Digital Forensics Magazine Issue 1

/ LEAD FEATURE
a reply. If your computer is using an older or badly conﬁ gured
wireless LAN, dangers can lurk within weak encryption mecha-
IP = 146.277.150.45
Before attack
IP = 146.277.150.245
nisms. A rogue wireless access point could be used by attack-
MAC = 00:19:d1:a4:22:f4 MAC = 00:14:f6:9a:29:f2
ers to lure you to your doom, another deadly Siren call that
IP = 146.277.150.46
MAC = 00:19:d1:a4:37:5f
can be hard to resist. Even if the network is trustworthy, the
During attack
response from the DNS server may not be. The protocol and
software used by DNS servers to exchange information can
be exploited to poison the DNS cache. This is a particularly
Figure 4. ARP poisoning in action (network diagram) deadly attack as it is so difﬁ cult for the users to detect. If you
carefully type in the URL of your favourite online bookshop,
Figures 4 and 5 show Ettercap ARP poisoning in action. your ISP’s DNS server responds to the browser’s DNS request
Figure 4 illustrates the network conﬁ guration, and Figure 5 with an IP address. How do you know whether the IP address
lists the packets captured using tcpdump. Working through is correct?
the packets shown in Figure 5, the ﬁ rst four show the attacker Poisoning a DNS server’s cache is known as pharming
(IP address ending with 46) requesting the MAC addresses and is a far more effective method of collecting credit card
of the victim and gateway, and the associated responses. details, login credentials and personal information than its
The next eight packets are malicious ARP replies not associ- more prevalent but less potent relative, the phishing attack,
ated with any speciﬁ c request. They target the victim and as seen in the many fake bank security emails and the equal-
gateway, and tell each of them that the other’s MAC address ly popular 419 scams “from the relatives of deposed African
is 00:19:d1:a4:37:5f, which just happens to be the attacker’s leaders”. Even if the DNS server is untouched, it is still pos-
MAC address. These packets are retransmitted every ten sec- sible to spoof a DNS response, especially if the transaction
onds to ensure that the ARP caches stay poisoned. The ﬁ nal ID is not very random. This is more likely to happen if the
two packets show Ettercap cleaning up after itself by using the operating system is virtualised, as virtualisation can reduce
same unrequested reply trick to correct the ARP caches on the the randomness of random number generators. This is also
victim and gateway. a problem for Initial Sequence Numbers (ISNs) used to keep
network conversations safe from session hijackers, which we
POISONING A DNS SERVER’S
shall discuss shortly.
So, when we receive our DNS response we’re ready to
CACHE IS KNOWN AS PHARMING
send out the HTTP GET request. We have the destination IP
AND IS A FAR MORE EFFECTIVE
address and since we are using HTTP, your browser will use
destination port 80 unless we tell it to use a different port by
METHOD OF COLLECTING CREDIT
including it after a colon in the URL (e.g. http://www.example.
CARD DETAILS
com:8080/). Your operating system will supply the MAC
address of your gateway. Our return details should also be in-
cluded: a source IP address, a source port number (chosen at
17:08:42.918718 arp who-has 146.227.150.45 tell 146.227.150.46 random by the operating system), and a source MAC address,
17:08:42.919203 arp reply 146.227.150.45 is-at 00:19:d1:a4:22:f4 (which will be included in the frame header). However, unlike
17:08:42.928903 arp who-has 146.227.150.254 tell 146.227.150.46 the previous DNS request which used UDP, an HTTP request
17:08:42.930510 arp reply 146.227.150.254 is-at 00:14:f6:9a:29:f2 will use the Transmission Control Protocol (TCP) and before we
17:08:43.940085 arp reply 146.227.150.254 is-at 00:19:d1:a4:37:5f see our GET request arriving at the destination Web server and
17:08:43.940093 arp reply 146.227.150.45 is-at 00:19:d1:a4:37:5f a response coming back, we need to understand a little more
… about TCP.
17:08:47.983375 arp reply 146.227.150.254 is-at 00:19:d1:a4:37:5f
17:08:47.983390 arp reply 146.227.150.45 is-at 00:19:d1:a4:37:5f / Getting Connected
17:08:57.993502 arp reply 146.227.150.254 is-at 00:19:d1:a4:37:5f If I want to send you a short message about my Caribbean
17:08:57.993525 arp reply 146.227.150.45 is-at 00:19:d1:a4:37:5f sailing holiday I’ll use a postcard. This is analogous to UDP – a
… method of communicating by sending one or more individually
17:09:58.055349 arp reply 146.227.150.254 is-at 00:19:d1:a4:37:5f addressed messages. If I want to send you a draft of my large
17:09:58.055372 arp reply 146.227.150.45 is-at 00:19:d1:a4:37:5f travel book, sending each page on a separate postcard would
17:09:58.107659 arp reply 146.227.150.254 is-at 00:14:f6:9a:29:f2 be confusing. Pages will arrive out of order and some may be
17:09:58.107672 arp reply 146.227.150.45 is-at 00:19:d1:a4:22:f4 missing. You would have to spend time receiving postcards,
sorting them, requesting duplicates of the missing ones etc.
Figure 5. ARP poisoning in action (packet capture) TCP solves this problem: it allows you to set up a connection,
and then TCP does all the sorting and requesting redeliveries,
/ Trust No One and even transforms the contents of individual packets into
After determining the correct MAC address to use, your oper- a stream of data. TCP is like a logical hosepipe – the sender
ating system can now send out the DNS request and receive shoves data in one end and the receiver sees it in the same
12 Digital / ForensicS
DF1_08-13_Lead Feature.indd 12 29/10/09 5:06:29 pm
Page 1 | Page 2 | Page 3 | Page 4 | Page 5 | Page 6 | Page 7 | Page 8 | Page 9 | Page 10 | Page 11 | Page 12 | Page 13 | Page 14 | Page 15 | Page 16 | Page 17 | Page 18 | Page 19 | Page 20 | Page 21 | Page 22 | Page 23 | Page 24 | Page 25 | Page 26 | Page 27 | Page 28 | Page 29 | Page 30 | Page 31 | Page 32 | Page 33 | Page 34 | Page 35 | Page 36 | Page 37 | Page 38 | Page 39 | Page 40 | Page 41 | Page 42 | Page 43 | Page 44 | Page 45 | Page 46 | Page 47 | Page 48 | Page 49 | Page 50 | Page 51 | Page 52