feat: add smtp auth login support

docs/reference/targets/smtp.md

@@ -65,7 +65,7 @@ Refuse to pass messages over plain-text connections.
 
 ---
 
-### auth `off` | `plain` _username_ _password_ | `forward`  | `external`
+### auth `off` | `plain` _username_ _password_ | `login` _username_ _password_ | `forward`  | `external`
 Default: `off`
 
 Specify the way to authenticate to the remote server.
@@ -74,6 +74,10 @@ Valid values:
 - `off` – No authentication.
 - `plain` – Authenticate using specified username-password pair.
   **Don't use** this without enforced TLS (`require_tls`).
+- `login` - Authenticate using specified username-password pair.
+  Uses obsolete SASL LOGIN mechanism instead of SASL PLAIN.
+  This is required for compatibility with some SMTP services (e.g. Office 365).
+  **Don't use** this without enforced TLS (`require_tls`).
 - `forward` – Forward credentials specified by the client.
   **Don't use** this without enforced TLS (`require_tls`).
 - `external` – Request "external" SASL authentication. This is usually used for

internal/target/smtp/sasl.go

@@ -57,12 +57,18 @@ func saslAuthDirective(_ *config.Map, node config.Node) (interface{}, error) {
 			}
 			return sasl.NewPlainClient("", msgMeta.Conn.AuthUser, msgMeta.Conn.AuthPassword), nil
 		}, nil
-	case "plain":
+	case "plain", "login":
 		if len(node.Args) != 3 {
 			return nil, config.NodeErr(node, "two additional arguments are required (username, password)")
 		}
 		return func(*module.MsgMetadata) (sasl.Client, error) {
-			return sasl.NewPlainClient("", node.Args[1], node.Args[2]), nil
+			if node.Args[0] == "plain" {
+				return sasl.NewPlainClient("", node.Args[1], node.Args[2]), nil
+			} else if node.Args[0] == "login" {
+				return sasl.NewLoginClient(node.Args[1], node.Args[2]), nil
+			} else {
+				return nil, config.NodeErr(node, "unknown authentication mechanism: %s", node.Args[0])
+			}
 		}, nil
 	case "external":
 		if len(node.Args) > 1 {

internal/target/smtp/sasl_test.go

@@ -96,6 +96,64 @@ func TestSASL_Plain_AuthFail(t *testing.T) {
 	}
 }
 
+func TestSASL_Login(t *testing.T) {
+	be, srv := testutils.SMTPServer(t, "127.0.0.1:"+testPort)
+	defer srv.Close()
+	defer testutils.CheckSMTPConnLeak(t, srv)
+
+	mod := &Downstream{
+		hostname: "mx.example.invalid",
+		endpoints: []config.Endpoint{
+			{
+				Scheme: "tcp",
+				Host:   "127.0.0.1",
+				Port:   testPort,
+			},
+		},
+		saslFactory: testSaslFactory(t, "login", "test", "testpass"),
+		log:         testutils.Logger(t, "target.smtp"),
+	}
+
+	testutils.DoTestDelivery(t, mod, "[email protected]", []string{"[email protected]"})
+	be.CheckMsg(t, 0, "[email protected]", []string{"[email protected]"})
+	if be.Messages[0].AuthUser != "test" {
+		t.Errorf("Wrong AuthUser: %v", be.Messages[0].AuthUser)
+	}
+	if be.Messages[0].AuthPass != "testpass" {
+		t.Errorf("Wrong AuthPass: %v", be.Messages[0].AuthPass)
+	}
+}
+
+func TestSASL_Login_AuthFail(t *testing.T) {
+	be, srv := testutils.SMTPServer(t, "127.0.0.1:"+testPort)
+	defer srv.Close()
+	defer testutils.CheckSMTPConnLeak(t, srv)
+
+	be.AuthErr = &smtp.SMTPError{
+		Code:         550,
+		EnhancedCode: smtp.EnhancedCode{5, 1, 2},
+		Message:      "Hey",
+	}
+
+	mod := &Downstream{
+		hostname: "mx.example.invalid",
+		endpoints: []config.Endpoint{
+			{
+				Scheme: "tcp",
+				Host:   "127.0.0.1",
+				Port:   testPort,
+			},
+		},
+		saslFactory: testSaslFactory(t, "login", "test", "testpass"),
+		log:         testutils.Logger(t, "target.smtp"),
+	}
+
+	_, err := testutils.DoTestDeliveryErr(t, mod, "[email protected]", []string{"[email protected]"})
+	if err == nil {
+		t.Error("Expected an error, got none")
+	}
+}
+
 func TestSASL_Forward(t *testing.T) {
 	be, srv := testutils.SMTPServer(t, "127.0.0.1:"+testPort)
 	defer srv.Close()

internal/testutils/smtp_server.go

@@ -29,6 +29,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/emersion/go-sasl"
 	"github.com/emersion/go-smtp"
 	"github.com/foxcpp/maddy/framework/exterrors"
 )
@@ -202,6 +203,19 @@ func SMTPServer(t *testing.T, addr string, fn ...SMTPServerConfigureFunc) (*SMTP
 
 	be := new(SMTPBackend)
 	s := smtp.NewServer(be)
+
+	// Enable AUTH LOGIN with a custom handler that just calls AuthPlain.
+	s.EnableAuth(sasl.Login, func(conn *smtp.Conn) sasl.Server {
+		return sasl.NewLoginServer(func(username, password string) error {
+			sess := conn.Session()
+			if sess == nil {
+				panic("No session when AUTH is called")
+			}
+
+			return sess.AuthPlain(username, password)
+		})
+	})
+
 	s.Domain = "localhost"
 	s.AllowInsecureAuth = true
 	for _, f := range fn {
@@ -282,6 +296,19 @@ func SMTPServerSTARTTLS(t *testing.T, addr string, fn ...SMTPServerConfigureFunc
 
 	be := new(SMTPBackend)
 	s := smtp.NewServer(be)
+
+	// Enable AUTH LOGIN with a custom handler that just calls AuthPlain.
+	s.EnableAuth(sasl.Login, func(conn *smtp.Conn) sasl.Server {
+		return sasl.NewLoginServer(func(username, password string) error {
+			sess := conn.Session()
+			if sess == nil {
+				panic("No session when AUTH is called")
+			}
+
+			return sess.AuthPlain(username, password)
+		})
+	})
+
 	s.Domain = "localhost"
 	s.AllowInsecureAuth = true
 	s.TLSConfig = &tls.Config{
@@ -341,6 +368,19 @@ func SMTPServerTLS(t *testing.T, addr string, fn ...SMTPServerConfigureFunc) (*t
 
 	be := new(SMTPBackend)
 	s := smtp.NewServer(be)
+
+	// Enable AUTH LOGIN with a custom handler that just calls AuthPlain.
+	s.EnableAuth(sasl.Login, func(conn *smtp.Conn) sasl.Server {
+		return sasl.NewLoginServer(func(username, password string) error {
+			sess := conn.Session()
+			if sess == nil {
+				panic("No session when AUTH is called")
+			}
+
+			return sess.AuthPlain(username, password)
+		})
+	})
+
 	s.Domain = "localhost"
 	for _, f := range fn {
 		f(s)