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.
