diff --git a/bin/check b/bin/check
index 646491f..da298f6 100755
--- a/bin/check
+++ b/bin/check
@@ -1,4 +1,20 @@
 #!/bin/bash
+#
+# Check if a service is running/working properly
+#
+# The script checks that the internal IP is reachable, the service
+# replies on expected ports, and any public ports are reachable on
+# the external IP.
+#
+# Further checks can be added by adding a
+# /federated/services/$SERVICE/check file
+#
+
+if [ "$#" != 1 ]; then
+	echo "Usage: $0 <service>"
+	exit 1
+fi
+
 SERVICE="$1"
 
 . /federated/lib/functions.sh