Static ARP Management
About ARP Entries
Each computer on your network has an IP address, but a computer's network interfaces have MAC addresses, not IP addresses. When on the same subnet (on the Untangle Server), if Computer A wants to send data to Computer B, it needs to know two things:
- Destination IP address for Computer B
- MAC address for Computer B
ARP (Address Resolution Protocol) is a protocol that associates a computer's IP address and MAC address so that Computer A has all the information it needs to communicate with Computer B. Basically, using an ARP broadcast, Computer A shouts out What's the MAC address for IP Address 22.214.171.124?, then Computer B shouts back, That's me! My MAC address is 00:90:1A:40:AA:4D. Talk to me baby!
Like other routers, the Untangle Server can automatically make this translation if Computer A and Computer B are on the same segment. But, sometimes you don't want to use this automatic mapping; you'd rather have fixed (static) associations between an IP address and a MAC address. The Untangle Server provides you the ability to add static ARP entries to:
- Protect your network against ARP spoofing. An unauthorized computer can sniff around, waiting for the right ARP request. Then, responds with its own MAC address, claiming to have the IP address of an important server or router.
- Prevent network confusion as a result of misconfigured computers. A host that has been misconfigured with the ip address of another router or server will confuse all the other computers on its subnet. A host with a static ARP entry for the true router or server will not be confused. Don't forget to clear the misconfigured client's cached ARP table.
- Enable computers to talk to primitive network devices. Some devices don't speak the ARP protocol, so their MAC addresses must add as ARP entries so that they can communicate with other network computers. This situation is rare these days.
Adding ARP Entries
To add an ARP entries:
- Retrieve the IP address and MAC address of the device that requires the ARP entry.
- From Windows Server command prompt, run ipconfig/all command.
- From Solaris and Linux command prompt, run ifconfig -a command.