Example DKIM proxy using the smtpd package.

examples/dkim-proxy/main.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"bytes"
+	"flag"
+	"io/ioutil"
+	"log"
+	"net/smtp"
+
+	"bitbucket.org/chrj/smtpd"
+	"github.com/eaigner/dkim"
+)
+
+var (
+	welcomeMsg  = flag.String("welcome", "DKIM-proxy ESMTP ready.", "Welcome message for SMTP session")
+	inAddr      = flag.String("inaddr", "localhost:10025", "Address to listen for incoming SMTP on")
+	outAddr     = flag.String("outaddr", "localhost:25", "Address to deliver outgoing SMTP on")
+	privKeyFile = flag.String("key", "", "Private key file.")
+	dkimS       = flag.String("s", "default", "DKIM selector")
+	dkimD       = flag.String("d", "", "DKIM domain")
+
+	dkimConf dkim.Conf
+	privKey  []byte
+)
+
+func handler(peer smtpd.Peer, env smtpd.Envelope) error {
+
+	d, err := dkim.New(dkimConf, privKey)
+	if err != nil {
+		log.Printf("DKIM error: %v", err)
+		return smtpd.Error{450, "Internal server error"}
+	}
+
+	// The dkim package expects \r\n newlines, so replace to that
+	data, err := d.Sign(bytes.Replace(env.Data, []byte("\n"), []byte("\r\n"), -1))
+	if err != nil {
+		log.Printf("DKIM signing error: %v", err)
+		return smtpd.Error{450, "Internal server error"}
+	}
+
+	return smtp.SendMail(
+		*outAddr,
+		nil,
+		env.Sender,
+		env.Recipients,
+		data,
+	)
+
+}
+
+func main() {
+
+	flag.Parse()
+
+	var err error
+
+	dkimConf, err = dkim.NewConf(*dkimD, *dkimS)
+	if err != nil {
+		log.Fatalf("DKIM configuration error: %v", err)
+	}
+
+	privKey, err = ioutil.ReadFile(*privKeyFile)
+	if err != nil {
+		log.Fatalf("Couldn't read private key: %v", err)
+	}
+
+	_, err = dkim.New(dkimConf, privKey)
+	if err != nil {
+		log.Fatalf("DKIM error: %v", err)
+	}
+
+	server := &smtpd.Server{
+		WelcomeMessage: *welcomeMsg,
+		Handler:        handler,
+	}
+
+	server.ListenAndServe(*inAddr)
+
+}