Back To Basics: Troubleshooting Proxy Server 2.0 -- Part 2 Page 3
WinSock Proxy Related Problems
The WinSock Proxy service is used to provide access for WinSock programs that are not able to use the Web Proxy service. Since the Web Proxy service supports only CERN compliant applications and only the FTP, HTTP, HTTPS and gopher protocols, you must use the WinSock Proxy service to support any other application layer protocols you want to put into service. Common examples would be for SMTP, POP3, and NNTP.
The WinSock Proxy service is able to accomplish this amazing feat by replacing the winsock.dll's on the client machines that need to use the WinSock Proxy service. This is one of the major sticking points for many administrators. They are loath to add client software because of concern over how the client software will interact with other software installed on the client machine. In our experience, the WinSock client has little or no effect on the overall performance of client workstations on which it is installed.
The WinSock Proxy client .dll's will pick up the request made from user agents on the client workstations and forward those requests to the WinSock Proxy service on the Proxy server. All this takes place in the background and the users are not aware of the process. There is no application configuration required in most cases.
WinSock Proxy related problems often can be traced back to the Local Address Table or LAT. The LAT is used to determine if a request should be handled by the WinSock Proxy service, or if the request can be forwarded directly to the server. The LAT therefore should contain address ranges that encompass your internal network. If a foreign address is included in the LAT, requests to that address will not be subjected to WinSock Proxy service access controls.
Common service requests such as DNS must go through the WinSock Proxy client software. If the machine tries to make a DNS query and it does not have permission to do so, the DNS query will fail. Normally, the DNS Server is on the internal network, therefore access permissions are not an issue. If DNS queries are failing, make sure that the DNS Server is included on the LAT so that WinSock Proxy access controls are not applied.
You can also use the chkwsp32.exe application on the WinSock Proxy clients to check out the connection status between the WinSock Proxy client and server. Often you'll find out that the WSP Service has been disabled in the Control Panel, and its just a matter of turning the client back on and everything is fine. Also, make sure that the Internet connection is actually functional by going to the Proxy Server itself and confirming that Internet access is possible.