Hi, I'm in the process of building a three-interface firewall and I have some questions about how the different chains see NAT packets and locally-generated packets.
Firstly, if I just do filtering in the INPUT/OUTPUT chains, NAT-ed packets will not traverse those chains, so I figure I should probably put similar filtering rules in the FORWARD chain? (For example, I'd like to be able to block all my internal users from accessing certain sites, or block incoming traffic sent by bad hosts from being port-forwarded to internal servers).
If I was trying to block incoming traffic from bad hosts, why not simply put the filters in the PREROUTING chain instead of both INPUT and FORWARD? Is it because the nat table is intended for just nat and doing filtering there would be ugly, or would it actually fail to work?
I read in http://davidcoulson.net/writing/lxf/14/iptables.pdf (on the netfilter.org documentation page) that nat's OUTPUT chain performs DNAT on outgoing packets originating from the server, and POSTROUTING performs SNAT on outgoing packets passing through the firewall from other hosts. If I have two Internet-facing IPs and would like to SNAT locally-generated traffic to one or the other, it would appear that iptables wouldn't let me do that very easily, right? What is the purpose of nat's OUTPUT chain (in other words, when would I want to DNAT locally-generated traffic)?
In what order does locally-generated traffic traverse the OUTPUT chains of filter and nat tables?
Lastly, aside from those issues, is the diagram below a reasonable representation? The only diagrams I found on chain traversal dealt with the nat and filter tables separately, but I'm hoping that it's possible to show them together. (I hope hotmail doesn't completely destroy this ascii hehe).
# -->n.PREROUT-->routing decision-->f.FORWARD-->n.POSTROUT--,--> # | ,-------------^ # v | # f.INPUT f.OUTPUT, n.OUTPUT # | ^ # `--->local process----'
Thanks!
--curby
http://joerg.fruehbrodt.bei.t-online.de/pics/abb3_netfilter_ablaufdiagramm.jpg
What about the mangle decisions, do you also want to include them :D?
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