proxy-from-env is a Node.js package that exports a function (getProxyForUrl) that takes an input URL (a string or url.parse's return value) and returns the desired proxy URL (also a string) based on standard proxy environment variables. If no proxy is set, an empty string is returned.

It is your responsibility to actually proxy the request using the given URL.

Installation:

npm install proxy-from-env

Example

This example shows how the data for a URL can be fetched via the http module, in a proxy-aware way.

var http = require('http');
var parseUrl = require('url').parse;
var getProxyForUrl = require('proxy-from-env').getProxyForUrl;

var some_url = 'http://example.com/something';

// // Example, if there is a proxy server at 10.0.0.1:1234, then setting the
// // http_proxy environment variable causes the request to go through a proxy.
// process.env.http_proxy = 'http://10.0.0.1:1234';
// 
// // But if the host to be proxied is listed in NO_PROXY, then the request is
// // not proxied (but a direct request is made).
// process.env.no_proxy = 'example.com';

var proxy_url = getProxyForUrl(some_url);  // <-- Our magic.
if (proxy_url) {
  // Should be proxied through proxy_url.
  var parsed_some_url = parseUrl(some_url);
  var parsed_proxy_url = parseUrl(proxy_url);
  // A HTTP proxy is quite simple. It is similar to a normal request, except the
  // path is an absolute URL, and the proxied URL's host is put in the header
  // instead of the server's actual host.
  httpOptions = {
    protocol: parsed_proxy_url.protocol,
    hostname: parsed_proxy_url.hostname,
    port: parsed_proxy_url.port,
    path: parsed_some_url.href,
    headers: {
      Host: parsed_some_url.host,  // = host name + optional port.
    },
  };
} else {
  // Direct request.
  httpOptions = some_url;
}
http.get(httpOptions, function(res) {
  var responses = [];
  res.on('data', function(chunk) { responses.push(chunk); });
  res.on('end', function() { console.log(responses.join(''));  });
});

Environment variables

The environment variables can be specified in lowercase or uppercase, with the lowercase name having precedence over the uppercase variant. A variable that is not set has the same meaning as a variable that is set but has no value.

NO_PROXY

NO_PROXY is a list of host names (optionally with a port). If the input URL matches any of the entries in NO_PROXY, then the input URL should be fetched by a direct request (i.e. without a proxy).

Matching follows the following rules:

NO_PROXY=* disables all proxies.
Space and commas may be used to separate the entries in the NO_PROXY list.
If NO_PROXY does not contain any entries, then proxies are never disabled.
If a port is added after the host name, then the ports must match. If the URL does not have an explicit port name, the protocol's default port is used.
Generally, the proxy is only disabled if the host name is an exact match for an entry in the NO_PROXY list. The only exceptions are entries that start with a dot or with a wildcard; then the proxy is disabled if the host name ends with the entry.

See test.js for examples of what should match and what does not.

*_PROXY

The environment variable used for the proxy depends on the protocol of the URL. For example, https://example.com uses the "https" protocol, and therefore the proxy to be used is HTTPS_PROXY (NOT HTTP_PROXY, which is only used for http:-URLs).

The library is not limited to http(s), other schemes such as FTP_PROXY (ftp:), WSS_PROXY (wss:), WS_PROXY (ws:) are also supported.

If present, ALL_PROXY is used as fallback if there is no other match.

External resources

The exact way of parsing the environment variables is not codified in any standard. This library is designed to be compatible with formats as expected by existing software. The following resources were used to determine the desired behavior:

cURL: https://curl.haxx.se/docs/manpage.html#ENVIRONMENT
https://github.com/curl/curl/blob/4af40b3646d3b09f68e419f7ca866ff395d1f897/lib/url.c#L4446-L4514
https://github.com/curl/curl/blob/4af40b3646d3b09f68e419f7ca866ff395d1f897/lib/url.c#L4608-L4638
wget: https://www.gnu.org/software/wget/manual/wget.html#Proxies
http://git.savannah.gnu.org/cgit/wget.git/tree/src/init.c?id=636a5f9a1c508aa39e35a3a8e9e54520a284d93d#n383
http://git.savannah.gnu.org/cgit/wget.git/tree/src/retr.c?id=93c1517c4071c4288ba5a4b038e7634e4c6b5482#n1278
W3: https://www.w3.org/Daemon/User/Proxies/ProxyClients.html
Python's urllib: https://github.com/python/cpython/blob/936135bb97fe04223aa30ca6e98eac8f3ed6b349/Lib/urllib/request.py#L755-L782
https://github.com/python/cpython/blob/936135bb97fe04223aa30ca6e98eac8f3ed6b349/Lib/urllib/request.py#L2444-L2479

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