how to make forward between two network cards in the same pc
 

hello everybody,
Actually i've a pc with two network cards and i want to make my pc as a netfilter firewall ,thus a packet enters in the NIC A and goes out from the NIC B and with the IP address i can filter so i can ACCEPT or DROP from the IP addresses and when a packet is accepted it passes from A card to B using Iptables.

thank you very match.

Heres an article on how to setup a "gateway".

http://www.linuxjournal.com/article/3866

This will be a very good thing learning wise. If you just prefere a out of the box solution there are linux firewall distribution. check out endian, shorewall, ipcop. There sure are more.

thank you zhjim for your answer,but if i want to forward a packet do i have to mention the IP address of card IN and then the card OUT????or when I put the rule in the FORWARD chain it will be automatically done.
because as I read in documents when a packet arrives not destined to the card A(IN) it will be routed ,but routed where?? so i have to mention the out IP address

As you normal define some condition witht the -s option of iptables. So to have all the ip's in the 192.168.0.0/24 range to be masqueraded you would do:

If you leave out the source option all the packages that arrive would match this rule.
I think what you are looking for is the -i option. This can be used to tell iptables to match on a certain incoming interface.
Example for eth0 as interface to local network
This would bring all the packets which match the ip('s) and the incoming interface to be masqueraded and forwarded to the internet. To match on the outgoing interface use -o.


So how or where something is routed depends on the rules and the according matches you setup.

*Sidenote*
I guess that if you just would do a
all packages coming into the machine would be masqueraded.


Totaly right on that one. You can use the -o options to tell where you want the packets to leave the machine.
I just read up on the tutorial link I gave you. It's neither clear nor suiting your needs (I just used the first link that came up from google). Here is a better one which just describes how to setup a linux gateway to connect your local network to the world wide web.

http://www.yolinux.com/TUTORIALS/Lin...rkGateway.html

Taken from netfilter.org


take a look at the link below to get some more inside into iptables and a bit of networking with linux.
http://www.netfilter.org/documentati...umentation-faq