[Eng] What is DNS Spoofing ?

What is DNS Spoofing ?

DNS Spoofing is the art of making a DNS entry to point to an another IP
than it would be supposed to point to. To understand better, let’s see
an example.You’re on your web browser and wish to see the news on
www.cnn.com, without to think of it, you just enter this URL in your
address bar and press enter.
Now, what’s happening behind the scenes
? Well… basically, your browser is going to send a request to a DNS
Server to get the matching IP address for www.cnn.com, then the DNS
server tells your browser the IP address of CNN, so your browser to
connect to CNN’s IP address and display the content of the main page.
Hold
on a minute… You get a message saying that CNN’s web site has closed
because they don’t have anymore money to pay for their web site. You’re
so amazed, you call and tell that to your best friend on the phone, of
course he’s laughing at you, but to be sure, he goes to CNN web site to
check by himself.
You are surprised when he tells you he can see the
news of the day as usual and you start to wonder what’s going on. Are
you sure you are talking to the good IP address ?Let’s check. You ask
your friend to fire up his favorite DNS resolving tool and to give you
the IP address he’s getting for www.cnn.com.Once you got it, you put it
in your browser URL bar :

http://212.153.32.65

You feel ridiculous and frustrated when you see CNN’s web page with its
daily news.
Well
you’ve just been the witness of a DNS hijacking scenario. You’re
wondering what happened, did the DNS Server told you the wrong IP
address ? Maybe… At least this is the most obvious answer coming to
our mind.
In fact there are two techniques for accomplishing this DNS hijacking.
Let’s see the first one, the „DNS ID Spoofing“ technique.

1) DNS Cache Poisoning

As
you can imagine, a DNS server can’t store information about all
existing names/IP on the net in its own memory space.That’s why DNS
server have a cache, it enables them to keep a DNS record for a while.
In
fact, A DNS Server has the records only for the machines of the domain
it has the authority, if it needs to know about machines out of his
domain, it has to send a request to the DNS Server which handles these
machines and since it doesn’t want to ask all the time about records,
it can store in its cache the replies returned by other DNS servers.
Now let’s see how someone could poison the cache of our DNS Server.
An
attacker his running is own domain (attacker.net) with his own hacked
DNS Server(ns.attacker.net) . Note that I said hacked DNS Server
because the attacker customized the records in his own DNS server, for
instance one record could be www.cnn.com=81.81.81.81
1) The attacker sends a request to your DNS Server asking it to resolve
www.attacker.net
2) Your DNS Server is not aware of this machine IP address, it doesn’t
belongs to his domain, so it needs to asks to the responsible name
server.
3) The hacked DNS Server is replying to your DNS server,
and at the same time, giving all his records (including his record
concerning www.cnn.com) Note : this process is called a zone transfer.
4) The DNS server is not „poisoned“.The attacker got his IP, but who
cares, his goal was not to get the IP address of his web server but to
force a zone transfer and make your DNS server poisoned as long as the
cache will not be cleared or updated.
5) Now if you ask your DNS
server, about www.cnn.com IP address it will give you 172.50.50.50,
where the attacker run his own web server. Or even simple, the attacker
could just run a bouncer forwarding all packets to the real web site
and vice versa,so you would see the real web site, but all your traffic
would be passing through the attacker’s web site.

2) DNS ID Spoofing

We
saw that when a machine X wants to communicate with a machine Y, the
former always needs the latter IP address. However in most of cases, X
only has the name of Y, in that case, the DNS protocol is used to
resolve the name of Y into its IP address.
Therefore, a DNS request
is sent to a DNS Server declared at X, asking for the IP address of the
machine Y. Meanwhile, the machine X assigned a pseudo random
identification number to its request which should be present in the
answer from the DNS server.Then when the answer from the DNS server
will be received by X, it will just have to compare both numbers if
they’re the same, in this case, the answer is taken as valid,otherwise
it will be simply ignored by X.
Does this concept is safe ? Not
completely. Anyone could lead an attack getting this ID number. If
you’re for example on LAN, someone who runs a sniffer could intercept
DNS requests on the fly, see the request ID number and send you a fake
reply with the correct ID number… but with the IP address of his
choice.Then, without to realize it, the machine X will be talking to
the IP of attacker’s choice thinking it’s Y.

By the way, the DNS
protocol relies on UDP for requests (TCP is used only for zone
transfers), which means that it is easy to send a packet coming from a
fake IP since there are no SYN/ACK numbers (Unlike TCP, UDP doesn’t
provide a minimum of protection against IP spoofing).

Nevertheless, there are some limitations to accomplish this attack.
In
my example above, the attacker runs a sniffer, intercept the ID number
and replies to his victim with the same ID number and with a reply of
his choice.
In the other hand, even if the attacker intercepted your
request, it will be transmitted to the DNS Server anyway which will
also reply to the request(unless the attacker is blocking the request
at the gateway or carry out ARP cache poisoning which would make the
attack possible on a switched network by the way).
That means that
the attacker has to reply BEFORE the real DNS server, which means that
to succeed this attack, the attacker MUST be on the same LAN so to have
a very quick ping to your machine, and also to be able to capture your
packets.

Practical example ( for
testing purposes ONLY)
To see yourself how to hijack a connection from a machine on your local
area network,we can do the followings :
First step :Poison the ARP cache of the victim’s machine (tools and explanations
for realizing this task can be found at http://www.arp-sk.org)
Second step :Now, outgoing packets of the target will be redirected to your host,but
you have to forward the traffic to the real gateway, this can be
achieved witha tool like Winroute Pro.
Third step :We then use WinDNSSpoof,
developed by valgasu (www.securiteinfo.org)
which isa tool that greatly help to carry out DNS ID Spoofing. (Before
to use this tool be sure you have the Winpcap library installed on your
machine, see http://winpcap.polito.it).We
run it in the cmd like :
wds -n www.cnn.com -i 123.123.123.123 -g 00-C0-26-DD-59-CF -v
This
will make www.cnn.com to point to 123.123.123.123 on the victim’s
machine. 00-C0-26-DD-59-C being the MAC Address of the gateway or DNS
server.