diff --git a/changelog.d/10578.feature b/changelog.d/10578.feature
new file mode 100644
index 0000000000..02397f0009
--- /dev/null
+++ b/changelog.d/10578.feature
@@ -0,0 +1 @@
+Add an admin API (`GET /_synapse/admin/username_available`) to check if a username is available (regardless of registration settings).
\ No newline at end of file
diff --git a/docs/admin_api/user_admin_api.md b/docs/admin_api/user_admin_api.md
index 33811f5bbb..4b5dd4685a 100644
--- a/docs/admin_api/user_admin_api.md
+++ b/docs/admin_api/user_admin_api.md
@@ -1057,3 +1057,23 @@ The following parameters should be set in the URL:
- `user_id` - The fully qualified MXID: for example, `@user:server.com`. The user must
be local.
+
+### Check username availability
+
+Checks to see if a username is available, and valid, for the server. See [the client-server
+API](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available)
+for more information.
+
+This endpoint will work even if registration is disabled on the server, unlike
+`/_matrix/client/r0/register/available`.
+
+The API is:
+
+```
+POST /_synapse/admin/v1/username_availabile?username=$localpart
+```
+
+The request and response format is the same as the [/_matrix/client/r0/register/available](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available) API.
+
+To use it, you will need to authenticate by providing an `access_token` for a
+server admin: [Admin API](../usage/administration/admin_api)
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index abf749b001..8a91068092 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -51,6 +51,7 @@ from synapse.rest.admin.rooms import (
)
from synapse.rest.admin.server_notice_servlet import SendServerNoticeServlet
from synapse.rest.admin.statistics import UserMediaStatisticsRestServlet
+from synapse.rest.admin.username_available import UsernameAvailableRestServlet
from synapse.rest.admin.users import (
AccountValidityRenewServlet,
DeactivateAccountRestServlet,
@@ -241,6 +242,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
ForwardExtremitiesRestServlet(hs).register(http_server)
RoomEventContextServlet(hs).register(http_server)
RateLimitRestServlet(hs).register(http_server)
+ UsernameAvailableRestServlet(hs).register(http_server)
def register_servlets_for_client_rest_resource(
diff --git a/synapse/rest/admin/username_available.py b/synapse/rest/admin/username_available.py
new file mode 100644
index 0000000000..2bf1472967
--- /dev/null
+++ b/synapse/rest/admin/username_available.py
@@ -0,0 +1,51 @@
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Tuple
+
+from synapse.http.servlet import RestServlet, parse_string
+from synapse.http.site import SynapseRequest
+from synapse.rest.admin._base import admin_patterns, assert_requester_is_admin
+from synapse.types import JsonDict
+
+if TYPE_CHECKING:
+ from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class UsernameAvailableRestServlet(RestServlet):
+ """An admin API to check if a given username is available, regardless of whether registration is enabled.
+
+ Example:
+ GET /_synapse/admin/v1/username_available?username=foo
+ 200 OK
+ {
+ "available": true
+ }
+ """
+
+ PATTERNS = admin_patterns("/username_available")
+
+ def __init__(self, hs: "HomeServer"):
+ self.auth = hs.get_auth()
+ self.registration_handler = hs.get_registration_handler()
+
+ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+ await assert_requester_is_admin(self.auth, request)
+
+ username = parse_string(request, "username", required=True)
+ await self.registration_handler.check_username(username)
+ return HTTPStatus.OK, {"available": True}
diff --git a/tests/rest/admin/test_username_available.py b/tests/rest/admin/test_username_available.py
new file mode 100644
index 0000000000..53cbc8ddab
--- /dev/null
+++ b/tests/rest/admin/test_username_available.py
@@ -0,0 +1,62 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import synapse.rest.admin
+from synapse.api.errors import Codes, SynapseError
+from synapse.rest.client.v1 import login
+
+from tests import unittest
+
+
+class UsernameAvailableTestCase(unittest.HomeserverTestCase):
+ servlets = [
+ synapse.rest.admin.register_servlets,
+ login.register_servlets,
+ ]
+ url = "/_synapse/admin/v1/username_available"
+
+ def prepare(self, reactor, clock, hs):
+ self.register_user("admin", "pass", admin=True)
+ self.admin_user_tok = self.login("admin", "pass")
+
+ async def check_username(username):
+ if username == "allowed":
+ return True
+ raise SynapseError(400, "User ID already taken.", errcode=Codes.USER_IN_USE)
+
+ handler = self.hs.get_registration_handler()
+ handler.check_username = check_username
+
+ def test_username_available(self):
+ """
+ The endpoint should return a 200 response if the username does not exist
+ """
+
+ url = "%s?username=%s" % (self.url, "allowed")
+ channel = self.make_request("GET", url, None, self.admin_user_tok)
+
+ self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+ self.assertTrue(channel.json_body["available"])
+
+ def test_username_unavailable(self):
+ """
+ The endpoint should return a 200 response if the username does not exist
+ """
+
+ url = "%s?username=%s" % (self.url, "disallowed")
+ channel = self.make_request("GET", url, None, self.admin_user_tok)
+
+ self.assertEqual(400, int(channel.result["code"]), msg=channel.result["body"])
+ self.assertEqual(channel.json_body["errcode"], "M_USER_IN_USE")
+ self.assertEqual(channel.json_body["error"], "User ID already taken.")
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