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I'd like to stop others from seeing my ports as filtered in the nmap standard scan (unprivileged). Let's say that I have the following ports open: 22, 3306, 995 and a firewall configured like this:

-A INPUT -p tcp -m tcp --dport 22 -j DROP
-A INPUT -p tcp -m tcp --dport 3306 -j DROP
-A INPUT -p tcp -m tcp --dport 995 -j DROP

This is the result of an nmap scan:

[+] Nmap scan report for X.X.X.X

    Host is up (0.040s latency).
    Not shown: 90 closed ports

    22/tcp   filtered ssh
    995/tcp  filtered pop3s
    3306/tcp filtered mysql

It displays these ports as filtered, because my server didn't reply RST for SYN. Is there a way to modify this behaviour? For example: if the iptables firewall blocks a port, reply RST for SYN, instead of remaining silent (not replying anything)?

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up vote 18 down vote accepted

Don't use DROP, that's easily identified as "filtered" if you know the box is up. Instead, you may use the following to send a RST. (as if there is a service listening, but it doesn't accept connections from you)

-A INPUT -p tcp -m tcp --dport 22 -j REJECT --reject-with tcp-reset

Or otherwise simply use the following to make the port look closed. (as if there is no service listening on it)

-A INPUT -p tcp -m tcp --dport 22 -j REJECT
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-A INPUT -p tcp -m tcp --dport 995 -j REJECT --reject-with tcp-reset

should be doing what you want (reply with RST).

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