1 files changed, 208 insertions, 0 deletions
diff --git a/internal/smtpclient/client.go b/internal/smtpclient/client.go
new file mode 100644
index 0000000..d4ee686
--- /dev/null
+++ b/internal/smtpclient/client.go
@@ -0,0 +1,208 @@
+package smtpclient
+
+import (
+	"bufio"
+	"context"
+	"crypto/tls"
+	"fmt"
+	"io"
+	"net"
+	"net/textproto"
+	"strings"
+	"time"
+)
+
+type DialContextFunc func(ctx context.Context, network, address string) (net.Conn, error)
+
+type Config struct {
+	Host          string
+	Port          int
+	Recipient     string
+	EnvelopeFrom  string
+	HELO          string
+	TLSServerName string
+	RequireTLS    bool
+	ImplicitTLS   bool
+	Timeout       time.Duration
+	DryRun        bool
+}
+
+type Client struct {
+	cfg  Config
+	dial DialContextFunc
+}
+
+type Message struct {
+	EnvelopeFrom string
+	Raw          string
+}
+
+func New(cfg Config, dial DialContextFunc) *Client {
+	return &Client{cfg: cfg, dial: dial}
+}
+
+func (c *Client) Send(ctx context.Context, msg Message) error {
+	if c.cfg.DryRun {
+		return nil
+	}
+	if c.dial == nil {
+		c.dial = (&net.Dialer{}).DialContext
+	}
+	timeout := c.cfg.Timeout
+	if timeout <= 0 {
+		timeout = 90 * time.Second
+	}
+	ctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	addr := net.JoinHostPort(c.cfg.Host, fmt.Sprintf("%d", c.cfg.Port))
+	conn, err := c.dial(ctx, "tcp", addr)
+	if err != nil {
+		return fmt.Errorf("dial smtp: %w", err)
+	}
+	defer conn.Close()
+	if deadline, ok := ctx.Deadline(); ok {
+		_ = conn.SetDeadline(deadline)
+	}
+	if c.cfg.ImplicitTLS {
+		tlsConn := tls.Client(conn, &tls.Config{
+			ServerName: c.cfg.TLSServerName,
+			MinVersion: tls.VersionTLS12,
+		})
+		if err := tlsConn.HandshakeContext(ctx); err != nil {
+			return fmt.Errorf("implicit tls handshake: %w", err)
+		}
+		conn = tlsConn
+	}
+
+	session := newSession(conn)
+	if _, _, err := session.read(220); err != nil {
+		return fmt.Errorf("smtp greeting: %w", err)
+	}
+
+	if err := session.ehlo(c.cfg.HELO); err != nil {
+		return err
+	}
+
+	if c.cfg.RequireTLS && !c.cfg.ImplicitTLS {
+		if _, _, err := session.cmd(220, "STARTTLS\r\n"); err != nil {
+			return fmt.Errorf("starttls: %w", err)
+		}
+		tlsConn := tls.Client(conn, &tls.Config{
+			ServerName: c.cfg.TLSServerName,
+			MinVersion: tls.VersionTLS12,
+		})
+		if err := tlsConn.HandshakeContext(ctx); err != nil {
+			return fmt.Errorf("tls handshake: %w", err)
+		}
+		session = newSession(tlsConn)
+		if err := session.ehlo(c.cfg.HELO); err != nil {
+			return err
+		}
+	}
+
+	envelopeFrom := sanitizeEnvelope(c.cfg.EnvelopeFrom)
+	if envelopeFrom == "" {
+		envelopeFrom = sanitizeEnvelope(msg.EnvelopeFrom)
+	}
+	if _, _, err := session.cmd(250, "MAIL FROM:<%s>\r\n", envelopeFrom); err != nil {
+		return fmt.Errorf("mail from rejected: %w", err)
+	}
+	if _, _, err := session.cmd(250, "RCPT TO:<%s>\r\n", sanitizeEnvelope(c.cfg.Recipient)); err != nil {
+		return fmt.Errorf("rcpt to rejected: %w", err)
+	}
+	if _, _, err := session.cmd(354, "DATA\r\n"); err != nil {
+		return fmt.Errorf("data rejected: %w", err)
+	}
+	if err := writeSMTPData(session.w, msg.Raw); err != nil {
+		return err
+	}
+	if _, _, err := session.read(250); err != nil {
+		return fmt.Errorf("message rejected: %w", err)
+	}
+	_, _, _ = session.cmd(221, "QUIT\r\n")
+	return nil
+}
+
+type session struct {
+	conn net.Conn
+	tp   *textproto.Reader
+	w    *bufio.Writer
+}
+
+func newSession(conn net.Conn) *session {
+	reader := bufio.NewReader(conn)
+	return &session{
+		conn: conn,
+		tp:   textproto.NewReader(reader),
+		w:    bufio.NewWriter(conn),
+	}
+}
+
+func (s *session) ehlo(helo string) error {
+	if helo == "" {
+		helo = "n2usenet.local"
+	}
+	if _, _, err := s.cmd(250, "EHLO %s\r\n", sanitizeAtom(helo)); err != nil {
+		if _, _, heloErr := s.cmd(250, "HELO %s\r\n", sanitizeAtom(helo)); heloErr != nil {
+			return fmt.Errorf("ehlo failed: %w", err)
+		}
+	}
+	return nil
+}
+
+func (s *session) cmd(expect int, format string, args ...any) (int, string, error) {
+	if _, err := fmt.Fprintf(s.w, format, args...); err != nil {
+		return 0, "", err
+	}
+	if err := s.w.Flush(); err != nil {
+		return 0, "", err
+	}
+	return s.read(expect)
+}
+
+func (s *session) read(expect int) (int, string, error) {
+	code, msg, err := s.tp.ReadResponse(expect)
+	if err != nil {
+		return code, msg, err
+	}
+	return code, msg, nil
+}
+
+func writeSMTPData(w *bufio.Writer, raw string) error {
+	raw = strings.ReplaceAll(raw, "\r\n", "\n")
+	raw = strings.ReplaceAll(raw, "\r", "\n")
+	for _, line := range strings.Split(raw, "\n") {
+		if strings.HasPrefix(line, ".") {
+			line = "." + line
+		}
+		if _, err := io.WriteString(w, line+"\r\n"); err != nil {
+			return fmt.Errorf("write smtp data: %w", err)
+		}
+	}
+	if _, err := io.WriteString(w, ".\r\n"); err != nil {
+		return fmt.Errorf("write smtp terminator: %w", err)
+	}
+	if err := w.Flush(); err != nil {
+		return fmt.Errorf("flush smtp data: %w", err)
+	}
+	return nil
+}
+
+func sanitizeEnvelope(v string) string {
+	v = strings.TrimSpace(v)
+	v = strings.ReplaceAll(v, "\r", "")
+	v = strings.ReplaceAll(v, "\n", "")
+	v = strings.Trim(v, "<>")
+	return v
+}
+
+func sanitizeAtom(v string) string {
+	v = strings.TrimSpace(v)
+	v = strings.ReplaceAll(v, "\r", "")
+	v = strings.ReplaceAll(v, "\n", "")
+	if v == "" {
+		return "n2usenet.local"
+	}
+	return v
+}