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Old 17th August 2011
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Real Name: Tom Purvis
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Default pf firewall, is it a bridge or router?

I'm getting my first OpenBSD firewall based on pf going. I have come to a place after a fair amount of struggle where I'm seeing how some things work, but am still hopelessly confused about others.

Intent of this firewall: to allow us to switch off firewall in our DSL Modem/router, put that device alone on one of this OpenBSD box's NICs, and hub the rest of our network onto the other NIC.

I started by going through FAQ6, Networking. I found FAQ6 as I worked to get a second NIC working on my machine. The second NIC discussion in that FAQ happens within the context of the section Setting up a network bridge in OpenBSD. So, I set up a bridge. I've got a hostname.bridge0 file and net.inet.ip.forwarding=1 set in my sysctl.conf and bridging works.

My test environment is a single Win Vista PC on the "internal" side of my network, connected to the rest of the world only through the rl1 nic, which I have at 192.168.1.254. The other nic, rl0 is at 192.168.0.1. I got confused when I set up the windows machine as a static IP (192.168.1.15 with default gateway 192.168.1.254). It would communicate with nobody but 192.168.1.254.

Then I set up my OpenBSD box to do DHCP and switched the win box to DHCP and it worked. But what I finally figured out was that it simply went to the DHCP server on the 192.168.0.* net that it had always used. Oh yeah, that OpenBSD machine is a bridge!

So now I have the win machine static at 192.168.0.15 with default gateway being 192.168.0.1, which is our DSL Modem/router (and DHCP server). And of course it's talking. Bridge.

Every time I put actual rules into my pf.conf file I start with something like

Code:
block all
pass out proto tcp to port { ssh, www, pop3 }
pass proto udp to port { domain }
And then my Vista box gets nothing. Comment out the "block all" and it works. Everything.

Finally I've noticed that some example pf rules apply to both nics, pass in/pass out statements per nic, so there are minimally four statements. Sometimes they are just a pair, with no mention of nic (like the example code above).

Finally it occurs to me, what if the basic firewall is not also a bridge?

The main documents I've been working on have been:

http://www.openbsd.org/faq/faq6.html
http://www.openbsd.org/faq/pf/index.html (PF User Guide) -and-
http://home.nuug.no/~peter/pf/ (Peter Hansteen's tutorial for PF)

Today I searched the PF User Guide for the word bridge and saw that it does not appear in that doc at all. I searched for it in Hansteen's tutorial and saw that he refers to a filtering bridge type firewall two thirds of the way down into his document and discusses its benefits and drawbacks. "The advantage of such a setup is that attacking the firewall itself is more difficult. The disadvantage is that all admin tasks must be performed at the firewall’s console, unless you configure a network interface which is reachable via a secured network of some kind, or even a serial console." (excerpt)

So, now I am a bit confused. Sounds like the default way to configure a firewall using OpenBSD/PF is to not have it configured as a bridge. In which case the pf daemon is routing? So, I would configure my little Vista test machine (and ultimately all of the machines on our network after this is working) to live on the 192.168.1.* subnet with the OpenBSD box's rl1 nic IP Address as its default gateway? And then remove the hostname.bridge0 file? Then the OpenBSD box should have it's gateway (mygate) set to the DSL Modem/router (192.168.0.1)?? What should the client machine's DNS Servers be set to? The OpenBSD machine has two nameserver IPs in resolv.conf...

I am happy to read more about this, but I find that most of the information out there assumes a certain amount of base knowledge that I don't seem to have. Any help greatly appreciated.
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