Whenever I try to telnet (just locally, to test) to my Linux box as root, it fails. If I try with another username, it works.
Is there some setting to prevent root logins via telnet?

Yes. It's a security hole to open root for telnet. But if you want to...

Look in /etc/securetty

Hope that helps,

jfk

Remark:
With telnet/rlogin your plain password will be transmitted over the network. So take ssh (secure shell)

But to your question:
Depending on your distribution

SuSE: rc.config -> ROOT_LOGIN_REMOTE
RedHat: ?

and eventually you have to edit in inetd.conf the line with in.rlogind

I found not being able to telnet as root at first an inconvionce. After thinking about it for a while I decided that I liked having the extra login/password combination for someone to gain full access to the system. You can telnet as another user and then use the su command to gain administrative priveledges.

As ST pointed out, a secure shell would solve the problem of having your root password vulerable to the network. I have gotten used to using the su command so I don't have to worry about someone unwanted trying to brute force my linux box as root.

Spammy

Thanks, this just goes to show "you learn something new every day"

:)

Cheers,
Milamber

Easiest thing to do would be to just su -
after logging in as a normal user. But I do recommend using Secure Shell as well. It's the safest alternative.