Network Security Hacks

Create a Static ARP Table

Use static ARP table entries to combat spoofing and other nefarious activities.

As discussed in [Hack #31] , a lot of bad things can happen if someone successfully poisons the ARP table of a machine on your network. The previous hack discussed how to monitor for this behavior, but how do we prevent the effects of someone attempting to poison an ARP table?

One way to prevent the ill effects of this behavior is to create static ARP table entries for all of the devices on your local network segment. When this is done, the kernel will ignore all ARP responses for the specific IP address used in the entry and use the specified MAC address instead.

To do this, you can use the arp command, which allows you to directly manipulate the kernel’s ARP table entries. To add a single static ARP table entry, run this:

            arp -s 
            ipaddr macaddr

If you know that the MAC address that corresponds to 192.168.0.65 is 00:50:BA:85:85:CA, you could add a static ARP entry for it like this:

# arp -s 192.168.0.65 00:50:ba:85:85:ca

For more than a few entries, this can be a time-consuming process. To be fully effective, you must add an entry for each device on your network on every host that allows you to create static ARP table entries.

Luckily, most versions of the arp command can take a file as input and use it to create static ARP table entries. Under Linux, this is done with the -f command-line switch. Now all you need to do is generate a file containing the MAC and IP address pairings, ...