dnswatcher/internal/tlscheck/tlscheck.go

// Package tlscheck provides TLS certificate inspection.
package tlscheck

import (
	"context"
	"crypto/tls"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"strconv"
	"time"

	"go.uber.org/fx"

	"sneak.berlin/go/dnswatcher/internal/logger"
)

const (
	defaultTimeout = 10 * time.Second
	defaultPort    = 443
)

// ErrUnexpectedConnType indicates the connection was not a TLS
// connection.
var ErrUnexpectedConnType = errors.New(
	"unexpected connection type",
)

// CertificateInfo holds information about a TLS certificate.
type CertificateInfo struct {
	CommonName              string
	Issuer                  string
	NotAfter                time.Time
	SubjectAlternativeNames []string
	SerialNumber            string
}

// Option configures a Checker.
type Option func(*Checker)

// WithTimeout sets the connection timeout.
func WithTimeout(d time.Duration) Option {
	return func(c *Checker) {
		c.timeout = d
	}
}

// WithTLSConfig sets a custom TLS configuration.
func WithTLSConfig(cfg *tls.Config) Option {
	return func(c *Checker) {
		c.tlsConfig = cfg
	}
}

// WithPort sets the TLS port to connect to.
func WithPort(port int) Option {
	return func(c *Checker) {
		c.port = port
	}
}

// Params contains dependencies for Checker.
type Params struct {
	fx.In

	Logger *logger.Logger
}

// Checker performs TLS certificate inspection.
type Checker struct {
	log       *slog.Logger
	timeout   time.Duration
	tlsConfig *tls.Config
	port      int
}

// New creates a new TLS Checker instance.
func New(
	_ fx.Lifecycle,
	params Params,
) (*Checker, error) {
	return &Checker{
		log:     params.Logger.Get(),
		timeout: defaultTimeout,
		port:    defaultPort,
	}, nil
}

// NewStandalone creates a Checker without fx dependencies.
func NewStandalone(opts ...Option) *Checker {
	checker := &Checker{
		log:     slog.Default(),
		timeout: defaultTimeout,
		port:    defaultPort,
	}

	for _, opt := range opts {
		opt(checker)
	}

	return checker
}

// CheckCertificate connects to the given IP address using the
// specified SNI hostname and returns certificate information.
func (c *Checker) CheckCertificate(
	ctx context.Context,
	ipAddress string,
	sniHostname string,
) (*CertificateInfo, error) {
	target := net.JoinHostPort(
		ipAddress, strconv.Itoa(c.port),
	)

	tlsCfg := c.buildTLSConfig(sniHostname)
	dialer := &tls.Dialer{
		NetDialer: &net.Dialer{Timeout: c.timeout},
		Config:    tlsCfg,
	}

	conn, err := dialer.DialContext(ctx, "tcp", target)
	if err != nil {
		return nil, fmt.Errorf(
			"TLS dial to %s: %w", target, err,
		)
	}

	defer func() {
		closeErr := conn.Close()
		if closeErr != nil {
			c.log.Debug(
				"closing TLS connection",
				"target", target,
				"error", closeErr.Error(),
			)
		}
	}()

	tlsConn, ok := conn.(*tls.Conn)
	if !ok {
		return nil, fmt.Errorf(
			"%s: %w", target, ErrUnexpectedConnType,
		)
	}

	return c.extractCertInfo(tlsConn), nil
}

func (c *Checker) buildTLSConfig(
	sniHostname string,
) *tls.Config {
	if c.tlsConfig != nil {
		cfg := c.tlsConfig.Clone()
		cfg.ServerName = sniHostname

		return cfg
	}

	return &tls.Config{
		ServerName: sniHostname,
		MinVersion: tls.VersionTLS12,
	}
}

func (c *Checker) extractCertInfo(
	conn *tls.Conn,
) *CertificateInfo {
	state := conn.ConnectionState()
	if len(state.PeerCertificates) == 0 {
		return &CertificateInfo{}
	}

	cert := state.PeerCertificates[0]

	sans := make([]string, len(cert.DNSNames))
	copy(sans, cert.DNSNames)

	return &CertificateInfo{
		CommonName:              cert.Subject.CommonName,
		Issuer:                  cert.Issuer.CommonName,
		NotAfter:                cert.NotAfter,
		SubjectAlternativeNames: sans,
		SerialNumber:            cert.SerialNumber.String(),
	}
}