C# in a Nutshell, Second Edition

Name

Dns

Synopsis

This type offers up a collection of static methods for resolving DNS (Domain Name Service) operations. Because raw IP addresses (such as 192.168.0.1) can be difficult for humans to remember, DNS servers take human-friendly names (such as “www.oreilly.com”) and in turn translate them into IP addresses and/or back again. This ability can be useful in a variety of scenarios, including the simple logging of clients who have visited a web site recently. (Note that this is not a secure way of tracking usage—even a simple attacker can spoof the return address of an IP packet, so don’t rely on this mechanism for any kind of security validation or audit trail.)

The GetHostByName( ) method takes a hostname (such as “www.oreilly.com”) and returns the corresponding IPHostEntry instance; this IPHostEntry instance is used later in several of the System.Net and System.Net.Sockets types. This method (along with the paired method Resolve( )) can be invoked asynchronously using the BeginGetHostByName( ) or BeginResolve( ) methods. Like all async methods in the .NET Framework, they take two additional parameters: an AsyncCallback object, and a generic object parameter that is passed to the AsyncCallback when the call completes.

At the surface, there would appear to be no difference between calling Resolve( ) or GetHostByName( ); in fact, the Resolve( ) method calls into GetHostByName( ) after doing a small amount of preprocessing to check if the string passed is either a standard host ...