Migrating ASP.NET applications to another Windows Domain

You may have to migrate Intranet software applications to another Windows Domain (e.g. Due to company restructure). As Intranet applications normally uses Domain account to authorize user access, changing domain means you have to change user group settings, sometimes source code and sometimes even worse your third-party applications will not work properly -- e.g. old data uses old domain account so that user cannot change their data using new domain account.

If your applications were based on ASP.NET framework, then there is a simple solution to ease the pain at least temporarily: you keep all those ASP.NET applications in original domain and add a HttpModule (defined in web.config) to map new domain account to old domain account. When users in new domain access those applications in old domain, the ASP.NET application will only see the old domain account so the users still work in the exact same way as before.

The main code logic for domain mapping HttpModule is as below:

// Get user's domain account
string strDomainUserAccount = HttpContext.Current.User.Identity.Name.ToUpper();

// Remove domain name from the account
string strUserAccount = strDomainUserAccount;
if (strDomainUserAccount.StartsWith("NewDomain\\")) {
 strUserAccount = strUserAccount.Substring(10);

// Fetch cached user mappings
Dictionary userMap = (Dictionary)HttpRuntime.Cache(USER_CACHE_KEY);
if (userMap == null) {
 // If no cache is available, reload the cache from database 
 // and try to get the item again
 userMap = (Dictionary)HttpRuntime.Cache(USER_CACHE_KEY);
 if (userMap == null) {
  // Throw exception here!!

// Get user's NTUserName from the mapping in cache 
// and create a new user for Http context
if (userMap.ContainsKey(strUserAccount)) {
 string strNewAccount = userMap[strUserAccount];
 if (!string.IsNullOrEmpty(strNewAccount)) {
  // New Identity
  string strAuthenticateType = HttpContext.Current.User.Identity.AuthenticationType;
  System.Security.Principal.GenericIdentity newID = 
   new System.Security.Principal.GenericIdentity(strNewAccount, strAuthenticateType);
  // Get cached roles in mapped domain. 
  // If no cache is available, get from AD and put into cache
  string[] strRoles = this.GetDomainGroupsForUserFromCache[strNewAccount];
  System.Security.Principal.GenericPrincipal newP = 
   new System.Security.Principal.GenericPrincipal(newID, strRoles);
  HttpContext.Current.User = newP;


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