keep-alive connection with Superagent

I'm running a node.js server A which uses superagent to issue HTTP requests to another server B.

I investigated the request on server B and saw the the header connection being close and the httpVersion being 1.1:

var http = require('http');
var request = require('superagent');

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.write('req.httpVersion seen on server:' + req.httpVersion);
  res.write('\nreq.headers.connection seen on server:' + req.headers.connection);
  res.end();
}).listen(1337, '0.0.0.0');

request
  .get('localhost:1337/helloword')
  .end(function (err, res) {
    console.log(res.text);
  });

This leads to:

req.httpVersion seen on server:1.1
req.headers.connection seen on server:close

However if I access the same server from a browser I get:

req.httpVersion seen on server:1.1
req.headers.connection seen on server:keep-alive

From https://www.rfc-editor.org/rfc/rfc2616#page-172 I learned that keep-alive is the default for HTTP 1.1 unless declared otherwise by using Connection: close.

So, my questions are:

Why does Superagent / Node.js explicitly set the request to not use keep-alive / persistent connections?
How can I force Superagent / Node.js to use keep-alive connections?
How can I further influence the exact behaviour (how many connections to keep open, timeouts etc.)?

Solution

It doesn't seem to be documented but you can pass an http agent to superagent with the function agent. So you could create a keep-alive agent with this module: https://www.npmjs.org/package/agentkeepalive and pass it to superagent.

Something like this:

util = require('util');
util.debuglog = require('debuglog');
var http = require('http');
var request = require('superagent');
var Agent = require('agentkeepalive');

var keepaliveAgent = new Agent({
  maxSockets: 100,
  maxFreeSockets: 10,
  timeout: 60000,
  keepAliveTimeout: 30000 // free socket keepalive for 30 seconds
});

http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.write('req.httpVersion seen on server:' + req.httpVersion);
  res.write('\nreq.headers.connection seen on server:' + req.headers.connection);
  res.end();
}).listen(1337, '0.0.0.0');

request
  .get('localhost:1337/helloword')
  .agent(keepaliveAgent)
  .end(function (err, res) {
    console.log(res.text);
  });