hostname alias

In solaris 9, how do I setup hostname alias, so essentially the machine has two hostnames...

It depends on the naming service being used, for example in NIS an entry would be added to the host map on the NIS master, in DNS a CNAME record (canonical name) would be added to the DNS zone.

would I need to do anything on the client side, or is this all within the DNS server itself.

That would all be within DNS, you would not need any change on the client.