diff --git a/nat-start.sh b/nat-start.sh
index af8c71f..46bdbd8 100755
--- a/nat-start.sh
+++ b/nat-start.sh
@@ -1,6 +1,16 @@
 #!/bin/bash
 
-iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
+function echoerr {
+	>&2 echo $1
+}
+
+if [ -z $1 ]; then
+	echoerr "syntax:"
+	echoerr "# $0 <iface>"
+	exit 1
+fi
+
+iptables -t nat -A POSTROUTING -o $1 -j MASQUERADE
 iptables -P FORWARD ACCEPT
 sysctl -w net.ipv4.ip_forward=1