PoC

README.md

@@ -1,2 +1,16 @@
-# smtp2shoutrrr
+# smtp2Shoutrrr
 
+A simple SMTP server that forwards incoming emails to a Shoutrrr service.
+
+## Development
+
+Run the server with:
+
+```
+go run main.go
+```
+
+Send a test email with:
+
+```
+go run ./cmd/sendmail/main.go

cmd/sendmail/main.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"log"
+	"log/slog"
+	"net/smtp"
+
+	"github.com/emersion/go-sasl"
+)
+
+// The ANONYMOUS mechanism name.
+const Anonymous = "ANONYMOUS"
+
+type anonymousClient struct {
+	Trace string
+}
+
+func (c *anonymousClient) Start(si *smtp.ServerInfo) (mech string, ir []byte, err error) {
+	mech = Anonymous
+	ir = []byte(c.Trace)
+	return
+}
+
+func (c *anonymousClient) Next(challenge []byte, b bool) (response []byte, err error) {
+	return nil, sasl.ErrUnexpectedServerChallenge
+}
+
+// A client implementation of the ANONYMOUS authentication mechanism, as
+// described in RFC 4505.
+func NewAnonymousClient(trace string) smtp.Auth {
+	return &anonymousClient{trace}
+}
+
+// The PLAIN mechanism name.
+const Plain = "PLAIN"
+
+type plainClient struct {
+	Identity string
+	Username string
+	Password string
+}
+
+func (a *plainClient) Start(si *smtp.ServerInfo) (mech string, ir []byte, err error) {
+	mech = "PLAIN"
+	ir = []byte(a.Identity + "\x00" + a.Username + "\x00" + a.Password)
+	return
+}
+
+func (a *plainClient) Next(challenge []byte, b bool) (response []byte, err error) {
+	slog.Info("Next: %v", challenge)
+	return nil, nil
+}
+
+// A client implementation of the PLAIN authentication mechanism, as described
+// in RFC 4616. Authorization identity may be left blank to indicate that it is
+// the same as the username.
+func NewPlainClient(identity, username, password string) smtp.Auth {
+	return &plainClient{identity, username, password}
+}
+
+func main() {
+	// hostname is used by PlainAuth to validate the TLS certificate.
+	hostname := "localhost"
+	auth := NewPlainClient("", "username", "password")
+	// auth := NewAnonymousClient("test")
+	recipients := []string{"test@test.com"}
+	msg := []byte("hello!")
+	from := "hello@localhost"
+	err := smtp.SendMail(hostname+":11025", auth, from, recipients, msg)
+	if err != nil {
+		log.Fatal(err)
+	}
+}

go.mod

@@ -0,0 +1,8 @@
+module git.nakama.town/fmartingr/smtp2shoutrrr
+
+go 1.23.3
+
+require (
+	github.com/emersion/go-sasl v0.0.0-20200509203442-7bfe0ed36a21
+	github.com/emersion/go-smtp v0.21.3
+)

go.sum

@@ -0,0 +1,4 @@
+github.com/emersion/go-sasl v0.0.0-20200509203442-7bfe0ed36a21 h1:OJyUGMJTzHTd1XQp98QTaHernxMYzRaOasRir9hUlFQ=
+github.com/emersion/go-sasl v0.0.0-20200509203442-7bfe0ed36a21/go.mod h1:iL2twTeMvZnrg54ZoPDNfJaJaqy0xIQFuBdrLsmspwQ=
+github.com/emersion/go-smtp v0.21.3 h1:7uVwagE8iPYE48WhNsng3RRpCUpFvNl39JGNSIyGVMY=
+github.com/emersion/go-smtp v0.21.3/go.mod h1:qm27SGYgoIPRot6ubfQ/GpiPy/g3PaZAVRxiO/sDUgQ=

main.go

@@ -0,0 +1,96 @@
+package main
+
+import (
+	"errors"
+	"io"
+	"log"
+	"log/slog"
+	"net/http"
+	"time"
+
+	"github.com/emersion/go-sasl"
+	"github.com/emersion/go-smtp"
+)
+
+// The Backend implements SMTP server methods.
+type Backend struct{}
+
+// NewSession is called after client greeting (EHLO, HELO).
+func (bkd *Backend) NewSession(c *smtp.Conn) (smtp.Session, error) {
+	return &Session{}, nil
+}
+
+// A Session is returned after successful login.
+type Session struct{}
+
+// AuthMechanisms returns a slice of available auth mechanisms; only PLAIN is
+// supported in this example.
+func (s *Session) AuthMechanisms() []string {
+	return []string{sasl.Plain}
+}
+
+// Auth is the handler for supported authenticators.
+func (s *Session) Auth(mech string) (sasl.Server, error) {
+	return sasl.NewPlainServer(func(identity, username, password string) error {
+		if username != "username" || password != "password" {
+			return errors.New("Invalid username or password")
+		}
+		return nil
+	}), nil
+}
+
+func (s *Session) Mail(from string, opts *smtp.MailOptions) error {
+	log.Println("Mail from:", from, opts)
+	return nil
+}
+
+func (s *Session) Rcpt(to string, opts *smtp.RcptOptions) error {
+	log.Println("Rcpt to:", to)
+	return nil
+}
+
+func (s *Session) Data(r io.Reader) error {
+	_, err := http.Post("https://ntfy.sh/fmartingr-dev", "text/plain", r)
+	if err != nil {
+		slog.Error("Error sending message:", slog.String("err", err.Error()))
+	}
+	return nil
+}
+
+func (s *Session) Reset() {}
+
+func (s *Session) Logout() error {
+	return nil
+}
+
+// ExampleServer runs an example SMTP server.
+//
+// It can be tested manually with e.g. netcat:
+//
+//	> netcat -C localhost 1025
+//	EHLO localhost
+//	AUTH PLAIN
+//	AHVzZXJuYW1lAHBhc3N3b3Jk
+//	MAIL FROM:<root@nsa.gov>
+//	RCPT TO:<root@gchq.gov.uk>
+//	DATA
+//	Hey <3
+//	.
+func main() {
+	be := &Backend{}
+
+	s := smtp.NewServer(be)
+
+	s.Addr = "localhost:11025"
+	s.Domain = "localhost"
+	s.WriteTimeout = 10 * time.Second
+	s.ReadTimeout = 10 * time.Second
+	s.MaxMessageBytes = 1024 * 1024
+	s.MaxRecipients = 50
+	s.AllowInsecureAuth = true
+
+	log.Println("Starting server at", s.Addr)
+	if err := s.ListenAndServe(); err != nil {
+		log.Fatal(err)
+	}
+}