17

What's the difference between:

iptables ... -m state --state NEW

and

iptables ... --syn

The first one should select NEW connections, but AFAIK new connections are made by sending a TCP syn flag. The other one means just that - packets with a syn flag.

Can you give any practical examples when the above commands return different results?

10

The --syn flag is useful to check TCP traffic, but the NEW state can be used for other protocols (including TCP) like UDP and ICMP. I can say that NEW is more general than --syn TCP option.

From the iptables manual, you can read:

NEW    meaning that the packet has started a new connection,
       or otherwise associated with a connection which has not seen packets in both directions

An example, a DNS request will match the NEW state, but it will not match a rule with --syn option. Simply, it is a UDP datagram.

Also, the --syn option can be used to check for TCP packets with bad flags combination to drop them.

Also, you can use both of these options together to check for NEW TCP flows without --syn as first packet and drop them such as:

$ sudo iptables -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j LOG --log-prefix "New not syn:"

Here, we are adding this types of packets to a user-defined chain called bad_tcp_packets to be dropped/logged, etc...

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