Don't explicitly sent Connection: close to upstream

[abhinavsingh]

Without proxy.py using telnet

$ telnet httpbin.org 80
Trying 3.219.197.134...
Connected to httpbin.org.
Escape character is '^]'.
GET /get HTTP/1.1
Host: httpbin.org

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Content-Type: application/json
Date: Sat, 21 Sep 2019 22:28:28 GMT
Referrer-Policy: no-referrer-when-downgrade
Server: nginx
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
Content-Length: 146
Connection: keep-alive

{
  "args": {}, 
  "headers": {
    "Host": "httpbin.org"
  }, 
  "origin": "1.2.3.4, 1.2.3.5", 
  "url": "https://httpbin.org/get"
}

^^^ Above connection hasn't dropped. As evident from server response Connection: keep-alive. Now, we can continue requesting further data over same connection until server drops the connection due to client inactivity or other reasons.

GET /get HTTP/1.1
Host: httpbin.org

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Content-Type: application/json
Date: Sat, 21 Sep 2019 22:28:38 GMT
Referrer-Policy: no-referrer-when-downgrade
Server: nginx
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
Content-Length: 146
Connection: keep-alive

{
  "args": {}, 
  "headers": {
    "Host": "httpbin.org"
  }, 
  "origin": "1.2.3.4, 1.2.3.5", 
  "url": "https://httpbin.org/get"
}
^]
telnet> quit
Connection closed.

With proxy.py using telnet

Same behavior doesn't work when pointing telnet to proxy.py

$ telnet localhost 8899
Trying ::1...
Connected to localhost.
Escape character is '^]'.
GET http://httpbin.org/get HTTP/1.1
Host: httpbin.org

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Content-Type: application/json
Date: Sat, 21 Sep 2019 22:34:11 GMT
Referrer-Policy: no-referrer-when-downgrade
Server: nginx
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
Content-Length: 146
Connection: Close

{
  "args": {}, 
  "headers": {
    "Host": "httpbin.org"
  }, 
  "origin": "1.2.3.4, 1.2.3.5", 
  "url": "https://httpbin.org/get"
}
^]
telnet> quit
Connection closed.

^^^ As evident from server header Connection: Close, connection was dropped by the server. Same can also be verified from proxy.py logs.

This is happening because currently proxy.py explicitly adds a Connection: Close header when sending requests to upstream server. We should fix this and only use Connection: Close if client has explicitly requested the same.

Cross reference https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Connection