Re: [fw-wiz] dirty packet tricks?

From: Stephen D. B. Wolthusen (
Date: 07/11/02

From: (Stephen D. B. Wolthusen)
Date: Thu Jul 11 16:50:13 2002


Ryan Russell <> writes:

> Wow, that's..not normal. OK. So, you want to build a hijacking router.
> So what do the route tables and subnet masks on the client machines look
> like, in theory? The clients have to believe that there is some route to
> the Internet, or they won't ever bother trying to get there. They either
> have to believe the Internet is all on the local segment (subnet mask
>, probably not workable..) or they have to think that it's through
> another router. I have to assume that if you don't want your transparent
> proxy thing to be the "official" router, then there must be another
> router, which means you must have multiple local subnets. Your firewall
> will have to have the ability to suck packets off all subnets you want to
> be able to reach the Internet, or have multiple ones, etc...

... phrased like that it is starting to sound a lot like a souped-up switch
(OK, multiport bridge). Sane switches treat multiple ARP responses (MAC
addresses) as fault conditions and isolate the port the offending frames
came from, so this probably won't go very far in most modern networks.

To catch all traffic (statically configured media address resolution tables
aren't that rare, it will save you a lot of headaches in some
fault-tolerant/clustered environments) and still meet the original
requirement, the firewall/monitor effectively has to act as a multiport
bridge (Lucent did this some years back and sell such a thing - if they're
still around by the time I'm writing this). This means the usual bandwidth
issues in switched/fabric environments, but the best bet probably hacking
up the switch OS, make port/VLAN mirroring a two-way street and then do
whatever you need on a host attached to the mirroring patch.

That's uglier than Saddam's hairy butt, and one of the reasons why
distributed firewalling/ID is probably the only way out of the bandwidth
mess (among others). Now there's some flame bait.

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