HAProxy: Route requests based on an environment variable

haproxy.cfg

frontend http
  bind 10.0.0.1:80
  mode http

  # Add a new header with the environment variable and the path concatenated
  http-request set-header X-ROUTING %[env(SERVER_AUTH)]::%[path]
  # Then compare the value using a regular expression with back-references
  # We need to use a named capture because of "reasons". Backreferences to normal captures seem to be rejected by HAProxy
  use_backend app if { req.hdr(X-ROUTING) -m reg ^(?<auth>.+)==\1 }
  # Cleanup in case we hadn't matched yet
  http-request del-header X-ROUTING
  
  default_backend default

backend app
  # ...
  
backend default
  # ...