DNS APIs in .NET

This section provides documentation on the classes and methods available in the .NET Framework for interacting with the Domain Name System (DNS).

Core DNS Classes

The primary classes for DNS operations reside in the System.Net.Dns namespace.

System.Net.Dns Class

Provides methods for resolving domain names and IP addresses. It's the central point for most DNS lookups.

Key Methods:

System.Net.IPHostEntry Class

Represents a DNS host, including its name, aliases, and IP addresses.

Properties:

• HostName: The primary DNS host name.
• Aliases: An array of aliases for the host.
• AddressList: An array of IPAddress objects associated with the host.

System.Net.IPAddress Class

Represents an Internet Protocol (IP) address.

While not exclusively a DNS class, it's crucial for interpreting DNS query results.

Key Members:

• Static properties like IPAddress.Any, IPAddress.Broadcast, IPAddress.Loopback.
• Methods like Parse(string s) and ToString().

Common DNS Operations and Examples

Resolving a Host Name to an IP Address

This is a fundamental DNS operation, converting a domain name like "www.example.com" into its corresponding IP addresses.

using System;
using System.Net;

public class DnsLookup
{
    public static void Main(string[] args)
    {
        try
        {
            string hostName = "www.microsoft.com";
            IPHostEntry ipHostInfo = Dns.GetHostEntry(hostName);

            Console.WriteLine($"Host Name: {ipHostInfo.HostName}");

            Console.WriteLine("IP Addresses:");
            foreach (IPAddress ipAddress in ipHostInfo.AddressList)
            {
                Console.WriteLine($"- {ipAddress}");
            }
        }
        catch (SocketException e)
        {
            Console.WriteLine($"SocketException caught: {e}");
        }
        catch (Exception e)
        {
            Console.WriteLine($"General Exception caught: {e}");
        }
    }
}

Getting the Local Host Name

Retrieve the name of the machine the application is running on.

string localHostName = Dns.GetHostName();
Console.WriteLine($"Local Host Name: {localHostName}");

Asynchronous DNS Operations

For better responsiveness in UI applications, asynchronous methods are recommended.

async Task ResolveHostAsync()
{
    try
    {
        string hostName = "www.google.com";
        IPHostEntry ipHostInfo = await Dns.GetHostEntryAsync(hostName);

        Console.WriteLine($"Async Host Name: {ipHostInfo.HostName}");
        foreach (IPAddress ipAddress in ipHostInfo.AddressList)
        {
            Console.WriteLine($"- {ipAddress}");
        }
    }
    catch (SocketException e)
    {
        Console.WriteLine($"SocketException caught: {e}");
    }
}

Advanced Topics

Reverse DNS Lookups

While Dns.GetHostEntry(string ipAddress) performs a reverse lookup if an IP address is provided, explicit reverse lookup mechanisms might be needed for specific scenarios (e.g., using DnsPermission).

DNS Permissions

Network operations, including DNS lookups, can be controlled using code access security (CAS) permissions, particularly System.Net.DnsPermission.

Custom DNS Resolution

For advanced scenarios, you might need to bypass the system's default DNS resolution and implement custom logic, possibly by implementing custom DNS client behavior or using lower-level socket operations.

See Also

• Socket Programming
• Networking Overview
• System.Net.Dns Class MSDN Reference
• System.Net.IPHostEntry Class MSDN Reference