Supporting WebFinger with Static Files and Nginx

A few weeks ago, I switched my website from WordPress to a staticly generated site. In doing so, I had to find alternative solutions to some of the things I was doing with WordPress that can’t be easily handled just using static files. One of those things is WebFinger (aka RFC 7033), which is a simple discovery protocol for URIs that might not be usable as locators otherwise, such as account or email URIs. It’s actually not too heavily used right now aside from OpenID Connect, but it’s still something I want to continue supporting.

WebFinger logo by Eran Hammer

In its simplest form, WebFinger involves sending a GET request to a well-known URL, /.well-known/webfinger, with an identifier passed in the resource query parameter. The server responds with a JSON document describing the requested resource. Given that the contents returned depend on the value of a query parameter, it can’t really be done with just static files, but it can with a little help from nginx.

Requirements

I wanted to follow the WebFinger spec as closely as possible, so the basic requirements for my setup were:

  • requests for any of my identifiers return my WebFinger document
  • requests for unrecognized identifiers return a 404 (required by section 4.2)
  • requests with a missing or malformed identifier return a 400 (required by section 4.2)
  • responses include a content type of “application/jrd+json” (required by section 4.2)
  • responses include an appropriate CORS header (required by section 5)

There were a few optional parts of the spec I explicitly chose not to support:

  • no support for content negotiation to request a different response format (allowed in section 4.2)
  • no support for the rel parameter to filter the contents of the response (allowed in section 4.3)

My setup

So, my final nginx configuration for supporting WebFinger is:

location = /.well-known/webfinger {
  if ($request_method !~ ^(GET|HEAD)$) { return 405; }
  set_by_lua $resource 'return ngx.unescape_uri(ngx.req.get_uri_args()["resource"])';
  if ($resource = "") { return 400; }
  if ($resource = "acct:will@willnorris.com")   { rewrite .* /webfinger.json last; }
  if ($resource = "mailto:will@willnorris.com") { rewrite .* /webfinger.json last; }
  if ($resource = "https://willnorris.com")     { rewrite .* /webfinger.json last; }
  if ($resource = "https://willnorris.com/")    { rewrite .* /webfinger.json last; }
}

location = /webfinger.json {
  types { application/jrd+json json; }
  add_header Access-Control-Allow-Origin "*";
}

My first location block is an exact match for the well-known WebFinger path. Within that block, I first enforce that only GET and HEAD requests are accepted, all others receive a 405 response. This isn’t required by the spec, but seems like a good idea.

Next, I use a small lua snippet to URL decode the resource query parameter and store it in a variable. By default, parameters are not URL decoded, but need to be for the string comparison in the next step. The embedded lua module is typically not included in standard nginx installations, but is part of the nginx-extras apt package in Debian and Ubuntu. Alternately, this could be done with the embedded perl module which may be more common, but is a little more work to use since the perl_set directive can’t be used inside location blocks. Or there is the ngx_set_misc module which provides a set_unescape_uri directive specifically for URL decoding values, but that typically requires building nginx from source, which I didn’t want to deal with.

Next, I compare the decoded resource value, returning a 400 response if empty, rewriting the request to my WebFinger document if it matches one of my identifiers, or (implicitly) returning a 404 response if nothing else matched. Because this is my personal domain, I can easily hardcode the identifiers I want to support, and rewrite these requests to a single webfinger.json resource. If I needed to support multiple users or identities, I would likely rewrite the request based on the resource value similar to what Aaron Parecki does with his Apache config.

Finally, I have a location block for my webfinger.json document which adds the correct content type and CORS header.

Possible alternatives

Instead of rewriting the original request, I could have just as easily returned a redirect to my webfinger.json document, but I chose to do it in a single request. Similarly, I also tried moving this file to /.well-known/webfinger directly which worked fine, but the nginx config to ensure that invalid resource values returned a 404 wasn’t quite as readable.

For Jekyll users, Eric Mill’s jekyll-webfinger plugin generates a WebFinger document from a yaml source file, which is really nice. It just focuses on file generation, so it still requires additional webserver configuration like the above to handle proper error codes and content types. Personally, I just edit the JSON file by hand.