I'd like to understand this rule.

This rule is correct?
iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST -m limit --limit 1/s -j ACCEPT

For what do you use it?

Sorry, I'm starting in Iptables

Who is Participating?
net_sec_guruConnect With a Mentor Commented:
iptables -A FORWARD (the FORWARD chain) used for ip forwarding / nat'ing

-p tcp (protocol)

Match when the TCP flags are as specified. The first argument is the flags which we should examine, written as a comma-separated list, and the second argument is a comma-separated list of flags which must be set. Flags are: SYN ACK FIN RST URG PSH ALL NONE. Hence the command

iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN - will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset.

-m limit (mark limit)
--limit rate
Maximum average matching rate: specified as a number, with an optional `/second', `/minute', `/hour', or `/day' suffix; the default is 3/hour.

-j ACCEPT (Jump -specifies target of the rule or action to take - in this case accept)

I believe this is correct.
ipsystemsAuthor Commented:

is this correct?
I need to close ALL ports and open only 25, 80 and 100

the final rule is problematic or can i use it ?  iptables -I INPUT 11 -i eth0 -j DROP

iptables -I INPUT 3 -m unclean -j DROP
iptables -I INPUT 4 -p icmp --icmp-type echo-request -j DROP
iptables -I INPUT 5 -p icmp --icmp-type echo-request -m limit --limit 1/s -j ACCEPT
iptables -I INPUT 6 -p tcp -m limit --limit 1/s -j ACCEPT
iptables -I INPUT 7 -p tcp --tcp-flags SYN,ACK,FIN,RST RST -m limit --limit 1/s -j ACCEPT
iptables -I INPUT 8 -i eth0 -p tcp --dport 80 -j ACCEPT
iptables -I INPUT 9 -i eth0 -p tcp --dport 110 -j ACCEPT
iptables -I INPUT 10 -i eth0 -p tcp --dport 25 -j ACCEPT
iptables -I INPUT 11 -i eth0 -j DROP
For just 25,80 and 100, you can use something like:

iptables -F INPUT
iptables -A INPUT -p tcp --dport 25 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 100 -j ACCEPT
iptables -A INPUT -j DROP
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