add Tls check

check/check.go

@@ -29,6 +29,7 @@ var Passive = []checkip.Check{
 var Active = []checkip.Check{
 	Ping,
 	TcpPorts,
+	Tls,
 }
 
 // na returns "n/a" if s is empty.

check/tls.go

@@ -0,0 +1,85 @@
+package check
+
+import (
+	"crypto/tls"
+	"encoding/json"
+	"fmt"
+	"net"
+	"strings"
+	"time"
+
+	"github.com/jreisinger/checkip"
+)
+
+const certTLSDialTimeout = 5 * time.Second
+
+type tlsinfo struct {
+	SAN     []string
+	version string
+}
+
+func (t tlsinfo) Summary() string {
+	s := strings.Join(t.SAN, ", ")
+	return fmt.Sprintf("version: %s, SAN: %s", na(t.version), na(s))
+}
+
+func (t tlsinfo) Json() ([]byte, error) {
+	return json.Marshal(t)
+}
+
+// Tls finds out TLS version and SANs by connecting to the ipaddr and TCP port
+// 443.
+func Tls(ipaddr net.IP) (checkip.Result, error) {
+	result := checkip.Result{
+		Name: "tls",
+		Type: checkip.TypeInfo,
+	}
+
+	address := net.JoinHostPort(ipaddr.String(), "443")
+	conn, err := tls.DialWithDialer(&net.Dialer{Timeout: certTLSDialTimeout}, "tcp", address, &tls.Config{InsecureSkipVerify: true})
+	if err != nil {
+		return result, newCheckError(err)
+	}
+	defer conn.Close()
+
+	// search only unique dns names
+	dnsSet := make(map[string]struct{})
+	var dnsNames []string
+	for _, cert := range conn.ConnectionState().PeerCertificates {
+		for _, dnsName := range cert.DNSNames {
+			if _, ok := dnsSet[dnsName]; ok {
+				continue
+			}
+			dnsNames = append(dnsNames, dnsName)
+			dnsSet[dnsName] = struct{}{}
+		}
+	}
+
+	t := tlsinfo{
+		SAN:     dnsNames,
+		version: tlsFormat(conn.ConnectionState().Version),
+	}
+
+	result.Info = t
+
+	return result, nil
+}
+
+func tlsFormat(tlsVersion uint16) string {
+	switch tlsVersion {
+	case 0:
+		return ""
+	case tls.VersionSSL30:
+		return "SSLv3 - Deprecated!"
+	case tls.VersionTLS10:
+		return "TLS 1.0 - Deprecated!"
+	case tls.VersionTLS11:
+		return "TLS 1.1 - Deprecated!"
+	case tls.VersionTLS12:
+		return "TLS 1.2"
+	case tls.VersionTLS13:
+		return "TLS 1.3"
+	default:
+		return "TLS Version %d (unknown)"
+	}
+}