diff --git a/p4app/sniff-host.sh b/p4app/sniff-host.sh
index 827b0dc..b97f0de 100755
--- a/p4app/sniff-host.sh
+++ b/p4app/sniff-host.sh
@@ -1,6 +1,12 @@
 #!/bin/sh
 
+if [ $# -ne 2 ]; then
+    echo "$0 host name"
+    exit 1
+fi
+
 set -x
+
 host=$1
 name=$2