Domain Forwarding?

Most all domain registrars allow you to forward your domain, to another domain with the option of masking as well. If I'm not making myself clear here's an example.

Say I have setup domain (URL) forwarding with my registrar. Now if I change my nameservers to THEIR nameservers, when a person goes to http://mydomain.com they actually get redirected to http://myotherdomain.com/somethingReallyLong/

If THEY can do this, why can't I do it as well? I'm curious because it seems as though there are a lot of people that have ISP's that block web servers and the such by blocking individual ports. However, if you set things up on a different port and add that damn annoying http://mydomain.com:xx the problem is solved. It'd be handy if I, myself, could redirect requests to any port I choose. I realize you can't do this in zone files (that I know of). But HOW DO THEY do it? Perhaps this isn't a DNS related question?

It's not a DNS related question but we forgive you (or at least I do. ). I provided URL forwarding for free though it isn't advertised. Some dude I know came to me with a long URL on a wierd port and said "can you make my domain go to this?" and I was like "sure". First I had his domain he registered and hosted through me point to my webserver. Then I added this to my httpd.conf:

<VirtualHost *>
ServerName www.abalmywinter.net
ServerAlias abalmywinter.net *.abalmywinter.net
Redirect 301 / http://tbr.balmy.cx:7081/va/balmy/
</VirtualHost>

Now

http://www.abalmywinter.net redirects to
http://tbr.balmy.cx:7081/va/balmy/

http://www.abalmywinter.net/test/directory redirects to
http://tbr.balmy.cx:7081/va/balmy/test/directory

Then he asked if there was any way to hide the URL it was redirected to. I said "sure, all I have to do is put it in frames" and he said "nah, I don't want to bother you" and I said "ok". The site is down now, so don't bother testing the above example.

What I currently do is URL forwarding and what he wanted me to do at first was "masking" the URL. Now I don't know if this is what those businesses actually do. I just know enough about DNS and HTTP to accomplish it. I have other ideas too, you could forward the request to a script which does the redirecting. It can check to see if the site is even online, and if not, return a default page letting the user know the site is down.

One last note, you cannot do URL forwarding for yourself to avoid blocked ports. The webserver that does the forwarding must be available via port 80.

Sweet. I like the script idea as well. I wasn't really aware of a web server having those capabilites. Thanks!