felhom-controller/controller/internal/mailrelay/server.go

// Package mailrelay is the in-process SMTP shim that gives deployed apps outbound
// email through one managed path. It accepts SMTP from apps on the Docker network
// (no Resend key on the box), validates the From-header domain, and forwards the raw
// MIME to the hub, which holds the Resend key and re-emits the message unchanged.
//
// Architecture: Shape 1 — the shim runs IN-PROCESS inside felhom-controller (operator-
// confirmed), reusing the controller's existing hub client. Listeners bind to the app
// Docker network only; they are NEVER published to the host or internet.
//
// Validated end-to-end in SPIKE-smtp-app-relay-2026-06-28.md (verdict READY).
package mailrelay

import (
	"context"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"fmt"
	"io"
	"log"
	"math/big"
	"net"
	"strings"
	"time"

	"github.com/emersion/go-sasl"
	"github.com/emersion/go-smtp"
)

// maxMessageBytes bounds a single message. Transactional app mail (password resets,
// invites) is small; 10 MiB leaves generous headroom for inline images/attachments.
const maxMessageBytes int64 = 10 << 20

// forwardTimeout bounds the synchronous hub forward (which itself dials Resend).
const forwardTimeout = 30 * time.Second

// Options configures the shim.
type Options struct {
	PlainAddr   string    // plaintext + STARTTLS listener (default ":2525")
	TLSAddr     string    // implicit-TLS listener (default ":2465")
	ServiceName string    // CN/SAN of the self-signed cert + SMTP greeting (e.g. "felhom-controller")
	Policy      *Policy   // From-domain allowlist (required)
	Forwarder   Forwarder // hub forwarder (required)
	Logger      *log.Logger
}

// Server runs the two SMTP listeners.
type Server struct {
	opts     Options
	tlsConf  *tls.Config
	plainSrv *smtp.Server
	tlsSrv   *smtp.Server
	plainLn  net.Listener
	tlsLn    net.Listener
}

// New builds the shim and its self-signed cert. It does not bind sockets — call Start.
func New(opts Options) (*Server, error) {
	if opts.Policy == nil {
		return nil, fmt.Errorf("mailrelay: Policy is required")
	}
	if opts.Forwarder == nil {
		return nil, fmt.Errorf("mailrelay: Forwarder is required")
	}
	if opts.PlainAddr == "" {
		opts.PlainAddr = ":2525"
	}
	if opts.TLSAddr == "" {
		opts.TLSAddr = ":2465"
	}
	if opts.ServiceName == "" {
		opts.ServiceName = "felhom-controller"
	}
	if opts.Logger == nil {
		opts.Logger = log.Default()
	}

	cert, err := selfSignedCert(opts.ServiceName)
	if err != nil {
		return nil, fmt.Errorf("mailrelay: generating self-signed cert: %w", err)
	}
	tlsConf := &tls.Config{Certificates: []tls.Certificate{cert}, MinVersion: tls.VersionTLS12}

	s := &Server{opts: opts, tlsConf: tlsConf}
	be := &backend{srv: s}

	// :2525 — plaintext with STARTTLS offered. AllowInsecureAuth lets an app present
	// AUTH before STARTTLS (apps send no creds, but some refuse to send without the offer).
	s.plainSrv = smtp.NewServer(be)
	s.plainSrv.Addr = opts.PlainAddr
	s.plainSrv.Domain = opts.ServiceName
	s.plainSrv.TLSConfig = tlsConf
	s.plainSrv.AllowInsecureAuth = true
	s.plainSrv.MaxMessageBytes = maxMessageBytes
	s.plainSrv.ReadTimeout = 60 * time.Second
	s.plainSrv.WriteTimeout = 60 * time.Second

	// :2465 — implicit TLS (the whole connection is TLS; for apps that force_tls).
	s.tlsSrv = smtp.NewServer(be)
	s.tlsSrv.Addr = opts.TLSAddr
	s.tlsSrv.Domain = opts.ServiceName
	s.tlsSrv.TLSConfig = tlsConf
	s.tlsSrv.MaxMessageBytes = maxMessageBytes
	s.tlsSrv.ReadTimeout = 60 * time.Second
	s.tlsSrv.WriteTimeout = 60 * time.Second

	return s, nil
}

// Start binds both listeners and serves them in background goroutines. A bind failure
// is returned synchronously. The listeners must reach the app Docker network ONLY —
// the caller is responsible for not publishing these ports to the host/internet.
func (s *Server) Start() error {
	pl, err := net.Listen("tcp", s.opts.PlainAddr)
	if err != nil {
		return fmt.Errorf("mailrelay: listen %s: %w", s.opts.PlainAddr, err)
	}
	tl, err := tls.Listen("tcp", s.opts.TLSAddr, s.tlsConf)
	if err != nil {
		pl.Close()
		return fmt.Errorf("mailrelay: listen TLS %s: %w", s.opts.TLSAddr, err)
	}
	s.plainLn, s.tlsLn = pl, tl

	s.opts.Logger.Printf("[INFO] [mailrelay] plaintext+STARTTLS listener on %s", pl.Addr())
	s.opts.Logger.Printf("[INFO] [mailrelay] implicit-TLS listener on %s", tl.Addr())

	go func() {
		if err := s.plainSrv.Serve(pl); err != nil && !isClosedErr(err) {
			s.opts.Logger.Printf("[ERROR] [mailrelay] plaintext listener stopped: %v", err)
		}
	}()
	go func() {
		if err := s.tlsSrv.Serve(tl); err != nil && !isClosedErr(err) {
			s.opts.Logger.Printf("[ERROR] [mailrelay] implicit-TLS listener stopped: %v", err)
		}
	}()
	return nil
}

// PlainAddr / TLSAddr return the bound addresses (resolved ports — useful for tests
// that listen on :0).
func (s *Server) PlainAddr() string {
	if s.plainLn != nil {
		return s.plainLn.Addr().String()
	}
	return s.opts.PlainAddr
}

func (s *Server) TLSAddr() string {
	if s.tlsLn != nil {
		return s.tlsLn.Addr().String()
	}
	return s.opts.TLSAddr
}

// Close stops both listeners.
func (s *Server) Close() error {
	var err error
	if s.plainSrv != nil {
		if e := s.plainSrv.Close(); e != nil {
			err = e
		}
	}
	if s.tlsSrv != nil {
		if e := s.tlsSrv.Close(); e != nil {
			err = e
		}
	}
	return err
}

func isClosedErr(err error) bool {
	return err != nil && strings.Contains(err.Error(), "use of closed network connection")
}

// ── go-smtp backend / session ────────────────────────────────────────────

type backend struct{ srv *Server }

func (b *backend) NewSession(_ *smtp.Conn) (smtp.Session, error) {
	return &session{srv: b.srv}, nil
}

// session implements smtp.Session + smtp.AuthSession.
type session struct {
	srv  *Server
	from string
	to   []string
}

func (s *session) Mail(from string, _ *smtp.MailOptions) error {
	s.from = from
	return nil
}

func (s *session) Rcpt(to string, _ *smtp.RcptOptions) error {
	s.to = append(s.to, to)
	return nil
}

// AuthMechanisms advertises PLAIN + LOGIN. We accept ANY credentials and ignore them
// (apps send none, but some refuse to send without an AUTH offer; no credential is a
// secret on the Docker network — the shim holds no Resend key).
func (s *session) AuthMechanisms() []string {
	return []string{sasl.Plain, sasl.Login}
}

func (s *session) Auth(mech string) (sasl.Server, error) {
	switch mech {
	case sasl.Plain:
		return sasl.NewPlainServer(func(_, _, _ string) error { return nil }), nil
	case sasl.Login:
		return &loginServer{}, nil
	default:
		return nil, smtp.ErrAuthUnsupported
	}
}

// Data reads the raw message, enforces the From policy BEFORE any hub call, then
// forwards it single-shot and maps the hub status to an SMTP reply.
func (s *session) Data(r io.Reader) error {
	raw, err := io.ReadAll(io.LimitReader(r, maxMessageBytes+1))
	if err != nil {
		s.srv.opts.Logger.Printf("[ERROR] [mailrelay] reading DATA: %v", err)
		return &smtp.SMTPError{Code: 451, EnhancedCode: smtp.EnhancedCode{4, 3, 0}, Message: "could not read message"}
	}
	if int64(len(raw)) > maxMessageBytes {
		return &smtp.SMTPError{Code: 552, EnhancedCode: smtp.EnhancedCode{5, 3, 4}, Message: "message too large"}
	}

	// From-policy gate (§7 B): reject with a clean 5xx BEFORE dialing the hub — never
	// spend a Resend call on a doomed message, and surface the misconfig loudly.
	if err := s.srv.opts.Policy.Check(raw); err != nil {
		s.srv.opts.Logger.Printf("[WARN] [mailrelay] rejected message (from=%q): %v", s.from, err)
		return &smtp.SMTPError{Code: 550, EnhancedCode: smtp.EnhancedCode{5, 7, 1}, Message: "sender address not permitted"}
	}

	ctx, cancel := context.WithTimeout(context.Background(), forwardTimeout)
	defer cancel()
	status, body, err := s.srv.opts.Forwarder.Forward(ctx, raw, s.from, s.to)
	if err != nil {
		// Transient: hub unreachable. Clean 4xx → the app shows the user an error; no
		// hang, no silent drop, no spool (§7 D / §9 rule 9).
		s.srv.opts.Logger.Printf("[ERROR] [mailrelay] forward to hub failed: %v", err)
		return &smtp.SMTPError{Code: 451, EnhancedCode: smtp.EnhancedCode{4, 4, 1}, Message: "upstream relay (hub) unavailable, try again later"}
	}
	if reply := mapStatusToSMTP(status, body); reply != nil {
		s.srv.opts.Logger.Printf("[WARN] [mailrelay] hub refused message (HTTP %d): %s", status, sanitizeReason(body))
		return reply
	}
	s.srv.opts.Logger.Printf("[INFO] [mailrelay] message relayed (from=%q rcpts=%d bytes=%d hub=%d)", s.from, len(s.to), len(raw), status)
	return nil
}

func (s *session) Reset() {
	s.from = ""
	s.to = nil
}

func (s *session) Logout() error { return nil }

// mapStatusToSMTP turns the hub's HTTP status into an SMTP reply so the app surfaces
// the real outcome (spike §6): 2xx → nil (250 OK), 4xx → 451 transient (incl. 429
// rate-limit — retry later), 5xx → 554 permanent. The hub's reason is relayed where
// present (sanitised to a single SMTP-safe line).
func mapStatusToSMTP(status int, body string) error {
	switch {
	case status >= 200 && status < 300:
		return nil
	case status == 429:
		return &smtp.SMTPError{Code: 451, EnhancedCode: smtp.EnhancedCode{4, 7, 0}, Message: msgOr(sanitizeReason(body), "rate limit exceeded, try again later")}
	case status >= 400 && status < 500:
		return &smtp.SMTPError{Code: 451, EnhancedCode: smtp.EnhancedCode{4, 4, 1}, Message: msgOr(sanitizeReason(body), "upstream rejected (temporary)")}
	default:
		return &smtp.SMTPError{Code: 554, EnhancedCode: smtp.EnhancedCode{5, 0, 0}, Message: msgOr(sanitizeReason(body), "upstream rejected")}
	}
}

// sanitizeReason extracts a short, SMTP-safe single line from the hub's response body.
// Never logs/relays a full body (which could echo addresses); caps at 200 chars.
func sanitizeReason(body string) string {
	body = strings.TrimSpace(body)
	if body == "" {
		return ""
	}
	if i := strings.IndexAny(body, "\r\n"); i >= 0 {
		body = body[:i]
	}
	if len(body) > 200 {
		body = body[:200]
	}
	return body
}

func msgOr(s, def string) string {
	if s == "" {
		return def
	}
	return s
}

// loginServer implements the SMTP AUTH LOGIN exchange as a sasl.Server. go-sasl ships
// a LOGIN *client* but no server (spike §2). It accepts ANY credentials and ignores
// them. The exchange: challenge "Username:" → username → challenge "Password:" →
// password → done.
type loginServer struct{ step int }

func (l *loginServer) Next(_ []byte) (challenge []byte, done bool, err error) {
	switch l.step {
	case 0:
		l.step++
		return []byte("Username:"), false, nil
	case 1:
		l.step++
		return []byte("Password:"), false, nil
	default:
		return nil, true, nil
	}
}

// selfSignedCert generates an in-process self-signed leaf for the shim (CN/SAN = the
// service name). Apps on the Docker network are told to accept invalid certs; the cert
// only terminates TLS for force_tls/starttls apps, it is not a trust anchor.
func selfSignedCert(serviceName string) (tls.Certificate, error) {
	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	if err != nil {
		return tls.Certificate{}, err
	}
	serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
	if err != nil {
		return tls.Certificate{}, err
	}
	tmpl := x509.Certificate{
		SerialNumber:          serial,
		Subject:               pkix.Name{CommonName: serviceName},
		DNSNames:              []string{serviceName, "localhost"},
		NotBefore:             time.Now().Add(-time.Hour),
		NotAfter:              time.Now().Add(10 * 365 * 24 * time.Hour),
		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		BasicConstraintsValid: true,
	}
	der, err := x509.CreateCertificate(rand.Reader, &tmpl, &tmpl, &key.PublicKey, key)
	if err != nil {
		return tls.Certificate{}, err
	}
	return tls.Certificate{Certificate: [][]byte{der}, PrivateKey: key}, nil
}