Add SMTP Honeypot

honeypot/smtp/mail.go

@@ -0,0 +1,12 @@
+package smtp
+
+import "time"
+
+type MailData struct {
+	From       string
+	Recipients []string
+	Data       string
+
+	RemoteAddr string
+	Timestamp  time.Time
+}

honeypot/smtp/metrics.go

@@ -0,0 +1,30 @@
+package smtp
+
+import "github.com/prometheus/client_golang/prometheus"
+
+type MetricsStore struct {
+	backend   SMTPStore
+	mailTotal *prometheus.CounterVec
+}
+
+func NewMetricsStore(backend SMTPStore) *MetricsStore {
+	store := &MetricsStore{backend: backend}
+
+	store.mailTotal = prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name:        "apiary_smtp_received_total",
+			Help:        "Total count of received emails.",
+			ConstLabels: prometheus.Labels{"service": "honeypot_smtp"},
+		},
+		[]string{"from"},
+	)
+	prometheus.MustRegister(store.mailTotal)
+
+	return store
+}
+
+func (s *MetricsStore) AddMailData(data *MailData) error {
+	s.mailTotal.WithLabelValues(data.From).Inc()
+
+	return s.backend.AddMailData(data)
+}

honeypot/smtp/server.go

@@ -0,0 +1,112 @@
+package smtp
+
+import (
+	"bytes"
+	"io"
+	"time"
+
+	smtplib "github.com/emersion/go-smtp"
+	"go.uber.org/zap"
+)
+
+type backend struct {
+	honeypot *SMTPHoneypot
+}
+
+func (b *backend) Login(state *smtplib.ConnectionState, username, password string) (smtplib.Session, error) {
+	sess := &session{
+		honeypot:      b.honeypot,
+		loginUsername: username,
+		loginPassword: password,
+		remoteAddr:    state.RemoteAddr.String(),
+	}
+	return sess, nil
+}
+func (b *backend) AnonymousLogin(state *smtplib.ConnectionState) (smtplib.Session, error) {
+	sess := &session{
+		loginAnonymous: true,
+		remoteAddr:     state.RemoteAddr.String(),
+	}
+	return sess, nil
+}
+
+type session struct {
+	honeypot *SMTPHoneypot
+
+	loginUsername  string
+	loginPassword  string
+	loginAnonymous bool
+	remoteAddr     string
+
+	mail MailData
+}
+
+func (sess *session) Reset() {
+}
+func (sess *session) Logout() error {
+	return nil
+}
+func (sess *session) Mail(from string, opts smtplib.MailOptions) error {
+	sess.mail.From = from
+	return nil
+}
+func (sess *session) Rcpt(to string) error {
+	sess.mail.Recipients = append(sess.mail.Recipients, to)
+	return nil
+}
+func (sess *session) Data(r io.Reader) error {
+	lr := io.LimitReader(r, 1024*1024)
+	var buf bytes.Buffer
+	n, err := io.Copy(&buf, lr)
+	if err != nil {
+		sess.honeypot.Logger.Debugw("Error reading email data from client.", "error", err, "remote_addr", sess.remoteAddr)
+	}
+
+	if n == 1024*1024 {
+		// There is probably more data, read the rest from the client
+		// We don't really care what happens to this, or if it errors
+		_, _ = io.Copy(io.Discard, r)
+	}
+
+	sess.mail.Data = buf.String()
+
+	if err := sess.honeypot.Store.AddMailData(&sess.mail); err != nil {
+		sess.honeypot.Logger.Warnw("Error storing maildata.", "error", err, "remote_addr", sess.remoteAddr)
+		return smtplib.ErrDataTooLarge
+	}
+	sess.honeypot.Logger.Infow("Mail received", "from", sess.mail.From, "remote_addr", sess.remoteAddr, "size", len(sess.mail.Data))
+
+	return nil
+}
+
+type SMTPHoneypot struct {
+	Addr   string
+	Logger *zap.SugaredLogger
+	Store  SMTPStore
+	server *smtplib.Server
+}
+
+func NewSMTPHoneypot() (*SMTPHoneypot, error) {
+	honeypot := &SMTPHoneypot{
+		Logger: zap.NewNop().Sugar(),
+	}
+
+	return honeypot, nil
+}
+
+func (sh *SMTPHoneypot) ListenAndServe() error {
+	server := smtplib.NewServer(&backend{
+		honeypot: sh,
+	})
+	server.Addr = sh.Addr
+	server.Domain = "apiary.t-juice.club"
+	server.AllowInsecureAuth = true
+	server.ReadTimeout = 10 * time.Second
+	server.WriteTimeout = 10 * time.Second
+	sh.server = server
+	return server.ListenAndServe()
+}
+
+func (sh *SMTPHoneypot) Shutdown() error {
+	return sh.server.Close()
+}

honeypot/smtp/store.go

@@ -0,0 +1,12 @@
+package smtp
+
+type SMTPStore interface {
+	AddMailData(mail *MailData) error
+}
+
+type DiscardStore struct {
+}
+
+func (s *DiscardStore) AddMailData(mail *MailData) error {
+	return nil
+}