httprequest.UserHostName in .net core

The accepted answer is not correct. someHttpContext.Request.Host corresponds to the Host header used in the HTTP request. For example: fetching http://example.org/test will result in example.org.

UserHostName used to return the DNS hostname of the client IP address making the request (which is someHttpContext.Connection.RemoteIpAddress). It looks as though the only recourse is to manually resolve the DNS hostname from the IP address with e.g. Dns.GetHostEntry.

In "old" ASP.NET this was relying on IIS, that puts user hostname into "REMOTE_HOST" server variable.

Since ASP.NET Core is cross platform, it does not have this feature.

But if you run on Windows, and on IIS, you can still access it via HttpContext

var svf = httpContext.Features.Get<IServerVariablesFeature>();
if (svf == null)
{
    //we're on linux, sorry, no luck
}
else
{
    var hosteName = svf["REMOTE_HOST"];
}