Using Proxy Server with Routing and Remote Access

• Microsoft Windows NT Server version 4.0
• Microsoft Proxy Server version 1.0
• Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0

SUMMARY

Microsoft Proxy Server is a network application that does not require routing. Operationally, this means that every packet that is transmitted to or from the proxy server is either sourced or destined with the proxy server's IP address.

For security reasons, the Proxy Server printed documentation recommends turning off IP forwarding on the computer on which Proxy Server is installed. However, when you install the Routing and Remote Access Update on a server running Microsoft Proxy Server, you now have IP forwarding turned on. With IP forwarding on, a computer running Windows NT Server is able to forward packets correctly from the Internet connection to the internal network. If IP forwarding is enabled on a proxy server, all security can be bypassed unless local host filters are configured.

MORE INFORMATION

For Microsoft Proxy Server, or any other application service, to work securely with Routing and Remote Access Services you must configure input and output filters for local host traffic. These filters are configured using the Routing and Remote Access Administrator tool.

Before any filters you set up will work, you must enable packet filtering on a global level.

To globally enable packet filtering:

1. In the IP Routing folder, right-click Summary and click Configure IP parameters.

2. On the General tab, select the Enable packet-filtering check box.

Adding Local Host Filters

A local host filter enables your computer to receive only traffic destined for the computer. A local host filter works by enabling users to access your computer, but not to route through your computer. After this filter is set, only traffic destined for this host will be allowed in the interface.

In this example, your Proxy server is configured with an Internet IP address of 192.168.1.1, with a subnet mask of 255.255.255.0. To add local host filters:

1. In the IP Routing folder, click Summary.

2. Right-click the interface over which you want to set the filter, and click on Configure Interface. This should be the external (Internet- connected) interface.

3. In the IP Configuration dialog box, click Input Filters.

4. In the IP Packet Filters Configuration dialog box, click Add. To allow packets with a destination address of your Proxy server, add a filter with the Destination IP address of 192.168.1.1 and the Destination subnet mask of 255.255.255.255. Then select Any as the type of protocol. Click OK, then select the 'Drop All Except Listed Below' option under 'Filter Action', and click OK again.

5. In the IP Configuration dialog box, click Output Filters.

6. In the IP Packet Filters Configuration dialog box, click Add. To allow packets leaving directly from your Proxy server, add a filter with the Source IP address of 192.168.1.1 and the Source subnet mask of 255.255.255.255. Then select Any as the type of protocol. Click OK, then select the 'Drop All Except Listed Below' option under 'Filter Action', and click OK again.

Configuring your Proxy Server/Routing and Remote Access server like this will also allow your server to now act as a PPTP server so that PPTP clients on the Internet can access your internal LAN.

For additional information, please see the following article(s) in the Microsoft Knowledge Base:

   ARTICLE-ID: Q161410
   TITLE     : How to Set Up a Private Network Over the Internet Using
               PPTP

Adding Advanced Filters

If you would like to make your Proxy Server’s Internet connection more secure than this, you can remove the Input filter that allows any packets address directly to your Proxy server and add individual input filters for each type of packet you would like to allow.

For example, you may want your Proxy server to only service WWW requests from Proxy clients on the LAN. To do this, you would remove the Input filter you added earlier with the Destination IP address of 192.168.1.1. Then you would add an Input filter allowing packets with the Destination IP address of 192.168.1.1, Protocol TCP, Source port 80, and Destination port 0. You would also have to add a second Input filter allowing packets with the Destination IP address of 192.168.1.1, Protocol UDP, Source port 53, and Destination port 0. This will allow the Proxy Server to resolve Internet names using DNS.

If you want Proxy clients to be able to use additional Proxy services, you would have to add Input filters allowing the correct protocol and port number that each service uses. If you want PPTP clients to be able to connect to your internal LAN, then you would need to add PPTP filters.

For additional information, please see the following article(s) in the Microsoft Knowledge Base:

   ARTICLE-ID: Q169890
   TITLE     : Enable PPTP Filtering Option No Longer Works