Re: An ethernet frame with two IP packets inside?

["Ryan Russell" <ryanr () sybase com>]

I can't actually think of any way to get two IP packets inside
a single Ethernet frame... custom crafted or not.

Certainly an Ethernet frame doesn't care what it's data
portion is...  It simply says here's my Ethertype, and I
have this many bytes.

This gets passed to an IP stack... IP stack wants to see an
IP header... IP header says how many bytes there are.
Even if there were a second IP header after the first
IP "packet" I don't think any IP stacks will get that far.
I'm pretty sure they will all either consider it a corrupt
single packet, or just dump the rest of the bytes.

I could be wrong.

Could such a beast be created, then two packets might make it through
some type of packet filter and into an behind the PF.

Sounds like it would fun to try, though.

                              Ryan





Keller <keller () wiesbaden netsurf de> on 10/23/98 04:51:39 PM

Please respond to Keller <keller () wiesbaden netsurf de>

To:   "firewall-wizards () nfr net" <firewall-wizards () nfr net>
cc:    (bcc: Ryan Russell/SYBASE)
Subject:  An ethernet frame with two IP packets inside?




Hi gurus and beardy wizards,

what happens if one ethernet frame contains two IP packets?

I know, it *shouldn't* happen, but I could construct one, right?
How will different tcpip stacks deal with the second IP packet?
Could it slip through the filtering rules on some routers?
Could it slip past static pattern matching firewalls (FW-1?) ?

Any ideas or pointers are greatly appreciated..

Cheers!

Stefan Keller

p.s.:
I'm aware that it would imply that the attacker sits directly
in front of the router/firewall server/whatever..
Then again, he could sit on a (compromised) Linux web server
with .. let's say SPAK.. downloaded to that machine.