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|
package main
import (
"bytes"
"context"
"crypto/tls"
"flag"
"io/ioutil"
"log"
"net"
"net/mail"
"net/smtp"
"net/textproto"
"strings"
"time"
"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)
var retryCount = 5
for retryCount > 0 {
err = smtp.SendMail(mxes[0].Host+":25", nil, `forwarder@localnet.cc`, []string{targetEmail}, builder.Bytes())
if err, ok := err.(*textproto.Error); ok {
if 400 <= err.Code && err.Code < 500 {
log.Printf(`retry sleep 120s count=%v code=%v`, retryCount, err.Code)
time.Sleep(120 * time.Second)
retryCount--
continue
}
}
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()
}
|
