We have several divisions where people mostly roam from one location to another be it a business trip or just a remote work. But as that’s all about doing their jobs they need the information they basically can access only right from the office. One of such types of information is surely their personal corporate mail. That’s how we work today. If we have no access to any collaboration services our work gets stuck. And the mail is the main thing there. So the core task for every system administrator today is how to provide the user with access to their corporate mail remotely from any place no matter where the user will decide to access it from.

How to do that?

One way is to create a Virtual Private Network (VPN). But what if by some reasons you can’t or simply don’t want to setup VPN to avoid making the things for users even more complex? What can you do here? What should you start with? The core term here is “RPC over HTTP“, where RPC is the Remote Procedure Call, a protocol that allows interprocess communications between client and server sides so that a component to be accessed remotely in such a way that we don’t even need to know any low-level information. This is the technology that allows Outlook users to connect to their Exchange mailbox from a remote place. And there’s no need to have a VPN connection. It allows accessing Exchange servers right through your default corporate LAN’s firewall using the basic ports used by browsers to access unsecure and secure contents on the internet. The ports that should be opened to allow access are the TCP port 80 used for basic unsecure connections and the SSL port 443 used for secure connections that are established using the Secure Sockets Layer protocol which is used as basis protocol for the Transport Layer Security (TLS) protocol which version 1.1. is defined in the RFC4346 document.

What should we do to enable all that for our users?

The process contains least two parts we should do to implement the functionality. As we are talking about client-server communications we need to prepare the configurations on both the server as the client. We will consider the Microsoft Exchange Server 2003 installed on the Windows Server 2003 Service Pack 1 and above to be the server side and Microsoft Office Outlook 2003 installed on the Windows XP Professional Service Pack 2 to be the client side.
Configuring Server Side
Let’s start configuring the setup from the server side. First of all we need to configure Exchange Server 2003 back-end server as an RPC proxy server. The process here starts with installing the additional component RPC over HTTP Proxy from the Windows Server Setup Disk. To do that:
1. Click Start and select Control Panel|Add or Remove Programs to start the Add or Remove Programs applet
2. In the Add or Remove Programs windows click Add/Remove Windows Components button
3. The Windows Components screen of the Windows Components Wizard will appear
4. Select Networking Sevices and click the Details button to open the Networking Sevices dialog
4. In the dialog box, check the RPC over HTTP Proxy checkbox and click OK

The RPC component will be installed on the system and the RPC virtual directory will be created on the IIS. Now we need to configure authentication and the encryption.

Configuring client authentication

Basic authentication will be used to authenticate users. This type of authentication has one very annoying property: it sends creadentials in the pure form as the plain text. That’s why we will need to configure SSL and implement the encryption to be used for passing the credentials.
To configure that
1. Click Start and select Programs|Administrative Tools|Internet Information Services (IIS) Manager to start the IIS manager
2. In the manager window navigate to Web Sites and select Default Web Site
3. Expand Default Web Site, right-click the RPC virtual directory, and select Properties command from the shortcut menu
4. In the RPC Virtual Directory Properties page switch to the Directory Security tab
5. Under Anonymous Access and Authentication Control pane click Edit button.
6. The Authentication Methods dialog box will appear
7. Uncheck the Enable Anonymous Access checkbox

That’s needed because by default RPC over HTTP doesn’t allow anonymous access

8. Under Authenticated access section, select the check box Basic authentication (password is sent in clear text)
9. You can also allow the NTLM Windows authentication and leave the Integrated Windows authentication checkbox checked

Microsoft has a note on this type of authentication:
It is recommended that you use Basic authentication over NTLM because of two reasons. First, RPC over HTTP currently supports only NTLM – it doesn’t support Kerberos. Second, if there is an HTTP Proxy or a firewall between the RPC over HTTP client and the RPC Proxy, which inserts via the pragma in the HTTP header, NTLM authentication will not work

10. End with the warning message and ensure that you have correct SSL certificate installed on your server

Now we need to enabled SSL to be used for the RPC Virtual Directory. To do that

1. On the same Directory Security tab mentioned above click Edit button under Secure communications
2. Check both the Require secure channel (SSL) and the Require 128-bit encryption check boxes
3. Click OK to save settings and close the window

See How to Configure the RPC Virtual Directory in IIS article for the detailed info

The next step is to configure the RPC proxy server on Exchange Server 2003 to use specified port range for RPC over HTTP. To do that:
1. Open registry editor by typing regedit in the Run dialog box
2. In the Regsitry Editor navigate to the path

HKLM\SOFTWARE\Microsoft\Rpc\RPCProxy

Create the ValidPorts string REG_SZ parameter and set it to the value the is built in the following manner

NETBIOSNAME:6001-6002;FQDNDNSNAME:6001-6002;NETBIOSNAME:6004;FQDNDNSNAME:6004;

to open the port range 6001-6002 and one single port 6004

Now we need to configure our Exchange 2003 back-end servers (the GC, Global Catalog servers) and set the NT Directory Services (NTDS) port on them. So we again should to specify registry parameter to do that. This time we need to open the

HKLM\SYSTEM\CurrentControlSet\Services\NTDS\Parameters

create a REG_MULTI_SZ ‘NSPI interface protocol sequences’ parameter and set it to value NCACN_HTTP:6004

We ended with the specific preliminary tasks on the server and can start with configuring client application (that is the Outlook 2003) profile to work with RPC over HTTPS. But that’s the story to be covered in the next part when we will talk about client side configuration.

Further info:
RPC over HTTP Interactions on the RPC Proxy Server
How RPC Works
Automatic Configuration of The Client Side
RPC over HTTP Authentication and Security

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