A new plugin for host-specific routing

The Dancer2 team decided this year to turn on GitHub Discussions, and the first new-feature idea that came over the transom was a way to have routes respond only for specific request hosts.

It is, after all, trivial to have nginx or most any other web server that you might be using as a proxy to your Dancer2 application only be sent information from a specific host:

upstream dancer_url {
  server unix:/srv/dancer_app/DEV/server.sock;
server {
  server_name  my_dancer_app.org;
  server_name  admin.my_dancer_app.org;

  location / {
    try_files $uri @proxy;

  location @proxy {
    proxy_pass       http://dancer_url;

...but how to make specific routes respond only to a specific host? My little testbed application, the JokeyBot, does this in a before hook:

hook before => sub {
   var dirty      => 0;
   if ( request->base =~ /dirty\.jokeybot/ ) { var dirty => 1; }

Thus, if you would like to see jokes that are, shall we say, less acceptable in polite company, you just go to https://dirty.jokeybot.com. The var in the hook is used in all the routes needed to inject search parameters for jokes that have been marked as "dirty." (As an aside, I must in all fairness warn you that I alone curate jokes into the JokeyBot, so the tags and sensibilities of a joke are all mine, and don't represent those of any professional comedian, my employer, or anyone else.)

Okay, so why a new plugin?

It may be that you want certain routes to only respond to a specific URL hostname, or that you may have a wildly different function for that hostname, that uses the same database. An example of this might be if you had some sort of administrative interface, at https://admin.my-dancer-app.org, and you wanted those routes to behave completely differently if you point at them from the main address.

Also, let's face it, TIMTOWTDI. A plugin that gave a route predicate for host-specific routing would be another way to do what I did on the JokeyBot. The idea had me curious, so I tinkered around for a few hours, and Dancer2::Plugin::HostSpecificRoute was born.

The host Predicate

Dancer2::Plugin::HostSpecificRoute introduced only one new keyword, host, which you use as a "predicate" in the introduction to a route, like so:

get '/' => host 'special.my-dancer-app.com' => sub {
   # special code here 

get '/' => sub {
   # default code here  

Using it couldn't be easier! You could, of course, extend this to as many routes as you need for as many "special" variants as your web server will respond to. There are a couple of tips I should give you, though:

  • Always put your default route (if any) last.

    You don't have to have a default fall-back route; if all of the routes with host predicates fail to match, the Dancer2 application will simply return a 404 error, as no route matches the request. If you choose to have a default behavior, put it last! For this reason, it's useful to keep them all in the same file, if you're splitting your routes across multiple files, just so include order doesn't trip you up.

  • You can use a regex for the host

    Simply state your predicate with a regex, like so: host qr/\.funkyhost.example$/, and then any request that otherwise matches the route that is adressed to *.funkyhost.example will match.

  • You don't have to use this for every route

    You probably want some routes to behave the same way for all hosts that might go there--that's fine, just don't include a host predicate on those; they'll behave as normal Dancer2 routes.


This little plugin was a simple one to write, and it made me very happy to be able to answer a request from a Dancer2 user for this feature. If it's something you can use in your own applications, I hope it brings you as much joy as I got out of building it. Happy Dancing!


This article was written by D Ruth Holloway for the Dancer Advent Calendar 2023.