diff --git a/changelog.d/10115.bugfix b/changelog.d/10115.bugfix
new file mode 100644
index 000000000000..e16f356e6819
--- /dev/null
+++ b/changelog.d/10115.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in Synapse v1.25.0 that prevented the `ip_range_whitelist` configuration option from working for federation and identity servers. Contributed by @mikure.
diff --git a/synapse/config/server.py b/synapse/config/server.py
index c290a35a9285..0833a5f7bcab 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -397,19 +397,22 @@ def read_config(self, config, **kwargs):
         self.ip_range_whitelist = generate_ip_set(
             config.get("ip_range_whitelist", ()), config_path=("ip_range_whitelist",)
         )
-
         # The federation_ip_range_blacklist is used for backwards-compatibility
-        # and only applies to federation and identity servers. If it is not given,
-        # default to ip_range_blacklist.
-        federation_ip_range_blacklist = config.get(
-            "federation_ip_range_blacklist", ip_range_blacklist
-        )
-        # Always blacklist 0.0.0.0, ::
-        self.federation_ip_range_blacklist = generate_ip_set(
-            federation_ip_range_blacklist,
-            ["0.0.0.0", "::"],
-            config_path=("federation_ip_range_blacklist",),
-        )
+        # and only applies to federation and identity servers.
+        if "federation_ip_range_blacklist" in config:
+            # Always blacklist 0.0.0.0, ::
+            self.federation_ip_range_blacklist = generate_ip_set(
+                config["federation_ip_range_blacklist"],
+                ["0.0.0.0", "::"],
+                config_path=("federation_ip_range_blacklist",),
+            )
+            # 'federation_ip_range_whitelist' was never a supported configuration option.
+            self.federation_ip_range_whitelist = None
+        else:
+            # No backwards-compatiblity requrired, as federation_ip_range_blacklist
+            # is not given. Default to ip_range_blacklist and ip_range_whitelist.
+            self.federation_ip_range_blacklist = self.ip_range_blacklist
+            self.federation_ip_range_whitelist = self.ip_range_whitelist
 
         # (undocumented) option for torturing the worker-mode replication a bit,
         # for testing. The value defines the number of milliseconds to pause before
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 629373fc475b..b8849c0150ae 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -318,7 +318,9 @@ def __init__(self, hs, tls_client_options_factory):
         # We need to use a DNS resolver which filters out blacklisted IP
         # addresses, to prevent DNS rebinding.
         self.reactor = BlacklistingReactorWrapper(
-            hs.get_reactor(), None, hs.config.federation_ip_range_blacklist
+            hs.get_reactor(),
+            hs.config.federation_ip_range_whitelist,
+            hs.config.federation_ip_range_blacklist,
         )  # type: ISynapseReactor
 
         user_agent = hs.version_string