HOWTO: Configure REMOTE_HOST to Perform a Reverse DNS Lookup in IIS

ID: Q245574

The information in this article applies to:

Microsoft Internet Information Server versions 4.0, 5.0

SUMMARY

By default, performing a Request.ServerVariables("REMOTE_HOST") in ASP returns a null value, which causes Internet Information Server (IIS) to return the value of REMOTE_ADDR, the IP address of the client. This is by design to increase performance for Web hosting. However, by changing a setting in the metabase, IIS performs a reverse DNS lookup and returns the host name of the client connecting. This metabase entry has no affect on the information written to the IIS logs or on the IP restriction reverse lookup.

NOTE: Enabling reverse lookups incurs extra processing overhead, that degrades the performance of your Web server. This information is not recommended for high capacity sites where performance is critical.

With the metabase setting enabled, every request for the REMOTE_HOST server variable causes IIS to pass a name resolution request off to the operating system, which attempts to execute a reverse DNS lookup. If the reverse DNS lookup fails to return the host name, the operating system then attempts name resolution using NetBIOS. If NetBIOS resolution fails an empty value is returned to IIS, which returns the REMOTE_ADDR value (client's IP address) to the original REMOTE_HOST request.

Reverse lookups can be set for the entire Web server or for individual Web sites, requiring administrative access to the metabase through the IIS Admin Objects. This article describes two methods for modifying the metabase with the IIS Admin Objects, using Active Server Pages and using Windows Scripting Host from the command-line. For the following Active Server Pages examples, create an .asp file in a Web site or virtual directory that has Script or Execute access enabled and copy the following code into the .asp file. The command-line method requires that the Admin Sample Scripts (installed during IIS setup) and Windows Scripting Host be installed (WSH installs with the Windows NT Option Pack, Windows 2000, Internet Explorer 5 and from:

Scripting Download Page

The command-line method requires that instructions be issued from a command prompt where the Adsutil.vbs script is located. The location of the Adsutil.vbs script depends on the version IIS, but is in the following directories by default:


   Internet Information Server 4.0 - C:\WinNT\System32\InetSrv\AdminSamples<BR/>
   Internet Information Server 5.0 - C:\InetPub\AdminScripts

Because improperly modifying the metabase can negatively impact Internet Information Server, Microsoft strongly recommends backing up the metabase before making any modifications.

Example 1 - Enabling Reverse Lookups for all sites:

This ASP code example enables all Web sites to perform a reverse lookup when a Request.ServerVariables("REMOTE_HOST") is issued. Due to the hierarchical design of the metabase, Web sites that do not explicitly have the EnableReverseDNS entry set inherit the value from the earlier node. ASP Method:


<%
Dim oIIS
Dim vEnableRevDNS
Dim vDisableRevDNS

vEnableRevDNS = 1
vDisableRevDNS = 0

Set oIIS = GetObject("IIS://localhost/w3svc")
oIIS.Put "EnableReverseDNS", vEnableRevDNS
oIIS.SetInfo
Set oIIS = Nothing

%>

Command-line Method:


cscript adsutil.vbs set /w3svc/EnableReverseDNS "TRUE"

Example 2 - Enabling Reverse Lookups for individual sites:

This example enables a reverse lookup for a specific Web site when a Request.ServerVariables("REMOTE_HOST") is issued. Web sites are referenced in the metabase by an integer value. Because the Default Web Site is the first site created, it receives the reference number 1. This example enables reverse lookups for only the Default Web Site, to allow reverse lookups on other Web sites replace 1 in the following line:


Set oIIS = GetObject("IIS://localhost/w3svc/1/ROOT")

with the numeric value of the Web site that needs to perform reverse lookups. The easiest way to determine the numeric value of a specific Web site is to look at the number listed at the end of the folder name where the log file name is specified. This can be accessed through the MMC/Internet Service Manager by selecting the Web site and choosing Action, Properties, Active Log Format and then Properties. The Default Web site's Log file name is W3SVC1\exyymmdd.log, corresponding to the /1/ROOT value. ASP Method:


<%
Dim oIIS
Dim vEnableRevDNS
Dim vDisableRevDNS

vEnableRevDNS = 1
vDisableRevDNS = 0

Set oIIS = GetObject("IIS://localhost/w3svc/1/ROOT")
oIIS.Put "EnableReverseDNS", vEnableRevDNS
oIIS.SetInfo
Set oIIS = Nothing

%>

Command-line Method:


cscript adsutil.vbs set /w3svc/1/ROOT/EnableReverseDNS "TRUE"

MORE INFORMATION

More information on the metabase can be found in the IIS documentation and in the article:

Q240941 An Introduction to the IIS Metabase

More information on the Scripting technologies listed in this article can be found at
http://msdn.microsoft.com/scripting

Additional query words:

Keywords : kbiis400 kbiis500 kbMetaBase
Version : winnt:4.0,5.0
Platform : winnt
Issue type : kbhowto