attempt anonymous only when logging in without JID and password

_example/example.go

@@ -32,7 +32,9 @@ func main() {
 	}
 	flag.Parse()
 	if *username == "" || *password == "" {
-		flag.Usage()
+		if *debug {
+			fmt.Fprintf(os.Stderr, "no username or password were given; attempting ANONYMOUS auth\n")
+		}
 	}
 
 	if !*notls {

xmpp.go

@@ -44,6 +44,10 @@ const (
 // Default TLS configuration options
 var DefaultConfig tls.Config
 
+func init() {
+	DefaultConfig.InsecureSkipVerify = true
+}
+
 // Cookie is a unique XMPP session identifier
 type Cookie uint64
 
@@ -272,11 +276,14 @@ func (c *Client) init(o *Options) error {
 		c.p = xml.NewDecoder(c.conn)
 	}
 
+	var domain string
 	a := strings.SplitN(o.User, "@", 2)
+        if len(o.User) > 0 {
 		if len(a) != 2 {
 			return errors.New("xmpp: invalid username (want user@domain): " + o.User)
 		}
-	domain := a[1]
+	    	domain = a[1]
+	} // Otherwise, we'll be attempting ANONYMOUS
 
 	// Declare intent to be a jabber client and gather stream features.
 	f, err := c.startStream(o, domain)
@@ -289,6 +296,19 @@ func (c *Client) init(o *Options) error {
 		return err
 	}
 
+	if o.User == "" && o.Password == "" {
+		found_anonymous := false
+		for _, m := range f.Mechanisms.Mechanism {
+			if m == "ANONYMOUS" {
+				fmt.Fprintf(c.conn, "<auth xmlns='%s' mechanism='ANONYMOUS' />\n", nsSASL)
+				found_anonymous = true
+				break
+			}
+		}
+		if !found_anonymous {
+			return fmt.Errorf("ANONYMOUS authentication is not an option and username and password were not specified")
+		}
+	} else {
 		// Even digest forms of authentication are unsafe if we do not know that the host
 		// we are talking to is the actual server, and not a man in the middle playing
 		// proxy.
@@ -370,7 +390,7 @@ func (c *Client) init(o *Options) error {
 		if mechanism == "" {
 			return fmt.Errorf("PLAIN authentication is not an option: %v", f.Mechanisms.Mechanism)
 		}
-
+	}
 	// Next message should be either success or failure.
 	name, val, err := next(c.p)
 	if err != nil {