title: GetHostEntry is very slow
body: <p>I have a WinForms app, and I'm trying to get reverse DNS entries for a list of IPs displayed on the form. </p>
  
  <p>The main issue I've run into is System.Net.Dns.GetHostEntry is ridiculously slow, particularly when no reverse DNS entry is found. With straight DNS, this should be fast, since the DNS server will return NXDOMAIN. Internally, it's calling <a href="http://msdn.microsoft.com/en-us/library/ms738532(VS.85).aspx" rel="noreferrer">ws2_32.dll getnameinfo()</a>, which states "Name resolution can be by the Domain Name System (DNS), a local hosts file, or by other naming mechanisms" - so I'm assuming it's those "other naming mechanisms" that's causing it to be so slow, but does anyone know what those mechanisms are?</p>
  
  <p>Generally this is taking 5 seconds per IP, unless it finds a reverse entry, and then it's almost immediate. I've partly worked around this using threads, but since I am doing a large list and I can only run so many threads at once, it still takes a while to get through them all.</p>
  
  <p>Is there a better way to find reverse DNS entries that is going to be faster? </p>