What does this iptables command do?

Question

Does iptables -I INPUT -j ACCEPT open all ports?

Answer 1

This will accept all traffic that is inbound and destined for this computer; all ports, all protocols, all users, all states. Remember that port isn't the only thing you have to think about when writing a firewall on a Linux System. Any traffic that is outbound or destined for another computer may have different rules. However if your rules are this, for example

iptables -I INPUT -j ACCEPT
iptables -I INPUT -j DROP

the second rule will end up being before the first and all traffic will be dropped. If you want to allow all traffic. I generally recommend using -A instead of -I when writing a firewall script so the order is obvious. Also if you just want to allow everything, you'd be better off setting the policy for the chain.

iptables -P INPUT ACCEPT

will tell everything that hasn't matched another rule at the end of INPUT to be allowed.

Answer 2

iptables -I INPUT -j ACCEPT

The above command will INSERT (-I) a new rule at chain position 1 (understood by omission) that matches every packet crossing the INPUT chain (because their are no filters) and will ACCEPT it.

Answer 3

Yes. This command creates a rule that will be checked first (-I) for incoming packets (INPUT); the rule matches every packet (no filter) and allows them (-j ACCEPT).

Answer 4

Yes, provided that the packet is not dropped in -t raw.