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Re: Thoughts about DNS...
From: vermont () GATE NET (Illuminati Primus)
Date: Sun, 27 Apr 1997 02:04:52 -0400


On Sun, 27 Apr 1997, Thomas H. Ptacek wrote:

There's a trivial and inexpensive denial of service attack associated with
this method. I can prevent any nameserver employing this mechanism from
ever being able to resolve "XXX.COM" by sending it a consistant stream of
"XXX.COM" requests with random invalid query IDs. You mention this, but
don't specifically state how it's done, so I'm simply clarifying.  The
attack is blind, doesn't require significant resources, and can be
performed almost untraceably from any dialup connection on the net.

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
reasonably assured that we're talking to the real server..  Someone
flooding us from a dialup modem will only cause our lookups to take
slightly more time.

Also, what does BIND do when it's caught up in a query->invalidate->retry
loop? Does it eventually give up and return NXDOMAIN? Does it cache the
NXDOMAIN (thus allowing for a far more severe denial of service attack)?
Does it hang indefinitely, allowing us to trick resolvers into thinking
the nameserver is dead?

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
replies.  I know BIND 8 wont cache unresolvable servers, so someone can
only kill a domain as long as he can deny access to that domain's
authoritative nameservers.

Can I tie arbitrary nameservers on the Internet up by employing this
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
being allocated in BIND to cover each request)?

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
attackers (providing routers were set up correctly)

The mechanism by which this is accomplished can be extremely simple - a
flag set off by default that gets set on the first time an invalid query
is received, and is turned off a configurable amount of time after the
last invalid request is received. As long as the flag is set on, all
requests must be completed with a "cookie" request to ensure the vitality
of the queried servers.

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.
Cookies would have to be supported on the server that is being spoofed,
or hidden somewhere in our queries (which is a bit of a kludge).

Although its not good to have even a small window of opportunity, what
percentage of the ID space could someone cross by fully saturating a T1
with forged DNS replies before the requesting server times out the

Assume every response to be 100 bytes long. I need 65536 queries to
completely span the ID-space. This is 6400k of network bandwidth. On a
heavily loaded T1, I manage 110 kb/s FTP, or roughly a minute to complete
the entire attack.

Remember, though, that there is /no window/ involved in schemes that rely
entirely on the ID to prove authenticity of responses. Figuring out how to
restrain a nameserver for 58.18 seconds is not a difficult problem. Bugs
in BIND may make it a trivial problem.

No, the problem is managing to force enough forged responses down a
server's throat before the lookup times out.. However, I didnt realized
it would be that quick and easy.  This is really a serious problem that
should be addressed.

(looking at bind code now)
-vermont () gate net , mongoloid programmer



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