DNS and C#

Summary

The Domain Name System (DNS) can be used by C# programs to determine the IP addresses of hostnames. DNS is a distributed hierarchical database system that assigns hostnames hierarchically, separating each level with a period. DNS servers are created to store the DNS database for individual domains. Clients can query the DNS servers to find the IP address associated with a hostname, or other domain information. If a local DNS server cannot answer the query, it goes to a higher-level DNS server until the query can be answered and returned to the client.

Windows systems can resolve hostnames in two ways. The first way is to use the hosts file to manually enter hostnames and provide quick hostname resolution. The second way is to configure DNS servers in the system to contact remote DNS servers and resolve DNS information. The C# language provides the RegistryKey class to query the local Registry for the addresses of DNS servers configured on the system.

Windows systems also include the nslookup program, which lets customers do manual DNS queries. The nslookup program has several options for altering the way it finds and displays DNS information. The debug option allows you to see the actual DNS query sent to the remote DNS server. The querytype option allows you to query for specific DNS record information for a domain.

C# contains the Dns class in the System.Net namespace to provide basic DNS query capability. The GetHostByName() and GetHostByAddress() methods allow C# programs to determine DNS information given particular host data. The Resolve() method can find DNS information for either a hostname or an IP address.

Also covered in this chapter are asynchronous Dns methods, for doing DNS queries in applications that don’t allow you to tie up system resources while waiting for an answer from the DNS server. The asynchronous methods use the standard C# delegate system to call a method when the DNS query has returned. This can greatly improve the performance of Windows-based network programs