is this is a child or parent portal? Child portals work by a directory added onto the existing portal i.e. www.primary.com and www.primary.com/child , and therfore require no further configuration. Parent portals work as seperate url's i.e. www.primary.com and www.child.com - in this case the new URL (www.child.com) needs to be added as a host header on the webserver itself - this is not something that dotnetnuke can do for you. Failed requests can be redirected to a failback portal (which is the primary portal).

Cathal

Edit:

I had created a second portal with alias www.domain2.com and www.domain2.com forwarded to http://www.domain1.com/dnninstalldir  It did not work until I instead forwarded it to http://www.domain1.com/dnninstalldir/Default.aspx?alias=www.domain2.com

This sort of works as it does direct toward the right site, but when you try to login it goes back to the primary portal.  I will post if I work it out.