by using somepattern matching strategy. It requires a linux kernel >= 2.6.14.        --algo  bm|kmp              Select the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)        --from offset              Set the offset from which it starts looking for anymatching. If not passed, default is 0.        --to offset              Set the offset to which it starts looking for anymatching. If not passed, default  is  the              packet size.        --string pattern              Matches the given pattern.  --hex-string patternMatches the given pattern in hex notation.