From 4d284312a368c7ec1e82f2fa418b33d3a7e21a3b Mon Sep 17 00:00:00 2001
From: Robert Sesek <rsesek@bluestatic.org>
Date: Sat, 6 Jun 2020 14:46:43 -0400
Subject: [PATCH] Format the Reporting-MTA in DSN messages per the spec.

---
 smtp/conn.go       | 12 +-----------
 smtp/relay.go      |  6 +-----
 smtp/relay_test.go |  7 +------
 smtp/server.go     | 17 +++++++++++++++++
 4 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/smtp/conn.go b/smtp/conn.go
index 5511799..9d807d4 100644
--- a/smtp/conn.go
+++ b/smtp/conn.go
@@ -510,17 +510,7 @@ func (conn *connection) handleSendAs(env *Envelope) {
 }
 
 func (conn *connection) getReceivedInfo(envelope Envelope) []byte {
-	rhost, _, err := net.SplitHostPort(conn.remoteAddr.String())
-	if err != nil {
-		rhost = conn.remoteAddr.String()
-	}
-
-	rhosts, err := net.LookupAddr(rhost)
-	if err == nil {
-		rhost = fmt.Sprintf("%s [%s]", rhosts[0], rhost)
-	}
-
-	base := fmt.Sprintf("Received: from %s (%s)\r\n        ", conn.ehlo, rhost)
+	base := fmt.Sprintf("Received: from %s (%s)\r\n        ", conn.ehlo, lookupRemoteHost(conn.remoteAddr))
 
 	with := "SMTP"
 	if conn.esmtp {
diff --git a/smtp/relay.go b/smtp/relay.go
index 81a91e3..ae90fe5 100644
--- a/smtp/relay.go
+++ b/smtp/relay.go
@@ -136,11 +136,7 @@ func deliverRelayFailure(server Server, env Envelope, log *zap.Logger, to, error
 	fmt.Fprintf(sw, "Original-Envelope-ID: %s\n", env.ID)
 	fmt.Fprintf(sw, "Reporting-UA: %s\n", env.EHLO)
 	if env.RemoteAddr != nil {
-		rhosts, err := net.LookupAddr(env.RemoteAddr.String())
-		if err == nil {
-			fmt.Fprintf(sw, "Reporting-MTA: %s\n", rhosts[0])
-		}
-		fmt.Fprintf(sw, "X-Remote-Address: %s\n", env.RemoteAddr)
+		fmt.Fprintf(sw, "Reporting-MTA: dns; %s\n", lookupRemoteHost(env.RemoteAddr))
 	}
 	fmt.Fprintf(sw, "Date: %s\n", env.Received.Format(time.RFC1123Z))
 
diff --git a/smtp/relay_test.go b/smtp/relay_test.go
index e486e82..1934173 100644
--- a/smtp/relay_test.go
+++ b/smtp/relay_test.go
@@ -191,12 +191,7 @@ func TestDeliveryFailureMessage(t *testing.T) {
 		t.Errorf("Missing %q in %q", expected, contentStr)
 	}
 
-	expected = "Reporting-MTA: localhost\n"
-	if !strings.Contains(contentStr, expected) {
-		t.Errorf("Missing %q in %q", expected, contentStr)
-	}
-
-	expected = "X-Remote-Address: 127.0.0.1\n"
+	expected = "Reporting-MTA: dns; localhost [127.0.0.1]\n"
 	if !strings.Contains(contentStr, expected) {
 		t.Errorf("Missing %q in %q", expected, contentStr)
 	}
diff --git a/smtp/server.go b/smtp/server.go
index 3b0fdfa..b991a22 100644
--- a/smtp/server.go
+++ b/smtp/server.go
@@ -71,6 +71,23 @@ func generateEnvelopeId(prefix string, t time.Time) string {
 	return fmt.Sprintf("%s.%d.%x", prefix, t.UnixNano(), idBytes)
 }
 
+// lookupRemoteHost attempts to reverse look-up the provided IP address. On
+// success, it returns the hostname and the IP as formatted for a receive
+// trace. If the lookup fails, it just returns the original IP.
+func lookupRemoteHost(addr net.Addr) string {
+	rhost, _, err := net.SplitHostPort(addr.String())
+	if err != nil {
+		rhost = addr.String()
+	}
+
+	rhosts, err := net.LookupAddr(rhost)
+	if err == nil {
+		rhost = fmt.Sprintf("%s [%s]", rhosts[0], rhost)
+	}
+
+	return rhost
+}
+
 type Server interface {
 	Name() string
 	TLSConfig() *tls.Config
-- 
2.22.5