1 files changed, 18 insertions, 1 deletions
diff --git a/tests/rest/client/v1/utils.py b/tests/rest/client/v1/utils.py
index 8231a423f3..946740aa5d 100644
--- a/tests/rest/client/v1/utils.py
+++ b/tests/rest/client/v1/utils.py
@@ -542,13 +542,30 @@ class RestHelper:
         if client_redirect_url:
             params["redirectUrl"] = client_redirect_url
 
-        # hit the redirect url (which will issue a cookie and state)
+        # hit the redirect url (which should redirect back to the redirect url. This
+        # is the easiest way of figuring out what the Host header ought to be set to
+        # to keep Synapse happy.
         channel = make_request(
             self.hs.get_reactor(),
             self.site,
             "GET",
             "/_matrix/client/r0/login/sso/redirect?" + urllib.parse.urlencode(params),
         )
+        assert channel.code == 302
+
+        # hit the redirect url again with the right Host header, which should now issue
+        # a cookie and redirect to the SSO provider.
+        location = channel.headers.getRawHeaders("Location")[0]
+        parts = urllib.parse.urlsplit(location)
+        channel = make_request(
+            self.hs.get_reactor(),
+            self.site,
+            "GET",
+            urllib.parse.urlunsplit(("", "") + parts[2:]),
+            custom_headers=[
+                ("Host", parts[1]),
+            ],
+        )
 
         assert channel.code == 302
         channel.extract_cookies(cookies)