How to improve slow DNS with HttpClient?-CodePudding

I have one request for a specific domain that take a really long time to complete : 22 seconds in average. The request itself does not return a lot of data.

var httpClient = new HttpClient();  //instantiated at app start and reused 
var request = new HttpRequestMessage(HttpMethod.Get, "http://www.somedomain.com");
var result = await httpClient.SendAsync(request); //take a really long time

After some debugging, it seems related to DNS. If I run the exact same request but replace domain name by IP, it's fast (< 200 ms).

It's also fast if I run the request a second time using same HttpClient instance. However, if I run that second request after more than one minute it's slow again (probably because connections are closed automatically after 1 minute).

Is there a way to improve this ? (eg: to make sure resolved DNS entries would stay in cache for a long period). Also why is DNS so slow for that domain when using HttpClient ? If I try to access that domain using Chrome it's blazing fast (even after flushing DNS and clearing cache). A DNS lookup of that domain using an online service is also very fast. Reported TTL is 60 minutes.

EDIT: I have tried to increase ServicePointManager.DnsRefreshTimeout (which is set by default to 2 minutes). But it does not help.

Calling Dns.GetHostEntry("somedomain.com") is instantaneous and return the right IP. I don't know why request with HttpClient are so slow.

CodePudding user response：

It's not only about caching DNS query. If you create new client for every request, then before every request both TCP and HTTP handshakes are made -- which makes your single request slow. This is probably the reason why subsequent requests with "old" agent are faster -- handshakes are only made before first request, but not before second one.

CodePudding user response：

I found what's wrong : That domain has a IPV6 address as well as IPV4. This can be confirmed by running the following code :

Dns.GetHostEntry("somedomain.com")

Connection using IPV6 throws an exception after some timeout (which explain why it's so slow). Solution I found so far is to force IPV4 by resolving domain myself :

var uri = new Uri("somedomain.com");
var builder = new UriBuilder(uri);
var addresses = await Dns.GetHostAddressesAsync(uri.Host);    
builder.Host = addresses
                 .First(x => x.AddressFamily == AddressFamily.InterNetwork) //force IPV4
                 .ToString(); 
var request = new HttpRequestMessage(HttpMethod.Get, builder.Uri);
request.Host = uri.Host;

Maybe there is a better way. I can't use SocketsHttpHandler since project is running .NET Framework 4.5