Many small networks still use the Routing Information Protocol (RIP), despite the fact that almost every alternative is better at loop prevention, fast convergence and security. Even so, if you're stuck with RIP, it doesn't mean you can't improve your system's security. Assuming you have a Cisco router and version of IOS released this century, this tip will explain the few easy commands required to prevent people from sending you bogus routes or corrupting your tables.
First, you need to create a key chain if you don't have one already. To create a key chain named "my-key-chain" from the config prompt, type:
key chain my-key-chain
Next, type the number of the key you want to create on the chain. The first key should be key 1. You can set up multiple keys if you have multiple adjacent routers and you want different keys for each, but that's just a management hassle. You can use the same key for all the routers. Type:
key 1
Now, you need the key string, which is the password. Type:
key-string mYpAsSw0rD
You should now have a key chain created. Now to enable the chain for RIP, you need to type the following command from the config interface prompt:
ip rip authentication key-chain my-key-chain
This will prevent accidental problems (like a server administrator starting the RIP service and advertising a default route to see what happens) because your router will only accept updates from routers that have the right key configured. However, this is in plain text and won't prevent anyone with malicious intentions from snooping the password and then sending you bad routes with the right password. To stop that sort of deviant behavior, you'll need to type the following:
ip rip authentication mode md5
Instead of sending your password in clear text over the wire, now you'll send an md5 hash of the password. Note that earlier this year, a flaw was found in the md5 algorithm so it's no longer considered "secure," but it's enough to deter most people.
Obviously, you need to type these same commands on each pair of router interfaces in your network. However, the key numbers do not have to be the same. For more information, you can search for key chains on Cisco's Website.
