Windows Sharepoint Service 2007 - Search Return Unknown Error

Found some strange problem today. Search function in Sharepoint Server which is installed in workgroup mode apparently not working when it is accessed from the Internet.

Below are the details of the setup:
- OS: Windows Server 2008 Data Center Edition
- Software: Windows Sharepoint Service 2007
- Machine: Amazon EC2 - Small Instance
- Authentication: Integrated Windows Authentication (No Domain)

The search function will work when it is accessed from a browser locally on the server, but return "unknown error" when accessed from Internet.

The solution of this problem:
1. Open Sharepoint Central Administration
2. Click Operations
3. Click Alternate access mappings under Global Configurations
4. Click Add Internal URLs
5. Under URL Protocol, host and port type: http://<domain name or ip address>
The domain name or ip address must be the one used by user to access from the Internet
6. Under Zone, select: Internet
7. Click Save