Re: Forward Chain: is Inbound traffic on eth0 not also Outbound depending on your view?

[Mart Frauenlob <mart.frauenlob@chello.at>]

paddy joesoap wrote:
> Dear Experts
>
> I am curious to know more about what FORWARD chain inbound and
> outbound actually mean.
>
> Example firewall set-up below:
>
> Internet --- Firewall --- PC
>
> Firewall has 2 interfaces: eth0 = External and eth1 = Internal
>
> >From what I can gather from the Netfilter website, all I need to do is
> create are inbound and outbound rules on the FORWARD chain.
>
> To allow inbound Internet access, I specify:
>
> FORWARD -i eth0
>
> To allow outbound PC access, I specify:
>
> FORWARD -o eth1
>
> The question is from whose perspective do you view what is inbound and
> what is outbound?
>
> For example, in the case of the Internet client, traffic flowing
> towards the firewall is indeed Inbound so naturally "FORWARD -i eth0"
> is required. However, isn't it also Outbound on eth1, given that it
> leaves interface eth1 to get to PC?
>
> Similarly, clients on the internal network think of their traffic as
> being outbound only, but when traffic is being "forwarded" from eth1
> to eth0 heading for the Internet, isn't that traffic classed as
> Inbound on eth0?
>
> Do I need to create rules for this scenario also or is Netfilter
> handling these implied situations?
>
> Beginner questions so apologies in advance.
> Paddy.
>   
Please read this carefully and if you still have questions, ask them 
afterwards:

http://www.frozentux.net/iptables-tutorial/iptables-tutorial.html#TRAVERSINGOFTABLES
http://jengelh.medozas.de/images/nf-packet-flow.png

but in short:
INPUT chain = packets destined to your host
OUTPUT chain = source of packets is your host
FORWARD chain = source is external - destination is external address 
(forwarded, routed)

regards

Mart