target.smtp: treat downstream DNS errors as temporary

internal/smtpconn/smtpconn.go

@@ -79,6 +79,13 @@ type C struct {
 	// "ADDRESS said: ..."
 	AddrInSMTPMsg bool
 
+	// Treat DNS resolution errors returned while connecting to the configured
+	// endpoint as temporary delivery errors.
+	//
+	// This is useful for explicitly configured downstream SMTP/LMTP targets where
+	// the endpoint is mail infrastructure, not recipient-domain DNS.
+	DNSErrorsTemporary bool
+
 	conn       net.Conn
 	serverName string
 	cl         *smtp.Client
@@ -135,9 +142,17 @@ func (c *C) wrapClientErr(err error, serverName string) error {
 			reason, misc := exterrors.UnwrapDNSErr(err)
 			misc["remote_server"] = err.Addr
 			misc["io_op"] = err.Op
+
+			code := exterrors.SMTPCode(err, 450, 550)
+			enhancedCode := exterrors.SMTPEnchCode(err, exterrors.EnhancedCode{0, 4, 4})
+			if c.DNSErrorsTemporary {
+				code = 451
+				enhancedCode = exterrors.EnhancedCode{4, 4, 4}
+			}
+
 			return &exterrors.SMTPError{
-				Code:         exterrors.SMTPCode(err, 450, 550),
-				EnhancedCode: exterrors.SMTPEnchCode(err, exterrors.EnhancedCode{0, 4, 4}),
+				Code:         code,
+				EnhancedCode: enhancedCode,
 				Message:      "DNS error",
 				Err:          err,
 				Reason:       reason,

internal/smtpconn/smtpconn_dns_test.go

@@ -0,0 +1,90 @@
+/*
+Maddy Mail Server - Composable all-in-one email server.
+Copyright © 2019-2020 Max Mazurov <fox.cpp@disroot.org>, Maddy Mail Server contributors
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+package smtpconn
+
+import (
+	"errors"
+	"net"
+	"testing"
+
+	"github.com/foxcpp/maddy/framework/exterrors"
+)
+
+func dnsOpError() error {
+	return &net.OpError{
+		Op:  "dial",
+		Net: "tcp",
+		Addr: &net.TCPAddr{
+			IP:   net.ParseIP("192.0.2.1"),
+			Port: 25,
+		},
+		Err: &net.DNSError{
+			Err:        "no such host",
+			Name:       "smtp.example.invalid",
+			IsNotFound: true,
+		},
+	}
+}
+
+func TestWrapClientErrDNSErrorDefaultClassification(t *testing.T) {
+	c := New()
+
+	err := c.wrapClientErr(dnsOpError(), "smtp.example.invalid")
+
+	var smtpErr *exterrors.SMTPError
+	if !errors.As(err, &smtpErr) {
+		t.Fatalf("expected *exterrors.SMTPError, got %T: %v", err, err)
+	}
+
+	if smtpErr.Code != 550 {
+		t.Fatalf("unexpected SMTP code: got %d, want 550", smtpErr.Code)
+	}
+
+	if smtpErr.EnhancedCode != (exterrors.EnhancedCode{5, 4, 4}) {
+		t.Fatalf("unexpected enhanced SMTP code: got %v, want 5.4.4", smtpErr.EnhancedCode)
+	}
+
+	if smtpErr.Temporary() {
+		t.Fatal("default DNS error classification should be permanent")
+	}
+}
+
+func TestWrapClientErrDNSErrorTemporaryClassification(t *testing.T) {
+	c := New()
+	c.DNSErrorsTemporary = true
+
+	err := c.wrapClientErr(dnsOpError(), "smtp.example.invalid")
+
+	var smtpErr *exterrors.SMTPError
+	if !errors.As(err, &smtpErr) {
+		t.Fatalf("expected *exterrors.SMTPError, got %T: %v", err, err)
+	}
+
+	if smtpErr.Code != 451 {
+		t.Fatalf("unexpected SMTP code: got %d, want 451", smtpErr.Code)
+	}
+
+	if smtpErr.EnhancedCode != (exterrors.EnhancedCode{4, 4, 4}) {
+		t.Fatalf("unexpected enhanced SMTP code: got %v, want 4.4.4", smtpErr.EnhancedCode)
+	}
+
+	if !smtpErr.Temporary() {
+		t.Fatal("DNS error should be temporary when DNSErrorsTemporary is enabled")
+	}
+}

internal/target/smtp/smtp_downstream.go

@@ -224,6 +224,7 @@ func (d *delivery) connect(ctx context.Context) error {
 	conn.Log = d.log
 	conn.Hostname = d.u.hostname
 	conn.AddrInSMTPMsg = false
+	conn.DNSErrorsTemporary = true
 	if d.u.connectTimeout != 0 {
 		conn.ConnectTimeout = d.u.connectTimeout
 	}