title: Questions about options in http settings "overide host name" and "pick host name from backend address"
body: <p>I did some test for http settings options "overide host name" and "pick host name from backend address", but got a little confused as the result was different for different type of backend pools.</p>
  
  <p>APPGW: test.cloudapp.net</p>
  
  <p>Backend pool1: VM1(IP1) (azure vm in same virtual network with APPGW); frontend url: test.cloudapp.net:8081</p>
  
  <p>Backend pool2: FQDN - test.azurewebsites.net(azure web app); frontend url: test.cloudapp.net:8082</p>
  
  <p>(1) test for option "override host name"
  For both 2 backend pools, set the value of option "override host name" in http settings to "test.azurewebsites.net" which is the FQDN of backend pool2.</p>
  
  <p>Try to access url test.cloudapp.net:8081 and test.cloudapp.net:8082, I thought it should return the same result from test.azurewebsites.net. But the result was that: 
  For test.cloudapp.net:8082, it returned content of test.azurewebsites.net which was expected.
  But for test.cloudapp.net:8081, it returned content on VM1. My question is that why it didn't return the content of test.azurewebsites.net. </p>
  
  <p>(2) test for option "pick host name from backend address"
  When this option was disabled by default, I can't visit url test.cloudapp.net:8082 while test.cloudapp.net:8081 was accessable with correct result.
  When this option was enabled, both url test.cloudapp.net:8081 and test.cloudapp.net:8082 were accessable with correct result.
  My question is: When this option is disabled, what's the host name in http header when APPGW send http request packet to backend pool1 and backend pool2?
  When this option is enabled, I think the host name in http packet sent to backend pool1 should be IP1 and to backend pool2 should be "test.azurewebsites.net", but not sure.</p>
  
  <p>Really appreciate if someone could provide detailed explanation for my test result and confusion. </p>