Proxying Golang Web Applications

2019-07-17

Recently someone posted an issue asking if Faktory could support putting nginx in front of Faktory’s Web UI. Normally you access the Web UI like http://localhost:7420/ but they wanted it to look something like http://somehost:8080/faktory. That’s quite common when trying to wrap multiple systems into something that looks like one website to the browser.

The issue is that the Web UI assumed it was at the root, so it hardcoded paths like /static/application.css. If you mount the Web UI at /faktory, you want that CSS URL to become /faktory/static/application.css.

After thirty minutes of Googling, I could find nothing on how to solve this problem so I put on my thinking cap and ground it out over 3-4 hours today. Here’s the scoop.

nginx Configuration

Here’s the nginx configuration to forward /faktory to Faktory at its default URL. We set a number of headers which might be useful in the future but only X-Script-Name is critical. Note the /faktory in the first two lines needs to stay in sync.

location /faktory {
  proxy_set_header X-Script-Name /faktory;

  proxy_pass   http://127.0.0.1:7420;
  proxy_set_header Host $host;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_set_header X-Scheme $scheme;
  proxy_set_header X-Real-IP $remote_addr;
}

SCRIPT_NAME is a legacy of CGI but it’s used by Python and Ruby apps to know the proxy prefix for requests coming to an app. We use a properly named HTTP header, X-Script-Name, in each request to signal this value to Faktory.

Abstract that Mux!

Go’s http package exposes a Mux type which acts as the router in a Go webapp. We tell it to route all /static/ requests like this:

app := http.NewServeMux()
app.HandleFunc("/static/", staticHandler)

But this has one very big problem: it doesn’t know about the /faktory prefix!

The trick I figured out is to add a special Mux before the application Mux which matches all requests and can unmangle any proxied requests so they look like an unproxied request to the app Mux:

app := http.NewServeMux()
app.HandleFunc("/static/", staticHandler)

proxy := http.NewServeMux()
proxy.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
  prefix := r.Header.Get("X-Script-Name")
  if prefix != "" {
    // this is super greasy, not sure it's optimal but Works For Me™
    r.RequestURI = strings.Replace(r.RequestURI, prefix, "", 1)
    r.URL.Path = r.RequestURI
  }
  app.ServeHTTP(w, r)
})

Every Link must be Relative

Within the HTML markup, I had to change every URL and path to use a helper to generate the relative path with any necessary prefix.

func fullpath(req *http.Request, relpath string) string {
	return fmt.Sprintf("%s%s", req.Header.Get("X-Script-Name"), relpath)
}

<link href="<%= fullpath(req, "/static/application.css") %>" ...>

req.Header.Get returns "" when there is no value set so the base case is handled smoothly.

Conclusion

No rocket science here but it was interesting to find a non-trivial HTTP edge case in Go without a blog post on the subject and very rewarding to solve it myself. Here’s the big commit that landed in Faktory to add proxy support. I hope this helps someone else!