Date: Mon, 21 Jul 2008 21:36:20 -0500
From: monsieur.aglie@...hmail.com
To: full-disclosure@...ts.grok.org.uk
Subject: The cat is indeed out of the bag

from chargen 19/udp by ecopeland

0.

The cat is out of the bag. Yes, Halvar Flake figured out the flaw 
Dan Kaminsky will announce at Black Hat.
1.

Pretend for the moment that you know only the basic function of DNS 
— that it translates WWW.VICTIM.COM into 1.2.3.4. The code that 
does this is called a resolver. Each time the resolver contacts the 
DNS to translate names to addresses, it creates a packet called a 
query. The exchange of packets is called a transaction. Since the 
number of packets flying about on the internet requires scientific 
notation to express, you can imagine there has to be some way of 
not mixing them up.

Bob goes to to a deli, to get a sandwich. Bob walks up to the 
counter, takes a pointy ticket from a round red dispenser. The 
ticket has a number on it. This will be Bob’s unique identifier for 
his sandwich acquisition transaction. Note that the number will 
probably be used twice — once when he is called to the counter to 
place his order and again when he’s called back to get his 
sandwich. If you’re wondering, Bob likes ham on rye with no onions.

If you’ve got this, you have the concept of transaction IDs, which 
are numbers assigned to keep different transactions in order. 
Conveniently, the first sixteen bits of a DNS packet is just such a 
unique identifier. It’s called a query id (QID). And with the 
efficiency of the deli, the QID is used for multiple transactions.
2.

Until very recently, there were two basic classes of DNS 
vulnerabilities. One of them involves mucking about with the QID in 
DNS packets and the other requires you to know the Deep Magic.

First, QIDs.

Bob’s a resolver and Alice is a content DNS server. Bob asks Alice 
for the address of WWW.VICTIM.COM. The answer is 1.2.3.4. Mallory 
would like the answer to be 6.6.6.0.

It is a (now not) secret shame of mine that for a great deal of my 
career, creating and sending packets was, to me, Deep Magic. Then 
it became part of my job, and I learned that it is surprisingly 
trivial. So put aside the idea that forging IP packets is the hard 
part of poisoning DNS. If I’m Mallory and I’m attacking Bob, how 
can he distinguish my packets from Alice’s? Because I can’t see the 
QID in his request, and the QID in my response won’t match. The QID 
is the only thing protecting the DNS from Mallory (me).

QID attacks began in the olden days, when BIND simply incremented 
the QID with every query response. If you can remember 1995, here’s 
a workable DNS attack. Think fast: 9372 + 1. Did you get 9372, or 
even miss and get 9373? You win, Alice loses. Mallory sends a 
constant stream of DNS responses for WWW.VICTIM.COM. All are 
quietly discarded —- until Mallory gets Bob to query for 
WWW.VICTIM.COM. If Mallory’s response gets to your computer before 
the legitimate response arrives from your ISP’s name server, you 
will be redirected where Mallory tells you you’re going.

Obvious fix: you want the QID be randomly generated. Now Alice and 
Mallory are in a race. Alice sees Bob’s request and knows the QID. 
Mallory has to guess it. The first one to land a packet with the 
correct QID wins. Randomized QIDs give Alice a big advantage in 
this race.

But there’s a bunch more problems here:

    *

      If you convince Bob to ask Alice the same question 1000 times 
all at once, and Bob uses a different QID for each packet, you made 
the race 1000 times easier for Mallory to win.
    *

      If Bob uses a crappy random number generator, Mallory can get 
Bob to ask for names she controls, like WWW.EVIL.COM, and watch how 
the QIDs bounce around; eventually, she’ll break the RNG and be 
able to predict its outputs.
    *

      16 bits just isn’t big enough to provide real security at the 
traffic rates we deal with in 2008.

Your computer’s resolver is probably a stub. Which means it won’t 
really save the response. You don’t want it to. The stub asks a 
real DNS server, probably run by your ISP. That server doesn’t know 
everything. It can’t, and shouldn’t, because the whole idea of DNS 
is to compensate for the organic and shifting nature of internet 
naming and addressing. Frequently, that server has to go ask 
another, and so on. The cool kids call this “recursion”.

Responses carry another value, too, called a time to live (TTL). 
This number tells your name server how long to cache the answer. 
Why? Because they deal with zillions of queries. Whoever wins the 
race between Alice and Mallory, their answer gets cached. All 
subsequent responses will be dropped. All future requests for that 
same data, within the TTL, come from that answer. This is good for 
whoever wins the race. If Alice wins, it means Mallory can’t poison 
the cache for that name. If Mallory wins, the next 10,000 or so 
people that ask that cache where WWW.VICTIM.COM is go to 6.6.6.0.
3.

Then there’s that other set of DNS vulnerabilities. These require 
you to pay attention in class. They haven’t really been talked 
about since 1997. And they’re hard to find, because you have to 
understand how DNS works. In other words, you have to be completely 
crazy. Lazlo Hollyfeld crazy. I’m speaking of course of RRset 
poisoning.

DNS has a complicated architecture. Not only that, but not all name 
servers run the same code. So not all of them implement DNS in 
exactly the same way. And not only that, but not all name servers 
are configured properly.

I just described a QID attack that poisons the name server’s cache. 
This attack requires speed, agility and luck, because if the “real” 
answer happens to arrive before your spoofed one, you’re locked 
out. Fortunately for those of you that have a time machine, some 
versions of DNS provide you with another way to poison the name 
server’s cache anyway. To explain it, I will have to explain more 
about the format of a DNS packet.

DNS packets are variable in length and consist of a header, some 
flags and resource records (RRs). RRs are where the goods ride 
around. There are up to three sets of RRs in a DNS packet, along 
with the original query. These are:

    *

      Answer RR’s, which contain the answer to whatever question 
you asked (such as the A record that says WWW.VICTIM.COM is 1.2.3.4)
    *

      Authority RR’s, which tell resolvers which name servers to 
refer to to get the complete answer for a question
    *

      Additional RR’s, sometimes called “glue”, which contain any 
additional information needed to make the response effective.

A word about the Additional RR’s. Think about an NS record, like 
the one that COM’s name server uses to tell us that, to find out 
where WWW.VICTIM.COM is, you have to ask NS1.VICTIM.COM. That’s 
good to know, but it’s not going to help you unless you know where 
to find NS1.VICTIM.COM. Names are not addresses. This is a chicken 
and egg problem. The answer is, you provide both the NS record 
pointing VICTIM.COM to NS1.VICTIM.COM, and the A record pointing 
NS1.VICTIM.COM to 1.2.3.1.

Now, let’s party like it’s 1995.

Download the source code for a DNS implementation and hack it up 
such that every time it sends out a response, it also sends out a 
little bit of evil — an extra Additional RR with bad information. 
Then let’s set up an evil server with it, and register it as 
EVIL.COM. Now get a bunch of web pages up with IMG tags pointing to 
names hosted at that server.

Bob innocently loads up a page with the malicious tags which 
coerces his browser resolve that name. Bob asks Alice to resolve 
that name. Here comes recursion: eventually the query arrives at 
our evil server. Which sends back a response with an unexpected 
(evil) Additional RR.

If Alice’s cache honors the unexpected record, it’s 1995 —- buy 
CSCO! —- and you just poisoned their cache. Worse, it will replace 
the “real” data already in the cache with the fake data. You asked 
where WWW.EVIL.COM was (or rather, the image tags did). But Alice 
also “found out” where WWW.VICTIM.COM was: 6.6.6.0. Every resolver 
that points to that name server will now gladly forward you to the 
website of the beast.
4.

It’s not 1995. It’s 2008. There are fixes for the attacks I have 
described.
Fix 1:

The QID race is fixed with random IDs, and by using a strong random 
number generator and being careful with the state you keep for 
queries. 16 bit query IDs are still too short, which fills us with 
dread. There are hacks to get around this. For instance, DJBDNS 
randomizes the source port on requests as well, and thus won’t 
honor responses unless they come from someone who guesses the ~16 
bit source port. This brings us close to 32 bits, which is much 
harder to guess.
Fix 2:

The RR set poisoning attack is fixed by bailiwick checking, which 
is a quirky way of saying that resolvers simply remember that if 
they’re asking where WWW.VICTIM.COM is, they’re not interested in 
caching a new address for WWW.GOOGLE.COM in the same transaction.

Remember how these fixes work. They’re very important.

And so we arrive at the present day.
5.

Let’s try again to convince Bob that WWW.VICTIM.COM is 6.6.6.0.

This time though, instead of getting Bob to look up WWW.VICTIM.COM 
and then beating Alice in the race, or getting Bob to look up 
WWW.EVIL.COM and slipping strychnine into his ham sandwich, we’re 
going to be clever (sneaky).

Get Bob to look up AAAAA.VICTIM.COM. Race Alice. Alice’s answer is 
NXDOMAIN, because there’s no such name as AAAAA.VICTIM.COM. Mallory 
has an answer. We’ll come back to it. Alice has an advantage in the 
race, and so she likely beats Mallory. NXDOMAIN for 
AAAAA.VICTIM.COM.

Alice’s advantage is not insurmountable. Mallory repeats with 
AAAAB.VICTIM.COM. Then AAAAC.VICTIM.COM. And so on. Sometime, 
perhaps around CXOPQ.VICTIM.COM, Mallory wins! Bob believes 
CXOPQ.VICTIM.COM is 6.6.6.0!

Poisoning CXOPQ.VICTIM.COM is not super valuable to Mallory. But 
Mallory has another trick up her sleeve. Because her response 
didn’t just say CXOPQ.VICTIM.COM was 6.6.6.0. It also contained 
Additional RRs pointing WWW.VICTIM.COM to 6.6.6.0. Those records 
are in-bailiwick: Bob is in fact interested in VICTIM.COM for this 
query. Mallory has combined attack #1 with attack #2, defeating fix 
#1 and fix #2. Mallory can conduct this attack in less than 10 
seconds on a fast Internet link.

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