by Sumit Dhar

If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.-- Sun Tzu in The Art of War

While I was at university, I once discussed the topic of sniffers with an experienced network administrator. He casually mentioned that he was not bothered by sniffers, as all his machines were connected to switches. This was someone who was paranoid about security and read almost all security newsgroups religiously, but he was living in complete ignorance about the threat to his network. Unfortunately, he is not alone as many experienced systems and network administrators feel switches are immune to being sniffed. Switches may be difficult to sniff, but they are certainly not immune.

Networking Basics

A computer connected to the LAN has two addresses. One is the MAC (media access control) address that uniquely identifies each node in a network and is stored on the network card itself. Each network card has a unique MAC address. It is the MAC address that gets used by the Ethernet protocol while building `frames' to transfer data to and from a machine. The other address is the IP address, which is used by applications.

The Ethernet header uses the MAC address of the destination machine and not the IP address. It is the job of the network layer to map a particular IP address to the corresponding MAC address, as required by the Data Link Protocol. It does this by initially looking up the MAC address of the destination machine in a table, usually called the ARP cache. If no entry is found for the IP address, the Address Resolution Protocol broadcasts a request packet (ARP request) to all machines on the network. The machine with that address responds to the source machine with its MAC address (ARP reply). This MAC address is added to the source machines ARP cache, and it is then used by the source machine in all its communications with the destination machine.

Ethernet Environments

Two basic types of Ethernets environments exist, and how sniffers work in each of them is different.

  • Shared Ethernet: In a shared Ethernet environment, all hosts are connected to the same bus and compete with one another for bandwidth. In such an environment packets meant for one machine are received by all the other machines. Thus, any machine in such an environment placed in promiscuous mode will be able to capture packets meant for other machines and can therefore listen to all the traffic on the network.

  • Switched Ethernet: An Ethernet environment in which the hosts are connected to a switch instead of a hub is called a Switched Ethernet. The switch maintains a table keeping track of each computer's MAC address and delivers packets destined for a particular machine to the port on which that machine is connected. The switch is an intelligent device that sends packets to the destined computer only and does not broadcast to all the machines on the network, as in the previous case. This switched Ethernet environment was intended for better network performance, but as an added benefit, a machine in promiscuous mode will not work here. As a result of this, most network administrators assume that sniffers don't work in a Switched Environment.

Methods to Sniff on a Switch

As mentioned earlier, a switch is certainly more secure than a hub when it comes to sniffing, but it is certainly not immune. The following methods are used to sniff the traffic on a switch:

  1. ARP Spoofing: We have explained earlier how ARP is used to obtain the MAC address of the destination machine with which we wish to communicate. The ARP is stateless, you can send a ARP reply, even if one has not been asked for and such a reply will be accepted. Ideally, when you want to sniff the traffic originating from a machine, you need to ARP spoof the gateway of the network. The ARP cache of that machine will now have a wrong entry for the gateway and is said to be "poisoned". This way all the traffic from that machine destined for the gateway will pass through your machine. Another trick that can be used is to poison a hosts ARP cache by setting the gateway's MAC address to FF:FF:FF:FF:FF:FF(also known as the broadcast MAC). There are various utilities available for ARP spoofing. An excellent tool for this is the arpspoof utility that comes with the dsniff suite. Using arpspoof to poison the ARP cache of a machine is accomplished by giving the command:

    [root@tachyon dhar]# arpspoof -t
    0:80:ad:7c:7:3a 52:54:5:f3:95:1 0806 42: arp reply is-at 0:80:ad:7c:7:3a
    0:80:ad:7c:7:3a 52:54:5:f3:95:1 0806 42: arp reply is-at 0:80:ad:7c:7:3a

    The -t flag specifies the target whose ARP cache we wish to poison, and the other argument is the IP address of the gateway that we wish to spoof. Now all the data destined for the gateway from the target machine will have to pass through our machine. Before you ARP spoof the gateway, however, it is essential to turn on IP Forwarding for your machine. This can be done by giving the command:

    [root@tachyon dhar]# echo 1 > /proc/sys/net/ipv4/ip_forward 
    [root@tachyon dhar]# cat /proc/sys/net/ipv4/ip_forward
    [root@tachyon dhar]#

    If the cat command returns a value of 1, then IP Forwarding has been enabled; if it returns 0, it means IP Forwarding has not been enabled.

  2. MAC Flooding: Switches keep a translation table that maps various MAC addresses to the physical ports on the switch. As a result of this, a switch can intelligently route packets from one host to another, but it has a limited memory for this work. MAC flooding makes use of this limitation to bombard the switch with fake MAC addresses until the switch can't keep up. The switch then enters into what is known as a `failopen mode', wherein it starts acting as a hub by broadcasting packets to all the machines on the network. Once that happens sniffing can be performed easily. MAC flooding can be performed by using macof, a utility which comes with dsniff suite.

    [root@tachyon dhar]# macof
    77:6b:e1:6e:5e:8c 93:2d:ed:45:f9:e3 > S 1847390231:1847390231(0) win 512
    84:a4:d3:57:ef:8 12:56:52:42:dc:95 > S 1484147693:1484147693(0) win 512
    88:f0:9:3f:18:89 d:86:53:53:d7:f8 > S 293820390:293820390(0) win 512
Detecting Sniffers

The reader is referred here to an earlier article for the basics of sniffer detection. When sniffers are working on switches, the chances of detecting them are higher. In such a scenario the sniffer is not a passive device; it performs certain activities by which it can be detected.

ARP spoofing can be detected using a program called ARP Watch. It is used to monitor the ARP cache of a machine to see if there is duplication. If there is, it could trigger alarms and lead to detection of sniffers. It can be obtained at online.securityfocus.com/data/tools/arpwatch.tar.Z


As is clear from the above sections, one method of sniffing in a switched environment is using ARP spoofing, and the machine that will most probably be ARP spoofed is the gateway. One thing that can be done is to add the MAC address of the gateway permanently to your ARP cache. This can be done by giving the -s flag to the arp command. Read more about this on the arp man page. Alternatively, you could use the /etc/ethers file for placing the MAC addresses of the important machines to prevent spoofing of those machines.

Final words of advice: Use encryption. Switch to SSH and SCP instead of Telnet and FTP.

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