Add clear exception if REMOTE_ADDR is empty

Certain configurations of Django when running behind a reverse proxy
that communicates with the app server over Unix sockets can lead to
REMOTE_ADDR being empty or missing. In these situations, IP-based
ratelimiting can behave unexpectedly or fail in unclear ways. This makes
the exception for an empty REMOTE_ADDR more clear and points to the
documentation.

CHANGELOG

@@ -14,6 +14,7 @@ Minor changes:
 --------------
 
 - Factor up _get_ip() logic into a single place (#218)
+- Exception on empty REMOTE_ADDR is clearer (#220)
 
 3.0.1
 =====

ratelimit/core.py

@@ -29,18 +29,24 @@
 def _get_ip(request):
     ip_meta = getattr(settings, 'RATELIMIT_IP_META_KEY', None)
     if not ip_meta:
-        return request.META['REMOTE_ADDR']
-    if callable(ip_meta):
-        return ip_meta(request)
-    if isinstance(ip_meta, str) and '.' in ip_meta:
+        ip = request.META['REMOTE_ADDR']
+        if not ip:
+            raise ImproperlyConfigured(
+                'IP address in REMOTE_ADDR is empty. This can happen when '
+                'using a reverse proxy and connecting to the app server with '
+                'Unix sockets. See the documentation for '
+                'RATELIMIT_IP_META_KEY: https://bit.ly/3iIpy2x')
+    elif callable(ip_meta):
+        ip = ip_meta(request)
+    elif isinstance(ip_meta, str) and '.' in ip_meta:
         ip_meta_fn = import_string(ip_meta)
-        return ip_meta_fn(request)
-    if ip_meta in request.META:
-        return request.META[ip_meta]
-    raise ImproperlyConfigured('Could not get IP address from "%s"' % ip_meta)
-
+        ip = ip_meta_fn(request)
+    elif ip_meta in request.META:
+        ip = request.META[ip_meta]
+    else:
+        raise ImproperlyConfigured(
+            'Could not get IP address from "%s"' % ip_meta)
 
-def ip_mask(ip):
     if ':' in ip:
         # IPv6
         mask = getattr(settings, 'RATELIMIT_IPV6_MASK', 64)
@@ -56,11 +62,11 @@ def ip_mask(ip):
 def user_or_ip(request):
     if request.user.is_authenticated:
         return str(request.user.pk)
-    return ip_mask(_get_ip(request))
+    return _get_ip(request)
 
 
 _SIMPLE_KEYS = {
-    'ip': lambda r: ip_mask(_get_ip(r)),
+    'ip': lambda r: _get_ip(r),
     'user': lambda r: str(r.user.pk),
     'user_or_ip': user_or_ip,
 }

ratelimit/tests.py

@@ -622,3 +622,10 @@ def test_callable_ip_key(self):
         req.META['MY_THING'] = '5.6.7.8'
 
         assert '5.6.7.8' == _get_ip(req)
+
+    def test_empty_ip(self):
+        req = rf.get('/')
+        req.META['REMOTE_ADDR'] = ''
+
+        with self.assertRaises(ImproperlyConfigured):
+            _get_ip(req)