Thanks Chris. Let me describe my problem more detailed, so maybe can get more detailed help.
Based on current situation, I have to install the web application and Database on ISA server. The domain name is www.abc.com, and the IP is 69.121.122.123.
Here are my I did:
On DNS:
1. Set www.abc.com to 69.121.122.123
On IIS:
2. Create virtual parth "webportal" under Default Web Site;
3. Set Local Path to "D:\webportal", and Enable Defaulto Document to "Default.aspx";
On ISA Server:
4. Set HTTP to 69.121.122.123, and set Port to 80
5. Set Internal Parth to "/", and External Parth to "/webportal/*"
On DNND Database:
5. Set HTTP alias to www.abc.com/webportal
Based on my test, I think the point is the parth setting on ISA confused the HTTP alias in DNN Database. But I tried all the combinations, and none of the work well.
Again, help please and many many thanks.