Fix IPv6 remote address formatting

remotes.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"net"
 	"net/smtp"
 	"net/url"
 )
@@ -55,7 +56,7 @@ func ParseRemote(remoteURL string) (*Remote, error) {
 		Scheme:   u.Scheme,
 		Hostname: hostname,
 		Port:     port,
-		Addr:     fmt.Sprintf("%s:%s", hostname, port),
+		Addr:     net.JoinHostPort(hostname, port),
 	}
 
 	if u.User != nil {

remotes_test.go

@@ -107,6 +107,26 @@ func TestValidRemoteUrls(t *testing.T) {
 	}, "starttls://user:pass@email.com:8425/sender@website.com?auth=login&skipVerify")
 }
 
+func TestIPv6RemoteUrls(t *testing.T) {
+	for _, test := range []struct {
+		url, scheme, port string
+	}{
+		{"smtp://[::1]", "smtp", "25"},
+		{"smtps://[::1]", "smtps", "465"},
+		{"starttls://[::1]", "starttls", "587"},
+		{"smtp://[::1]:2525", "smtp", "2525"},
+	} {
+		t.Run(test.url, func(t *testing.T) {
+			AssertRemoteUrlEquals(t, &Remote{
+				Scheme:   test.scheme,
+				Hostname: "::1",
+				Port:     test.port,
+				Addr:     "[::1]:" + test.port,
+			}, test.url)
+		})
+	}
+}
+
 func TestMissingScheme(t *testing.T) {
 	_, err := ParseRemote("http://user:pass@email.com:8425/sender@website.com")
 	assert.NotNil(t, err, "Err must be present")

smtp_test.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+	"fmt"
+	"net"
+	"net/textproto"
+	"testing"
+	"time"
+)
+
+func TestSendMailIPv6Remote(t *testing.T) {
+	listener, err := net.Listen("tcp6", "[::1]:0")
+	if err != nil {
+		t.Skipf("IPv6 loopback unavailable: %v", err)
+	}
+	defer listener.Close()
+	if err := listener.(*net.TCPListener).SetDeadline(time.Now().Add(5 * time.Second)); err != nil {
+		t.Fatal(err)
+	}
+
+	result := make(chan error, 1)
+	go func() {
+		result <- serveTestSMTP(listener)
+	}()
+
+	remote, err := ParseRemote("smtp://" + listener.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := SendMail(remote, "sender@example.invalid", []string{"recipient@example.invalid"}, []byte("Subject: loopback test\r\n\r\nsynthetic fixture\r\n")); err != nil {
+		t.Fatal(err)
+	}
+	if err := <-result; err != nil {
+		t.Fatal(err)
+	}
+}
+
+// This sink accepts only the synthetic transaction used above, never relaying it.
+func serveTestSMTP(listener net.Listener) error {
+	conn, err := listener.Accept()
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+	if err := conn.SetDeadline(time.Now().Add(5 * time.Second)); err != nil {
+		return err
+	}
+	text := textproto.NewConn(conn)
+	if err := text.PrintfLine("220 loopback test"); err != nil {
+		return err
+	}
+	commands := []struct{ command, response string }{
+		{"EHLO " + *hostName, "250 loopback test"},
+		{"MAIL FROM:<sender@example.invalid>", "250 OK"},
+		{"RCPT TO:<recipient@example.invalid>", "250 OK"},
+		{"DATA", "354 End with a dot"},
+	}
+	for _, step := range commands {
+		line, err := text.ReadLine()
+		if err != nil {
+			return err
+		}
+		if line != step.command {
+			return fmt.Errorf("command = %q, want %q", line, step.command)
+		}
+		if err := text.PrintfLine("%s", step.response); err != nil {
+			return err
+		}
+	}
+	data, err := text.ReadDotBytes()
+	if err != nil {
+		return err
+	}
+	if string(data) != "Subject: loopback test\n\nsynthetic fixture\n" {
+		return fmt.Errorf("unexpected DATA: %q", data)
+	}
+	if err := text.PrintfLine("250 OK"); err != nil {
+		return err
+	}
+	line, err := text.ReadLine()
+	if err != nil {
+		return err
+	}
+	if line != "QUIT" {
+		return fmt.Errorf("command = %q, want QUIT", line)
+	}
+	return text.PrintfLine("221 Bye")
+}