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While adding iptables rules, I couldn't understand what is the difference between connmark and mark? If anyone explains it with detailed explanations, I will be appreciated.

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up vote 6 down vote accepted

MARK associates "marks" with packets. CONNMARK associates "marks" with connections. The second one is useful because you can mark all the packets of a connection or related to a connection with the same mark (for example, FTP). Another usefull use of CONNMARK is that you can mark packets using the criteria that only matches with the first packet.

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Agreed. You almost always want to use CONNMARK, unless you know why you don't. If in doubt, use CONNMARK. – bahamat Jul 13 '12 at 18:51

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