Enumerating Domain Controllers

Prior to Windows XP, an application could only obtain a single domain controller in a domain by calling DsGetDcName. There was no way to predict which domain controller would be retrieved or to obtain a list of the domain controllers. In Windows XP, the operating system enables an application to enumerate the domain controllers in a domain by using the DsGetDcOpen, DsGetDcNext, and DsGetDcClose functions.

A domain controller enumeration operation is begun by calling DsGetDcOpen. This function takes parameters that define the domain to enumerate and other enumeration options. DsGetDcOpen provides a domain enumeration context handle that is used to identify the enumeration operation when DsGetDcNext and DsGetDcClose are called.

The DsGetDcNext function is called with the domain enumeration context handle to retrieve the next domain controller in the enumeration. The first time this function is called, the first domain controller in the enumeration is retrieved. The second time this function is called, the second domain controller in the enumeration is retrieved. This process is repeated until DsGetDcNext returns ERROR_NO_MORE_ITEMS, which indicates the end of the enumeration has been reached.

The DsGetDcNext function will enumerate the domain controllers in two groups. The first group contains the domain controllers that cover the site of the computer where the function is executed and the second group contains the domain controllers that do not cover the site of the computer where the function is executed. If the DS_NOTIFY_AFTER_SITE_RECORDS flag is specified in the OptionFlags parameter in DsGetDcOpen, the DsGetDcNext function will return ERROR_FILEMARK_DETECTED after all of the site-specific domain controllers have been retrieved. DsGetDcNext will then begin enumerating the second group, which contains all domain controllers in the domain, including the site-specific domain controllers contained in the first group.

Domain controllers that cover the site of the computer where the function is executed are enumerated first followed by the domain controllers that do not cover the site of the computer where the function is executed. A domain controller is said to cover a site if the domain controller is configured to reside in that site or if the domain controller resides in a site that is nearest to the site in question in terms of the configured inter-site link cost. If there are any domain controllers in both the group of domain controllers that cover and the group of domain controllers that do not cover the machine site, domain controllers are returned within the group in order of their configured priorities and weights that are specified in DNS. Domain controllers that have lower numeric priority are returned within a group first. If within a site-related group there is a subgroup of several domain controllers with the same priority, domain controllers are returned in a weighted random order where domain controllers with higher weight have more probability to be returned first. The sites, priorities, and weights are configured by the domain administrator to achieve effective performance and load balancing among multiple domain controllers available in the domain. Because of this, applications that use the DsGetDcOpen/DsGetDcNext/DsGetDcClose functions automatically take advantage of these optimizations.

When the enumeration is complete or is no longer required, the enumeration must be closed by calling DsGetDcClose with the domain enumeration context handle.

To reset the enumeration, it is necessary to close the current enumeration by calling DsGetDcClose and then reopen the enumeration by calling DsGetDcOpen again.

Example

The following code example shows how to use these functions to enumerate the domain controllers in the local domain.

DWORD dwRet;
PDOMAIN_CONTROLLER_INFO pdcInfo;

// Get a domain controller for the domain this computer is on.
dwRet = DsGetDcName(NULL, NULL, NULL, NULL, 0, &pdcInfo);
if(ERROR_SUCCESS == dwRet)
{
	HANDLE hGetDc;

	// Open the enumeration.
	dwRet = DsGetDcOpen(	pdcInfo->DomainName,
							DS_NOTIFY_AFTER_SITE_RECORDS,
							NULL,
							NULL,
							NULL,
							0,
							&hGetDc);
	if(ERROR_SUCCESS == dwRet)
	{
		LPTSTR pszDnsHostName;

		// Enumerate each domain controller and print its name to the debug window.
		while(TRUE)
		{
			ULONG ulSocketCount;
			LPSOCKET_ADDRESS rgSocketAddresses;

			dwRet = DsGetDcNext(hGetDc, &ulSocketCount, &rgSocketAddresses, &pszDnsHostName);
		
			if(ERROR_SUCCESS == dwRet)
			{
				OutputDebugString(pszDnsHostName);
				OutputDebugString(TEXT("\n"));
			
				// Free the allocated string.
				NetApiBufferFree(pszDnsHostName);

				// Free the socket address array.
				LocalFree(rgSocketAddresses);
		}
			else if(ERROR_NO_MORE_ITEMS == dwRet)
			{
				// The end of the list has been reached.
				break;
		}
			else if(ERROR_FILEMARK_DETECTED == dwRet)
			{
				/*
				DS_NOTIFY_AFTER_SITE_RECORDS was specified in DsGetDcOpen and the end 
				of the site-specific records was reached.
				*/
				OutputDebugString(TEXT("End of site-specific domain controllers\n"));
				continue;
		}
			else
			{
				// Some other error occurred.
				break;
		}
	}
	
		// Close the enumeration.
		DsGetDcClose(hGetDc);
}

	// Free the DOMAIN_CONTROLLER_INFO structure. 
	NetApiBufferFree(pdcInfo);
}