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Re: Thoughts about DNS...
From: tqbf () ENTERACT COM (Thomas H. Ptacek)
Date: Sun, 27 Apr 1997 03:02:57 -0500


True, and I just thought up a solution: Once a server notices fake
responses arriving, it can reattempt the lookup via TCP.  Then we can be

You can't guarantee that every nameserver in the world will support TCP.

[ re: the possibility of manipulating servers into caching NXDOMAIN
      as a result of forgery protection ]

Well since we are dealing with these problems right now, we can be
intelligent and not cache a host/IP as broken when we receive spoofed

Ok, just bringing it up. However, we've still prevented a legitimate query
from resolving, so the only choices we have are to play dead or to
incorrectly return an NXDOMAIN (right?).

mechanism on multiple servers in parallel? What happens if I open up N
recursive queries to a nameserver, following them up with a stream of
invalid requests? Can I starve the nameserver of resources (some state is

To protect against this we can refuse recursive queries from outside the
subnets that the server is set up to serve, protecting us against outside

That doesn't work. This is a blind attack; I can make the queries come
from any address in the world I want them to come from. I don't think
anything that relies on router configuration is a solution - do you?

[ re: TCP vs Cookie Response ]
I think connecting to the servers via TCP would be the better solution,
since it is a capability built into almost every DNS server in existence.

Responding NXDOMAIN to a query is a capability built into every DNS server
in existance. TCP DNS is not. It's an excellent idea, though!

Cookies would have to be supported on the server that is being spoofed,

No, the point behind the "cookie" support is that they don't require
support. We don't need the server to UNDERSTAND the cookie, we just need
it to acknowledge it in a response. We're embedding the cookie in a
legitimate query, and expecting to receive an NXDOMAIN back. The NXDOMAIN
confirms that the server is alive.

[ the problem is... ]
server's throat before the lookup times out.. However, I didnt realized

What happens when the server times out?

We try again.

Does the server cache an NXDOMAIN simply due to a timeout?

It shouldn't.

If it doesn't, I can continue trying this attack indefinitely until I win.

it would be that quick and easy.  This is really a serious problem that
should be addressed.

Agreed. I think the combination of D.O.S. with the ID prediction attack is
the most significant issue here.

----------------
Thomas Ptacek at EnterAct, L.L.C., Chicago, IL [tqbf () enteract com]
----------------
"If you're so special, why aren't you dead?"



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