add LOGIN auth support

auth.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"fmt"
+	"net/smtp"
+)
+
+type loginAuth struct {
+	username, password string
+}
+
+// LoginAuth provides a simple implementation of LOGIN authorization of SMTP as
+// described here: https://www.ietf.org/archive/id/draft-murchison-sasl-login-00.txt
+func LoginAuth(username, password string) smtp.Auth {
+	return &loginAuth{username, password}
+}
+
+func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	return "LOGIN", []byte{}, nil
+}
+
+func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		switch string(fromServer) {
+		case "User Name", "Username:":
+			return []byte(a.username), nil
+		case "Password", "Password:":
+			return []byte(a.password), nil
+		default:
+			return nil, fmt.Errorf("unknown server response \"%s\"", string(fromServer))
+		}
+	}
+	return nil, nil
+}