Arping with pfSense: Installation and Use

Arping

Arping in action under pfSense 2.1.3.

Arping is a computer software tool that is used to discover hosts on a computer network, and is available as a package for pfSense. The program tests whether a given IP address is in use on the local network, and it can get additional information about the device using that address. The utility is similar to the ping utility, which has been discussed on this site in an earlier posting. Whereas ping probes hosts using the Internet Control Message Protocol (a routable protocol that operates on the network layer of the OSI model), arping operates entirely on the data link layer.

There are two popular arping implementations. One of them, part of the Linux iputils suite, cannot resolve MAC addresses to IP addresses. However, the version of this utility that is available as a package for pfSense was written by Thomas Habets and can ping hosts by MAC address as well as by IP address.


Installing Arping

Installing this utility is easy. In the pfSense web GUI, navigate to System -> Packages and click on the “Available Packages” tab. Arping should be on the list. Scroll down to arping and click on the “plus” button on the right side to install arping. The pfSense package installer will ask you to confirm that you want to install arping; press the “Confirm” button. The package installer status window will provide information about the installation and let you know when installation is complete. once it is, arping should appear on the “Installed Packages” tab.

Using Arping

Once arping is installed, you can access arping by navigating to Services -> Arping. From there, you can enter a host ip or MAc address and press the “ARPing” button to ARP ping.

What is it good for, given that the utility essentially replicates the functionality of ping? One case where arping is helpful is when the host you want to ping is firewalled and will not respond to a ping request. Even a firewalled host will respond to ARP.

Another case is when you do not have network layer (layer 3) connectivity to the host you wish to ping (possibly because you want to find out if an IP is taken), but you have data link layer (layer 2) connectivity. Without network layer connectivity, you won’t be able to ping a host, but you can use ARP (since ARP is a data link layer protocol), albeit only for hosts on the local subnet. One note of caution is that on networks employing repeaters that use proxy ARP, the ARP response may be coming from a proxy host and not from the probed target.


External Links:

Arping website for Thomas Habets’ arping

Arping on Wikipedia

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