Hosting Multiple IIS Web Sites with Host Headers

Most IIS administrators are aware that IIS (versions 4.0, 5.0—and soon—6.0) can host multiple Web sites on one machine. The most common implementation of hosting multiple Web sites is to dedicate a single IP address to each Web site. The least common approach is to assign a unique port number to each Web site. A third and very viable alternative is to assign host header names (often referred to as host headers) to each Web site. You can use host headers to host multiple domain names from a single IP address. Host headers let IIS snatch a host name out of the HTTP header that the browser sends to the server. For example, if I type in my browser, the IIS 5.0 server that services the request extrapolates as the host header and routes the request to the proper Web site. Some very old browsers don't support the HTTP 1.1 protocol, so they can't send host names in the header. But those browsers are few and far between; even the Macintosh version of Netscape 2.0.2 supports host headers.

You'll need to configure and host some form of name resolution, such as DNS, to make sure requests from your site are properly routed to your Web server. You'll also need to stop Default Web Site because it responds to all assigned IP addresses by default. Or you can assign a host header to Default Web Site and force it to respond to a single IP address. I don't recommend this approach because IIS add-on packages (such as Microsoft Proxy Server) expect the default Web site to use the IP address of All Unassigned.

Follow these steps to assign host headers:

  1. Run Internet Service Manager (ISM) (Start, Programs, Administrative Tools, Internet Service Manager).
  2. To add each Web site to your server, right-click the server name in ISM, click New, then click Web Site. Follow the wizard to create the site(s).
  3. Right-click one of the Web sites that will use host headers, then click Properties.
  4. On the Web Site tab, select the IP address all the sites will use. You'll probably keep the default of TCP Port 80, the port that services the HTTP requests.
  5. Click Advanced next to the IP Address. In the "Multiple identities for this Web Site" list, select the Web site identity, click Edit, then add the desired host header (e.g., NOTE: If you want this Web site to respond to more than one host header (e.g., and, use the Add button to add other identities to this list using the same IP address and port. You'll need to set up some form of name resolution (such as DNS) for each identity.
  6. Click OK three times to apply the change(s).
  7. Stop, then start the Web site.
  8. Repeat steps 3 through 7 for the remaining Web sites.

Now you're ready to run a test. Grab a browser and navigate to your sites using the host headers. The browser will open the appropriate Web sites as applicable.

Using host headers has one drawback—IIS doesn't support Secure Sockets Layer (SSL) with host headers. IIS encrypts the HTTP header when it routes the request, so it can't determine the host header it needs to route to the correct Web site. Because IIS has to decrypt the SSL key anyway, it should be smart enough to perform the encryption during routing by rotating through the key types until it successfully decrypts the host header from the body of the encrypted HTTP header. But it doesn't. A really sharp Internet Server API (ISAPI) developer could probably do just that, but that level of development pain probably isn't worth the effort; simply dedicating an IP address to each Web site overcomes the SSL problem.

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