Accessing Exchange over the Internet without using a VPN

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RPC over HTTP is a new feature found in Windows Server 2003. Exchange 2003 introduces support for this feature and does enable you to use Outlook over the Internet without a VPN.

Before you will be able to use RPC over HTTP, you'll first have to upgrade a few of your components. First, only Exchange 2003 on Windows 2003 supports this feature. Therefore, youll need to upgrade your Exchange 2003 server to Windows 2003. Next, only Outlook 2003 running on Windows XP SP1 (plus the hotfix from Microsoft Knowledge Base article 331320) can be used, so you will have to update your Outlook client and install this hotfix (and upgrade to Windows XP SP1 if you have not done so already).

The feature really should be called RPC over HTTPs because you'll definitely want to use SSL to prevent someone from using a network sniffer to capture user names and passwords. Because RPC over HTTP only supports Basic authentication, usernames and passwords will be transmitted in Base64 encoding and can be decoded and captured if SSL is not used.

There are two basic deployment options for implementing RPC over HTTP. One option involves using Microsoft Internet Security & Acceleration (ISA) Server as a perimeter firewall and locating the RPC Proxy server on your corporate network. In this scenario an ISA Server in a perimeter network passes the packets to an internal Exchange front-end server that also functions as an RPC Proxy server. This option will be most attractive to current ISA Server users, as they may not need to deploy additional servers in order to leverage this feature.

Instead of using ISA Server, the second option uses an Exchange front-end server in a perimeter network that also acts as an RPC Proxy server.

For more information on configuring RPC over HTTP, see this Microsoft article.