No support for relative server_port overrides

haproxy supports relative port addresses, prefixed by a "+" or "-".

If this is set the server port is determined by adding the value to the client's port. In the base service_watcher there is currently a check to verify that the override port is an integer which breaks existing configurations.

I assume nginx doesn't support a similar behaviour, would you accept a patch or should we just maintain our own version?

Start with the base service_watcher check that requires server_port overrides to be integers. Compare HAProxy's +/− relative-port behavior with the existing configuration handling; done means valid relative overrides no longer fail validation while existing configurations remain supported.

Written by the indexing model from the issue text.