Re: simple questions to finally understand netfilter

[Ralf Spenneberg <lists@spenneberg.org>]

Am Don, 2003-11-27 um 16.41 schrieb Juan Hernandez:

> 1) This rule tells netfilter to drop any packet forwarding I guess
> iptables -P FORWARD DROP
This is a default rule. All packets not accepted or dropped by other
rules will be dropped by this one.

> 
> 2) This one only accepts related and stablished packets
> iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
correct.

> 3) This one forwards everything from the mailserver to the outside i
> guess
> iptables -A FORWARD -d 192.168.0.2 -p tcp --dport 25 -j ACCEPT
No. This allows all packets destined for 192.168.0.2:25 through.

> 4) and this rule redirects everything in port 25 to the mailserver
> iptables -t nat -A PREROUTING -d 192.168.0.1 -p tcp --dport 25 -j DNAT
> --to-destination 192.168.0.2:25
This rule redirects all packets targeted at 192.168.0.1:25 to 192.168.0.2:25. 

> Now these are the questions I have about netfilter
> 
> 1) What does the first rules do exactly?? 
> 
> 2) How come if I add this rule to redirect everything from port 666 to
> the mailserver's ssh port it doesnt work?? 
> 
> iptables -t nat -A PREROUTING -d 192.168.0.1 -p tcp --dport 666 -j DNAT
> --to-destination 192.168.0.2:22 
You need a rule which will allow these packets through the FORWARD
chain:
iptables -A FORWARD -d 192.168.0.2 -p tcp --dport 22 -j ACCEPT

> 3) How do I use DROP with a range of port in oder to close everyhing
> else to the outside? or is there other way to do it??
Everything else is closed by the Default rule (1)

Cheers,

Ralf
-- 
Ralf Spenneberg
RHCE, RHCX

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