From 7e420c5d5b2bb4c638fe2278ac69a69e4a6c06e6 Mon Sep 17 00:00:00 2001
From: Yves Fischer <yvesf+git@xapek.org>
Date: Wed, 17 Feb 2021 22:58:31 +0100
Subject: smtp-forward

---
 main.go | 142 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 142 insertions(+)
 create mode 100644 main.go

(limited to 'main.go')

diff --git a/main.go b/main.go
new file mode 100644
index 0000000..37ebedc
--- /dev/null
+++ b/main.go
@@ -0,0 +1,142 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"crypto/tls"
+	"flag"
+	"io/ioutil"
+	"log"
+	"net"
+	"net/mail"
+	"net/smtp"
+	"net/textproto"
+	"strings"
+
+	"github.com/mhale/smtpd"
+	"github.com/pkg/errors"
+)
+
+var flagListen = flag.String(`l`, `:25`, `Address to listen on`)
+var flagHostname = flag.String(`h`, `HOSTNAME-NOT-SET`, `Server flagHostname`)
+var flagMap = flag.String(`m`, ``, `-m prefix-matcher1:target@targethost,prefix-matcher2:target@targethost`)
+var flagCertFile = flag.String(`c`, ``, ``)
+var flagKeyFile = flag.String(`k`, ``, ``)
+
+func logWrapper(handler smtpd.Handler) smtpd.Handler {
+	return func(remoteAddr net.Addr, from string, to []string, data []byte) (err error) {
+		log.Printf(`received email remoteAddr=%v from=%v`, remoteAddr, from)
+		err = handler(remoteAddr, from, to, data)
+		if err != nil {
+			log.Printf(`failed to forward: %v`, err.Error())
+			return err
+		}
+		log.Printf(`forwarded email remoteAddr=%v from=%v`, remoteAddr, from)
+		return nil
+	}
+}
+
+func transformEmail(data []byte) (headers mail.Header, msgData []byte, err error) {
+	msg, err := mail.ReadMessage(bytes.NewReader(data))
+	if err != nil {
+		return nil, nil, errors.Wrap(err, `failed to parse email`)
+	}
+	headers = make(mail.Header)
+
+	msgData, err = ioutil.ReadAll(msg.Body)
+	if err != nil {
+		return nil, nil, errors.Wrap(err, `failed to parse email body`)
+	}
+
+	for headerKey, headerVal := range msg.Header {
+		headers[headerKey] = headerVal
+	}
+
+	return headers, msgData, nil
+}
+
+func forward(targetEmail string, data []byte) error {
+	toAddress := strings.SplitN(targetEmail, `@`, 2)
+	if len(toAddress) != 2 {
+		return errors.New(`invalid targetEmail address: ` + targetEmail)
+	}
+	mxes, err := net.DefaultResolver.LookupMX(context.Background(), toAddress[1])
+	if err != nil || len(mxes) == 0 {
+		return errors.Wrap(err, `failed targetEmail resolve mx`)
+	}
+
+	headers, msgData, err := transformEmail(data)
+	if err != nil {
+		return errors.Wrap(err, `failed targetEmail transform email`)
+	}
+	headers[textproto.CanonicalMIMEHeaderKey(`To`)] = []string{targetEmail}
+	headers[textproto.CanonicalMIMEHeaderKey(`From`)] = []string{`forwarder@localnet.cc`}
+	headers[textproto.CanonicalMIMEHeaderKey(`Subject`)] = []string{`Forwarded: ` + headers.Get(`subject`)}
+
+	var builder bytes.Buffer
+	for headerName, headerValues := range headers {
+		for _, value := range headerValues {
+			builder.WriteString(textproto.CanonicalMIMEHeaderKey(headerName))
+			builder.WriteString(`: `)
+			builder.WriteString(value)
+			builder.WriteString("\r\n")
+		}
+	}
+	builder.WriteString("\r\n")
+	builder.Write(msgData)
+
+	err = smtp.SendMail(mxes[0].Host+":25", nil, `forwarder@localnet.cc`, []string{targetEmail}, builder.Bytes())
+	if err != nil {
+		return errors.Wrap(err, `failed targetEmail send mail via smtp`)
+	}
+	log.Printf("forwarded targetEmail=%v", targetEmail)
+
+	return nil
+}
+
+func makeHandleEmail(mapping map[string]string) smtpd.Handler {
+	return func(remoteAddr net.Addr, from string, to []string, data []byte) error {
+		for prefix, targetEmail := range mapping {
+			for _, to := range to {
+				if strings.HasPrefix(to, prefix) {
+					err := forward(targetEmail, data)
+					if err != nil {
+						log.Print(`forwarded failed `, to, ` to `, targetEmail, err.Error())
+					}
+				}
+			}
+		}
+		return nil
+	}
+}
+
+func main() {
+	flag.Parse()
+	var mapping = make(map[string]string)
+	for _, m := range strings.Split(*flagMap, `,`) {
+		if m == `` {
+			continue
+		}
+		m := strings.SplitN(m, `:`, 2)
+		if len(m) != 2 {
+			panic(`invalid flag -m: ` + *flagMap)
+		}
+		mapping[m[0]] = m[1]
+	}
+
+	var tlsConfig *tls.Config
+	if *flagCertFile != `` && *flagKeyFile != `` {
+		cert, err := tls.LoadX509KeyPair(*flagCertFile, *flagKeyFile)
+		if err != nil {
+			panic(err)
+		}
+		tlsConfig = &tls.Config{Certificates: []tls.Certificate{cert}}
+	}
+	server := &smtpd.Server{
+		TLSConfig: tlsConfig,
+		Addr:      *flagListen,
+		Hostname:  *flagHostname,
+		Handler:   logWrapper(makeHandleEmail(mapping)),
+	}
+	_ = server.ListenAndServe()
+}
-- 
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