docs: def lookup(self):
          """
          The meat of this middleware.
  
          Returns None and sets settings.SITE_ID if able to find a Site
          object by domain and its subdomain is valid.
  
          Returns an HttpResponsePermanentRedirect to the Site's default
          subdomain if a site is found but the requested subdomain
          is not supported, or if domain_unsplit is defined in
          settings.HOSTNAME_REDIRECTS
  
          Otherwise, returns False.
          """
  
          # check to see if this hostname is actually a env hostname
          if self.domain:
              if self.subdomain:
                  self.domain_unsplit = '%s.%s' % (self.subdomain, self.domain)
              else:
                  self.domain_unsplit = self.domain
              self.domain_requested = self.domain_unsplit
  
          # check cache
          cache_key = 'site_id:%s' % self.domain_unsplit
          site_id = cache.get(cache_key)
          if site_id:
              SITE_ID.value = site_id
              try:
                  self.site = Site.objects.get(id=site_id)
              except Site.DoesNotExist:
                  # This might happen if the Site object was deleted from the
                  # database after it was cached.  Remove from cache and act
                  # as if the cache lookup failed.
                  cache.delete(cache_key)
              else:
                  return None
  
          # check database
          try:
              self.site = Site.objects.get(domain=self.domain)
          except Site.DoesNotExist:
              return False
          if not self.site:
              return False
  
          SITE_ID.value = self.site.pk
          cache.set(cache_key, SITE_ID.value, 5*60)
          return None