checker-smtp/checker/smtp.go

package checker

import (
	"bufio"
	"crypto/tls"
	"fmt"
	"io"
	"net"
	"strconv"
	"strings"
	"time"
)

// smtpConn is a minimal ESMTP client focused on *observation*, not on
// transmitting mail: we never reach DATA, and every transaction is torn
// down with RSET + QUIT. Writing our own client (instead of using
// net/smtp) gives us access to the raw banner text, per-line extension
// responses, and the timing of each step.
type smtpConn struct {
	raw     net.Conn
	br      *bufio.Reader
	wr      io.Writer
	timeout time.Duration
}

func newSMTPConn(c net.Conn, timeout time.Duration) *smtpConn {
	return &smtpConn{
		raw:     c,
		br:      bufio.NewReader(c),
		wr:      c,
		timeout: timeout,
	}
}

// touch extends the per-turn deadline on the underlying connection.
func (s *smtpConn) touch() {
	if s.timeout > 0 {
		_ = s.raw.SetDeadline(time.Now().Add(s.timeout))
	}
}

// swap replaces the underlying connection (used after STARTTLS).
func (s *smtpConn) swap(c net.Conn) {
	s.raw = c
	s.br = bufio.NewReader(c)
	s.wr = c
	s.touch()
}

// writeLine writes a single CRLF-terminated line.
func (s *smtpConn) writeLine(line string) error {
	s.touch()
	_, err := io.WriteString(s.wr, line+"\r\n")
	return err
}

// readResponse reads a multiline SMTP response ("xxx-...\r\nxxx ...\r\n")
// and returns (code, full-joined-text, raw-lines, error).
//
// Any transport error is surfaced immediately; a malformed line (missing
// 3-digit code, missing separator) yields an error with the offending
// text so callers can surface it verbatim.
func (s *smtpConn) readResponse() (code int, text string, lines []string, err error) {
	for {
		s.touch()
		line, rerr := s.br.ReadString('\n')
		if rerr != nil && line == "" {
			return 0, "", lines, rerr
		}
		line = strings.TrimRight(line, "\r\n")
		if len(line) < 4 {
			return 0, line, append(lines, line), fmt.Errorf("short SMTP line %q", line)
		}
		cStr := line[:3]
		sep := line[3]
		rest := line[4:]
		c, nerr := strconv.Atoi(cStr)
		if nerr != nil {
			return 0, line, append(lines, line), fmt.Errorf("bad SMTP code in %q", line)
		}
		if code == 0 {
			code = c
		}
		lines = append(lines, line)
		if sep == ' ' {
			text += rest
			return code, text, lines, nil
		}
		if sep == '-' {
			text += rest + "\n"
			continue
		}
		return 0, line, lines, fmt.Errorf("bad separator %q in %q", sep, line)
	}
}

// cmd writes a command and returns the server response.
func (s *smtpConn) cmd(line string) (int, string, []string, error) {
	if err := s.writeLine(line); err != nil {
		return 0, "", nil, err
	}
	return s.readResponse()
}

// close attempts a graceful QUIT then closes the socket. Errors are
// swallowed; the caller has already captured everything interesting.
func (s *smtpConn) close() {
	_ = s.writeLine("QUIT")
	_, _, _, _ = s.readResponse()
	_ = s.raw.Close()
}

// parseBanner teases the announced hostname out of the 220 greeting.
// ESMTP convention is "220 <hostname> <greeting-text>", but a number of
// servers deviate, so we return the first whitespace-delimited token that
// looks like a FQDN. Empty string when nothing looks plausible.
func parseBanner(text string) string {
	for f := range strings.FieldsSeq(text) {
		// Skip things that are obviously not a hostname.
		if strings.Contains(f, "@") {
			continue
		}
		if !strings.Contains(f, ".") {
			continue
		}
		// Strip trailing punctuation.
		f = strings.TrimRight(f, ",;:.")
		if looksLikeHostname(f) {
			return f
		}
	}
	return ""
}

func looksLikeHostname(s string) bool {
	if s == "" || len(s) > 253 {
		return false
	}
	// A hostname has at least one dot and no invalid characters.
	if strings.ContainsAny(s, " \t\r\n<>\"()[]") {
		return false
	}
	// We tolerate '_' even though RFC 1123 forbids it in hostnames: this
	// helper only classifies tokens parsed out of an SMTP banner for
	// display, never for routing or certificate matching, and a number of
	// real-world MTAs announce names with underscores.
	for _, r := range s {
		if r == '.' || r == '-' || r == '_' ||
			(r >= '0' && r <= '9') || (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') {
			continue
		}
		return false
	}
	return true
}

// parseEHLO splits an EHLO response into its keyword/arg lines. The
// first line is the greeting hostname, subsequent lines are extensions.
func parseEHLO(lines []string) (greeting string, extensions []string) {
	for i, l := range lines {
		if len(l) < 4 {
			continue
		}
		payload := strings.TrimSpace(l[4:])
		if i == 0 {
			greeting = payload
			continue
		}
		extensions = append(extensions, payload)
	}
	return greeting, extensions
}

// extensionLookup indexes parsed EHLO extensions by their (uppercased)
// keyword, preserving the argument portion unchanged.
type extensionLookup map[string]string

func buildExtensions(exts []string) extensionLookup {
	m := extensionLookup{}
	for _, e := range exts {
		kw, arg, _ := strings.Cut(e, " ")
		m[strings.ToUpper(strings.TrimSpace(kw))] = strings.TrimSpace(arg)
	}
	return m
}

func (m extensionLookup) has(kw string) bool {
	_, ok := m[kw]
	return ok
}

// parseSize extracts the integer argument of the SIZE extension (0 when
// absent or unparseable). SIZE may be advertised without an argument;
// we treat that as "no limit declared".
func (m extensionLookup) parseSize() uint64 {
	v, ok := m["SIZE"]
	if !ok || v == "" {
		return 0
	}
	n, err := strconv.ParseUint(strings.Fields(v)[0], 10, 64)
	if err != nil {
		return 0
	}
	return n
}

// parseAuth returns the SASL mechanisms advertised under the AUTH
// extension, upper-cased for easy comparison. Empty slice when AUTH is
// not advertised.
func (m extensionLookup) parseAuth() []string {
	v, ok := m["AUTH"]
	if !ok || v == "" {
		return nil
	}
	out := strings.Fields(v)
	for i := range out {
		out[i] = strings.ToUpper(out[i])
	}
	return out
}

// tlsProbeConfig mirrors the XMPP checker's stance: certificate
// verification is checker-tls' job, so we skip it here.
func tlsProbeConfig(serverName string) *tls.Config {
	return &tls.Config{
		ServerName:         serverName,
		InsecureSkipVerify: true, //nolint:gosec (cert validation is the TLS checker's job)
		MinVersion:         tls.VersionTLS10,
	}
}