How do you block new incoming tcp connections on X port? Needs to be done with iptables. I actually have a working iptables command but we always reach ip_conntrack_max even when ip_conntrack_max isset at very high. There a way to do it without keeping track?


If you want to block attempts to establish new sessions to a given port, but still allow packets to established sessions through, you'd need to do something like:

iptables -A INPUT -j DROP -p tcp --syn --destination-port dport

This should allow any connection initiated from the local machine, that happens to use dport as its local port number.


this should block the traffic without involving conn_track:

iptables -A INPUT -j DROP -p tcp --destination-port <your port>

connection tracking should only do its job when you specify -m state or --state in your rules.

  • 1
    The question was for blocking incoming TCP connections - that should have been a huge clue to filter on SYN – PP. Mar 10 '10 at 14:23
  • as i understand it: when you drop syn packets, there will be no connections, so you can drop every connection to the port. – Christian Mar 10 '10 at 14:50
  • If, for some reason, you have attempted to initiate a connection using that port number as the local port, dropping everything would drop the returning SYNACK, dropping SYN-only packets just stops non-locally-initiated packets. Admittedly, less of a problem when you have a specified destination port, but in the general case... – Vatine Mar 16 '10 at 11:29

Dropping --syn will stop new connections and there shouldn't be any half-open connections to track. In general filtering "without keeping track" is possible at the -t raw -I PREROUTING stage.


You could accept everything other than SYN packets. One way to do it would be:

iptables -A INPUT -p tcp '!' --syn --destination-port <your-port> -j ACCEPT

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