checker-tls/checker/starttls_ldap.go

package checker

import (
	"bufio"
	"fmt"
	"io"
	"net"
)

func init() {
	registerStartTLS("ldap", starttlsLDAP)
}

// starttlsLDAP implements RFC 2830 StartTLS.
//
// It sends a single ExtendedRequest with OID 1.3.6.1.4.1.1466.20037 on
// messageID=1 and reads back the ExtendedResponse. On resultCode 0 the
// connection is ready for the TLS handshake. On resultCode 2
// (protocolError) or 53 (unwillingToPerform) we wrap errStartTLSNotOffered
// -- the server is reachable but cannot upgrade -- so the caller can surface
// that as a missing-STARTTLS issue rather than a handshake failure.
func starttlsLDAP(conn net.Conn, sni string) error {
	// Fixed LDAPMessage:
	//   SEQUENCE {
	//     INTEGER messageID = 1,
	//     [APPLICATION 23] SEQUENCE {
	//       [0] OCTET STRING "1.3.6.1.4.1.1466.20037"
	//     }
	//   }
	request := []byte{
		0x30, 0x1d,
		0x02, 0x01, 0x01,
		0x77, 0x18,
		0x80, 0x16,
		'1', '.', '3', '.', '6', '.', '1', '.', '4', '.', '1', '.',
		'1', '4', '6', '6', '.', '2', '0', '0', '3', '7',
	}
	if _, err := conn.Write(request); err != nil {
		return fmt.Errorf("write StartTLS request: %w", err)
	}

	r := bufio.NewReader(conn)

	tag, err := r.ReadByte()
	if err != nil {
		return fmt.Errorf("read response: %w", err)
	}
	if tag != 0x30 {
		return fmt.Errorf("unexpected LDAP response tag 0x%02x", tag)
	}
	length, err := readBERLength(r)
	if err != nil {
		return fmt.Errorf("read response length: %w", err)
	}
	// 16 KiB comfortably accommodates an ExtendedResponse with a verbose
	// diagnosticMessage while still bounding memory against a hostile peer.
	const maxLDAPResponseBytes = 16 * 1024
	if length <= 0 || length > maxLDAPResponseBytes {
		return fmt.Errorf("unreasonable LDAP response length %d", length)
	}
	body := make([]byte, length)
	if _, err := io.ReadFull(r, body); err != nil {
		return fmt.Errorf("read response body: %w", err)
	}

	// messageID INTEGER -- skip.
	pos := 0
	if pos >= len(body) || body[pos] != 0x02 {
		return fmt.Errorf("expected INTEGER messageID")
	}
	pos++
	if pos >= len(body) {
		return fmt.Errorf("truncated messageID length")
	}
	msgIDLen := int(body[pos])
	pos++
	pos += msgIDLen
	if pos >= len(body) {
		return fmt.Errorf("truncated LDAP response")
	}

	// [APPLICATION 24] constructed = 0x78.
	if body[pos] != 0x78 {
		return fmt.Errorf("expected ExtendedResponse tag, got 0x%02x", body[pos])
	}
	pos++
	// Skip extendedResp length (possibly multi-byte).
	if pos >= len(body) {
		return fmt.Errorf("truncated ExtendedResponse length")
	}
	if body[pos] < 0x80 {
		pos++
	} else {
		n := int(body[pos] & 0x7f)
		pos += 1 + n
	}

	// resultCode ENUMERATED (tag 0x0a).
	if pos+2 > len(body) || body[pos] != 0x0a {
		return fmt.Errorf("expected resultCode ENUMERATED")
	}
	rcLen := int(body[pos+1])
	if rcLen < 1 || pos+2+rcLen > len(body) {
		return fmt.Errorf("invalid resultCode length %d", rcLen)
	}
	rc := 0
	for i := 0; i < rcLen; i++ {
		rc = (rc << 8) | int(body[pos+2+i])
	}
	switch rc {
	case 0:
		return nil
	case 2, 53:
		return fmt.Errorf("%w: LDAP StartTLS refused (resultCode=%d)", errStartTLSNotOffered, rc)
	default:
		return fmt.Errorf("server refused StartTLS (LDAP resultCode=%d)", rc)
	}
}

// readBERLength reads a definite-form BER length (short or long form).
func readBERLength(r *bufio.Reader) (int, error) {
	b, err := r.ReadByte()
	if err != nil {
		return 0, err
	}
	if b < 0x80 {
		return int(b), nil
	}
	n := int(b & 0x7f)
	if n == 0 {
		return 0, fmt.Errorf("indefinite-form length not supported")
	}
	if n > 4 {
		return 0, fmt.Errorf("length octet count %d too large", n)
	}
	length := 0
	for i := 0; i < n; i++ {
		bb, err := r.ReadByte()
		if err != nil {
			return 0, err
		}
		length = (length << 8) | int(bb)
	}
	return length, nil
}