http://stackoverflow.com/questions/24949055/default-value-fo

# routing.yml

app:
    resource: @AppBundle/Controller/
    type:     annotation
    prefix:   /{_locale}
    host:     "{domain}.example.com"
    requirements:
        _locale: "%available_locales%"
        domain: "[a-z0-9]+"

# services.yml

services:
    listener.pathparameters:
        class: AppBundle\EventListener\PathParameters
        arguments: [ @service_container ]
        tags:
            - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest }

# AppBundle\EventListener\PathParameters

<?php

namespace AppBundle\EventListener;

use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
use Symfony\Component\HttpFoundation\Cookie;
use Symfony\Component\DependencyInjection\ContainerInterface;

class PathParameters
{
    protected $container;

    public function __construct(ContainerInterface $container) // this is @service_container
    {
        $this->container = $container;
    }

    public function onKernelRequest(GetResponseEvent $event)
    {
        $request   = $event->getRequest();
        $this->container->get('router.request_context')->setParameter('domain', $request->attributes->get('domain'));
    }
}