Is it possible to set custom DNS resolver in C#'s HttpClient

Question

what exactly I want :

public static CustomDnsResolver : Dns 
{
   .....
}

public static void Main(string[] args) 
{
    var httpClient = new HttpClient();
    httpClient.Dns(new CustomDnsResolver());
}

basically I just want to use my custom DNS resolver in HttpClient instead of System default, Is there any way to achieve it?

Answer 1

The use case you have is exactly why Microsoft build the HttpClient stack. It allow you to put your business logic in layered class with the help of HttpMessageHandler class. You can find some sample in ms docs or visualstudiomagazine

async Task Main()
{
    var dnsHandler = new DnsHandler(new CustomDnsResolver());
    var client = new HttpClient(dnsHandler);
    var html = await client.GetStringAsync("http://google.com");
}

public class DnsHandler : HttpClientHandler
{
    private readonly CustomDnsResolver _dnsResolver;

    public DnsHandler(CustomDnsResolver dnsResolver)
    {
        _dnsResolver = dnsResolver;
    }
    
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var host = request.RequestUri.Host;
        var ip = _dnsResolver.Resolve(host);
        
        var builder = new UriBuilder(request.RequestUri);
        builder.Host = ip;
        
        request.RequestUri = builder.Uri;
        
        return base.SendAsync(request, cancellationToken);
    }
}

public class CustomDnsResolver
{
    public string Resolve(string host)
    {
        return "127.0.0.1";
    }
}

More advanced version based on Meziantou blog post

async Task Main()
{
    var dnsResolver = new CustomDnsResolver();

    var sockerHttpHandler = new SocketsHttpHandler()
    {
        ConnectCallback = async (context, cancellationToken) =>
        {
            var ipAddress = dnsResolver.Resolve(context.DnsEndPoint.Host);
            

            var socket = new Socket(SocketType.Stream, ProtocolType.Tcp)
            {
                // Turn off Nagle's algorithm since it degrades
                // performance in most HttpClient scenarios.
                NoDelay = true
            };

            try
            {
                await socket.ConnectAsync(
                    ipAddress,
                    context.DnsEndPoint.Port,
                    cancellationToken);
                return new NetworkStream(socket, ownsSocket: true);
            }
            catch
            {
                socket.Dispose();
                throw;
            }
        },
    };

    var client = new HttpClient(sockerHttpHandler);
    var html = await client.GetStringAsync("https://google.com");
}

public class CustomDnsResolver
{
    public IPAddress Resolve(string host)
    {
        return IPAddress.Parse("216.58.214.164");
    }
}