fix #2142

Allow specifying TCP4 or TCP6 for outgoing email sending, or choosing a
specific local address to send from.

default.yaml

@@ -405,6 +405,10 @@ accounts:
             sender: "admin@my.network"
             require-tls: true
             helo-domain: "my.network" # defaults to server name if unset
+            # set to `tcp4` to force sending over IPv4, `tcp6` to force IPv6:
+            # protocol: "tcp4"
+            # set to force a specific source/local IPv4 or IPv6 address:
+            # local-address: "1.2.3.4"
             # options to enable DKIM signing of outgoing emails (recommended, but
             # requires creating a DNS entry for the public key):
             # dkim:

irc/email/email.go

@@ -75,6 +75,9 @@ type MailtoConfig struct {
 	Sender                 string
 	HeloDomain             string `yaml:"helo-domain"`
 	RequireTLS             bool   `yaml:"require-tls"`
+	Protocol               string `yaml:"protocol"`
+	LocalAddress           string `yaml:"local-address"`
+	localAddress           net.Addr
 	VerifyMessageSubject   string `yaml:"verify-message-subject"`
 	DKIM                   DKIMConfig
 	MTAReal                MTAConfig       `yaml:"mta"`
@@ -159,6 +162,25 @@ func (config *MailtoConfig) Postprocess(heloDomain string) (err error) {
 		}
 	}
 
+	config.Protocol = strings.ToLower(config.Protocol)
+	if config.Protocol == "" {
+		config.Protocol = "tcp"
+	}
+	if !(config.Protocol == "tcp" || config.Protocol == "tcp4" || config.Protocol == "tcp6") {
+		return fmt.Errorf("Invalid protocol for email sending: `%s`", config.Protocol)
+	}
+
+	if config.LocalAddress != "" {
+		ipAddr := net.ParseIP(config.LocalAddress)
+		if ipAddr == nil {
+			return fmt.Errorf("Could not parse local-address for email sending: `%s`", config.LocalAddress)
+		}
+		config.localAddress = &net.TCPAddr{
+			IP:   ipAddr,
+			Port: 0,
+		}
+	}
+
 	if config.MTAConfig.Server != "" {
 		// smarthost, nothing more to validate
 		return nil
@@ -241,6 +263,6 @@ func SendMail(config MailtoConfig, recipient string, msg []byte) (err error) {
 
 	return smtp.SendMail(
 		addr, auth, config.HeloDomain, config.Sender, []string{recipient}, msg,
-		config.RequireTLS, implicitTLS, config.Timeout,
+		config.RequireTLS, implicitTLS, config.Protocol, config.localAddress, config.Timeout,
 	)
 }

irc/smtp/smtp.go

@@ -55,17 +55,18 @@ type Client struct {
 
 // Dial returns a new Client connected to an SMTP server at addr.
 // The addr must include a port, as in "mail.example.com:smtp".
-func Dial(addr string, timeout time.Duration, implicitTLS bool) (*Client, error) {
+func Dial(protocol, addr string, localAddress net.Addr, timeout time.Duration, implicitTLS bool) (*Client, error) {
 	var conn net.Conn
 	var err error
 	dialer := net.Dialer{
-		Timeout: timeout,
+		Timeout:   timeout,
+		LocalAddr: localAddress,
 	}
 	start := time.Now()
 	if !implicitTLS {
-		conn, err = dialer.Dial("tcp", addr)
+		conn, err = dialer.Dial(protocol, addr)
 	} else {
-		conn, err = tls.DialWithDialer(&dialer, "tcp", addr, nil)
+		conn, err = tls.DialWithDialer(&dialer, protocol, addr, nil)
 	}
 	if err != nil {
 		return nil, err
@@ -341,7 +342,7 @@ var testHookStartTLS func(*tls.Config) // nil, except for tests
 // functionality. Higher-level packages exist outside of the standard
 // library.
 // XXX: modified in Ergo to add `requireTLS`, `heloDomain`, and `timeout` arguments
-func SendMail(addr string, a Auth, heloDomain string, from string, to []string, msg []byte, requireTLS, implicitTLS bool, timeout time.Duration) error {
+func SendMail(addr string, a Auth, heloDomain string, from string, to []string, msg []byte, requireTLS, implicitTLS bool, protocol string, localAddress net.Addr, timeout time.Duration) error {
 	if err := validateLine(from); err != nil {
 		return err
 	}
@@ -350,7 +351,7 @@ func SendMail(addr string, a Auth, heloDomain string, from string, to []string,
 			return err
 		}
 	}
-	c, err := Dial(addr, timeout, implicitTLS)
+	c, err := Dial(protocol, addr, localAddress, timeout, implicitTLS)
 	if err != nil {
 		return err
 	}

traditional.yaml

@@ -378,6 +378,10 @@ accounts:
             sender: "admin@my.network"
             require-tls: true
             helo-domain: "my.network" # defaults to server name if unset
+            # set to `tcp4` to force sending over IPv4, `tcp6` to force IPv6:
+            # protocol: "tcp4"
+            # set to force a specific source/local IPv4 or IPv6 address:
+            # local-address: "1.2.3.4"
             # options to enable DKIM signing of outgoing emails (recommended, but
             # requires creating a DNS entry for the public key):
             # dkim: