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In iptables i would to add logging to every place where DROP is called.

However since I could encounter different tables in a production environment, I would just like to extend all rules that contain the DROP target, I don't want to go through the tedious process of rewriting every rule manually. Hence, I wonder - Is there a way to just change or add a log-prefix in a number of matching rules in order to add a custom string, say "DROPPED" ...? This way, if I add a similar entry in the PREROUTING table I can detect all the packets in a range of ports where PREROUTING is not followed by DROPPED, which would indicate a packet passthrough, which is ultimately what I would like to detect.

Either this way, or if there is another better way...

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Here is one of the best options.

iptables -N myDrop
iptables -A myDrop -j LOG --log-prefix "dropping " --log-level 7 --log-tcp-sequence --log-tcp-options --log-ip-options
iptables -A myDrop -j DROP

Now instead of doing a -j DROP do -j myDrop and it will be logged.

You can setup ulogd and log to a number of other destinations including a database.

iptables -A myDrop -j NFLOG --nflog-prefix  banned_hosts --nflog-group 1
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  • That is kind of what I am doing. Problem is DROP but also REJECT (DROP with ICMP response) are all over the place which forces me to manipulate the firewall extensively and manually, thereby also risk making a mistake. – JohnyTex Jun 7 '18 at 8:03
  • by starting a new chain you only have to reference the new chain, and it will log and drop for you. There shouldn't be DROP and REJECT rules everywhere. It should be -j myDrop and -j myReject and the chain with the matching name does all the heavy lifting. – cybernard Jun 7 '18 at 11:35

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