resolve hostnames passed in IPList flags

util/flag.go

@@ -18,11 +18,17 @@ func (l *ipList) String() string {
 }
 
 func (l *ipList) Set(value string) error {
-	ip := net.ParseIP(value)
-	if ip == nil {
-		return fmt.Errorf("Unable to parse IP address \"%s\"", value)
+	if ip := net.ParseIP(value); ip != nil {
+		*l = append(*l, ip)
+		return nil
 	}
-	*l = append(*l, ip)
+
+	// Value is not an IP address, try to resolve it.
+	ips, err := net.LookupIP(value)
+	if err != nil {
+		return fmt.Errorf("Unable to parse IP address \"%s\": %v", value, err)
+	}
+	*l = append(*l, ips...)
 	return nil
 }