You need to set the hostname for your system.
This thread should do the trick.
You may also want to have a look at
this more general thread about setting up a server, because if you missed that one you likely missed some other things. Things that can be far more dangerous.
Why is it important? Because this morning alone I had to report 17 other people's machines which have become "owned" by some script kiddie, based upon SSH hack attempts against the lowly two servers I watch over. While a larger number than normal for me --it's usually 3-6 per day-- this was
one day's total of hacked servers that were trying to break into mine.
I'm more amazed every day at how many servers out there are basically sitting open for anyone who wants to use as they wish. Many don't even have a firewall installed! I fear it's going to get much worse before it gets better. So many servers being managed by folks who have never done it before and don't have the resources we have around here on the forums.