diff --git a/src/wormhole/server/cli_args.py b/src/wormhole/server/cli_args.py
index 4bc87e1..f6b32b5 100644
--- a/src/wormhole/server/cli_args.py
+++ b/src/wormhole/server/cli_args.py
@@ -24,6 +24,8 @@ sp_start.add_argument("--transit", default="tcp:3001", metavar="tcp:PORT",
                       help="endpoint specification for the transit-relay port")
 sp_start.add_argument("--advertise-version", metavar="VERSION",
                       help="version to recommend to clients")
+sp_start.add_argument("--signal-error", metavar="ERROR",
+                      help="force all clients to fail with a message")
 sp_start.add_argument("--blur-usage", default=None, type=int,
                       metavar="SECONDS",
                       help="round logged access times to improve privacy")
diff --git a/src/wormhole/server/cmd_server.py b/src/wormhole/server/cmd_server.py
index 938d451..02e6c66 100644
--- a/src/wormhole/server/cmd_server.py
+++ b/src/wormhole/server/cmd_server.py
@@ -13,7 +13,9 @@ class MyPlugin:
         from .server import RelayServer
         return RelayServer(self.args.rendezvous, self.args.transit,
                            self.args.advertise_version,
-                           "relay.sqlite", self.args.blur_usage)
+                           "relay.sqlite", self.args.blur_usage,
+                           signal_error=self.args.signal_error,
+                           )
 
 class MyTwistdConfig(twistd.ServerOptions):
     subCommands = [("XYZ", None, usage.Options, "node")]
diff --git a/src/wormhole/server/server.py b/src/wormhole/server/server.py
index 166a6ba..e182e1d 100644
--- a/src/wormhole/server/server.py
+++ b/src/wormhole/server/server.py
@@ -26,7 +26,8 @@ class PrivacyEnhancedSite(server.Site):
 
 class RelayServer(service.MultiService):
     def __init__(self, rendezvous_web_port, transit_port,
-                 advertise_version, db_url=":memory:", blur_usage=None):
+                 advertise_version, db_url=":memory:", blur_usage=None,
+                 signal_error=None):
         service.MultiService.__init__(self)
         self._blur_usage = blur_usage
 
@@ -42,6 +43,8 @@ class RelayServer(service.MultiService):
             }
         if advertise_version:
             welcome["current_version"] = advertise_version
+        if signal_error:
+            welcome["error"] = signal_error
 
         rendezvous = Rendezvous(db, welcome, blur_usage)
         rendezvous.setServiceParent(self) # for the pruning timer