How Short Links Work

In some organizations, you may have seen “short links” like

http://go/foo-bar

or

http://x/12345.

These are short for (and resolve to) something else, like https://example.com/go/foo-bar or https://example.com/x/reports/12345.

One way to set this up is to use a proxy and something called Proxy Auto Configuration (PAC). A PAC file is a small snippet of Javascript that directs browsers to use a proxy for certain hostnames. For more details, see MDN.

Here’s a simple setup for macOS using the Caddy proxy that will let you type http://g/foo bar and it will take you to http://google.com/search?q=foo+bar. (This example is a bit of a contrived, since all browsers today support querying a search engine in the URL bar, but it gets the idea across).

Install Caddy:

$ brew install caddy

In a new file proxyconfig.pac:

function FindProxyForURL(url, host) {
  if (host == "g") {
    return "PROXY localhost:9999";
  } else {
    return "DIRECT";
  }
}

For details on how the PAC file works, see the aforementioned MDN page.

Next, we’ll configure Caddy to (a) serve a static PAC file, and (b) proxy http://g requests to Google. In a new file caddyfile in the same directory:

# Serve up the PAC file.
http://localhost:9999 {
    log
    handle /proxyconfig.pac {
        file_server {
        }
    }
}
# Rewrite http://g requests.
http://g:9999 {
    log
    @mymatcher {
        path_regexp /(.*)
    }
    redir @mymatcher http://www.google.com/search?q={http.regexp.1}
}
# Redirect requests for Google.
http://, https:// {
    redir {scheme}{hostport}{uri}
}

(I’m new to Caddy so the above may not be good form, secure, etc. You’ve been warned.)

Start Caddy:

$ caddy run

Install the PAC file on macOS by opening System Preferences, go to Network > Advanced > Proxies > Automatic Proxy Configuration. Set the Proxy Configuration File URL to http://localhost:9999/proxyconfig.pac.¹

Now, you should be able to type in g/foo bar into your browser, hit enter, and it should direct you to Google’s search results for “foo bar”. At least in Chrome, when you type it in, there may be two results in the drop-down suggestion list - you need to make sure to select the blue one (you should only need to do this once).

If you modify the PAC file, your browser may not immediately pick the change up. To force Chrome to do so, go to chrome://net-internals#proxy and click ‘Re-apply settings’.