Understanding connection tracking in iptables

Posted by Matt on Server Fault See other posts from Server Fault or by Matt
Published on 2012-11-26T22:59:31Z Indexed on 2012/11/26 23:05 UTC
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linux

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iptables

I'm after some clarification of the state/connection tracking in iptables.

What is the difference between these rules?

iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
Is connection tracking turned on when a packet is first matched containing -m state --state BLA , or is connection tracking always on?
Can/Should connection state be used for fast matching like below?

e.g. suppose this is some sort of router/firewall (no nat).

# Default DROP policy
iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

# Drop invalid
iptables -A FORWARD -m state --state INVALID -j DROP

# Accept established,related connections
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow ssh through, track connection 
iptables -A FORWARD -p tcp --syn --dport 22 -m state --state NEW -j ACCEPT

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