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Did you know that yourdomain.com and www.yourdomain.com are actually different sites? Are they both serving the same content? If so, it may be negatively impacting your search engine rankings.

Subdomains and the Synonymous 'WWW'

Sub-domains are the prefix to a domain (http://subdomain.yourdomain.com), and are treated by browsers, computers, domain name systems (DNS), search engines, and the general internet as separate, individual web sites. Google's primary web presence, http://www.google.com, is very different than Google Mail, http://mail.google.com, or Google Documents, http://docs.google.com, all because of subdomains. However, what many do not realize is that www is, itself, a subdomain.

A domain, on its own, requires no www prefix; a subdomain-less http://yourdomain.com should be sufficient for serving up a web site. And since www is a subdomain, dropping the prefix could potentially return a different response. There are some sites that will fail to return without the prefix, and some sites that fail with it, but the most common practice is that the www subdomain is synonymous for no subdomain at all.

The Synonymous WWW and SEO

The issue with having two synonymous URLs (http://yourdomain.com and http://www.yourdomain.com) is that search engines may interpret them as separate sites, even if they are serving the same content. The two addresses are technically independent and are potentially serving unique content; to a cautious search engine, even if pages appear to contain the same content, there may be something different under the covers. This means your audience's search results returns two entries for the same content. Some users will happen to click on yourdomain.com while others navigate to www.yourdomain.com, splitting your traffic, your page hits, your search ranking between two sites, unnecessarily.

HTTP Redirects will cure the issue. If you access http://google.com, your browser is instantly redirected to http://www.google.com. This is done through a HTTP 301 permanent redirect. Search Spiders recognize HTTP response codes, and understand the 301 as a "use this other URL instead" command. Many search engines, such as Google, will then update all page entries for the original URI (http://yourdomain.com) and replace it with the 301's destination URL (http://www.yourdomain.com). If there is already an entry for the destination URL, the two entries will be merged together. The search entries for yourdomain.com and www.yourdomain.com will now share traffic, share page hits, and share search ranking. Instead of having two entries on the second and third pages of search results, combining these entries may be just enough to place you on the first page of results.

In addition to combining search entries for subdomains, you can also combine root-level domains through HTTP 301. On this site, in addition to adding the www prefix if no subdomain is specified, captainloadtest.com will HTTP 301 redirect to www.cptloadtest.com.

Combining the Synonyms

We need a way to implement an HTTP 301 redirect at the domain level for all requests to a site; however, often we are using applications that may not grant us access to the source, or we don't have the access into IIS through our host to set up redirects for ourselves. URL Rewrite, Part 2 covers a great drop-in redirect module by Fritz Onion that uses a stand-alone assembly with a few additions in web.config to HTTP 301 redirect paths in your domain (it also supports HTTP 302 redirects). This module is perfect for converting a WordPress blog post URL, such as cptloadtest.com/?p=56, to a DasBlog blog post URL like cptloadtest.com/2006/05/31/VSNetMacroCollapseAll.aspx. However, to redirect domains and subdomains, the module must go a step further and redirect based on matches against the entire URL, such as directing http:// to https:// or captainloadtest.com to cptloadtest.com, which it does not support. It's time for some modifications.

private void OnBeginRequest(object src, EventArgs e) {
  HttpApplication app = src as HttpApplication;
  string reqUrl = app.Request.Url.AbsoluteUri;
  redirections redirs
    = (redirections) ConfigurationManager.GetSection("redirections");

  foreach (Add a in redirs.Adds) {
    Regex regex = new Regex(a.targetUrl, RegexOptions.IgnoreCase);
    if (regex.IsMatch(reqUrl)) {
      string targetUrl = regex.Replace(reqUrl, a.destinationUrl, 1);

      if (a.permanent) {
        app.Response.StatusCode = 301; // make a permanent redirect
        app.Response.AddHeader("Location", targetUrl);


By converting app.Request.RawURL to app.Request.AbsoluteUri, the regular expression will now match against the entire URL, rather than just the requested path. There is one downside to this change: the value is the actual path processed, not necessarily what was in the originally requested URL. To this effect, the value of AbsoluteUri for requesting http://www.cptloadtest.com?p=56 is actually http://www.cptloadtest.com/default.aspx?p=56; by requesting the root directory, the default page is being processed, not the directory itself, so default.aspx is added to the URL. Keep this in mind when setting up your redirection rules. Also, the original code converted the URL to lower case; with my modifications, I chose to maintain the case of the URL, since sometimes case matters, and instead ignore case in the regular expression match using RegexOptions.IgnoreCase. Finally, I made some other minor enhancements, like using the ConfigurationManager, since ConfigurationSettings is now obsolete, and reusing the matching Regex instance for replacements.

Download: RedirectModule.zip


  • Source code for the drop-in Redirect Module
  • Sample web.config that uses the module
  • Compiled version of redirectmodule.dll

The code is based on the original Redirect Module by Fritz Onion and the Xml Serializer Section Handler by Craig Andera. As always, this code is provided with no warranties or guarantees. Use at your own risk. Your mileage may vary. Thanks to Fritz Onion for the original work, and allowing me extend his code further.

The usage is the same as Fritz Onion's original module. Drop the assembly into your site's bin, and place a few lines into the web.config. The example below contains the rules as they would apply to this site, 301 redirecting http://www.captainloadtest.com to http://www.cptloadtest.com, and adding the www subdomain to any domain requests that have no subdomain.

<?xml version="1.0"?>
    <section name="redirections"
      type="Pluralsight.Website.XmlSerializerSectionHandler, redirectmodule" />
  <!-- Redirect Rules -->
  <redirections type="Pluralsight.Website.redirections, redirectmodule">
    <!-- Domain Redirects //-->
    <add targetUrl="captainloadtest\.com/Default\.aspx"
      destinationUrl="cptloadtest.com/" permanent="true" />
    <add targetUrl="captainloadtest\.com"
      destinationUrl="cptloadtest.com" permanent="true" />

    <!-- Add 'WWW' to the domain request //-->
    <add targetUrl="://cptloadtest\.com/Default\.aspx"
      destinationUrl="://www.$1.com/" permanent="true" />
    <add targetUrl="://cptloadtest\.com"
      destinationUrl="://www.$1.com" permanent="true" />

    <!-- ...More Redirects -->
      <add name="RedirectModule"
        type="Pluralsight.Website.RedirectModule, redirectmodule" />

The component is easy to use, and can redirect your site traffic to any URL you choose. Neither code changes to the application nor configuration changes to IIS are needed. By using this module to combine synonymous versions of your URLs, such as alternate domains or subdomains, you will improve your page ranking through combining duplicate search result entries. One more step towards your own search engine optimization goals.

URL Rewrite

Thursday, 04 December 2008 16:43:10 (Eastern Standard Time, UTC-05:00)  #    Comments [4] - Trackback

Thursday, 04 December 2008 22:12:59 (Eastern Standard Time, UTC-05:00)
Great article. I wrote on this exact same topic here: http://www.dotnetblogger.com/post/2008/12/03/ASPNET-Force-Website-to-use-WWW.aspx
Tuesday, 16 December 2008 13:27:45 (Eastern Standard Time, UTC-05:00)
>> the most common practice is that the www subdomain is synonymous for no subdomain at all.

That is technically not exactly right. (or to be more exact, it's backwards)

The first segment of the URL is actually the host, i.e., one particular computer in that domain, so that john.example.com is John's computer, and jack.example.com is Jack's. And www.example.com is the server at example.com they named "www" to handle WWW requests. (or at least that's the way it was originally intended)

The host is necessary, but can be defaulted. Hence that line should be written "the most common practice is that no host name is synonymous for the www host."

Subdomains would come between the host name and domain name (ie, sam.uk.example.com is Sam's computer at example.com's UK office). Subdomain's are truly optional- if not specified there is no default. But you can have a specified subdomain and a defaulted host, so it get a bit confusing)
Thursday, 07 January 2010 15:11:10 (Eastern Standard Time, UTC-05:00)
will you please help me
i want to create a website which give facility to make subdomain on site
like - abc.maindomain.com
could it be possible to redirect that request to my maindomain with query string or as - maindomain.com/subdomain/abc/
sir i am new at this so please help me with some code
Thursday, 04 February 2010 05:47:26 (Eastern Standard Time, UTC-05:00)
Its not working with IIS7
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