check_ssl_certificate - SMTP STARTTLS support

* Refactored code so that obtaining TLS certificates from server can
  have different implementations
* Added SMTP-specific implementation that will send a HELO followed
  by a STARTTLS.

cmd/check_ssl_certificate/main.go

@@ -4,6 +4,8 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"fmt"
+	"net"
+	"net/textproto"
 	"os"
 	"strings"
 	"time"
@@ -14,6 +16,72 @@ import (
 	"github.com/karrick/golf"
 )
 
+//--------------------------------------------------------------------------------------------------------
+
+// Interface that can be implemented to fetch TLS certificates.
+type certGetter interface {
+	getCertificate(tlsConfig *tls.Config, address string) (*x509.Certificate, error)
+}
+
+// Full TLS certificate fetcher
+type fullTLSGetter struct{}
+
+func (f fullTLSGetter) getCertificate(tlsConfig *tls.Config, address string) (*x509.Certificate, error) {
+	conn, err := tls.Dial("tcp", address, tlsConfig)
+	if err != nil {
+		return nil, err
+	}
+	defer conn.Close()
+	if err := conn.Handshake(); err != nil {
+		return nil, err
+	}
+	return conn.ConnectionState().PeerCertificates[0], nil
+}
+
+// SMTP STARTTLS getter
+type smtpGetter struct{}
+
+func (f smtpGetter) cmd(tcon *textproto.Conn, expectCode int, text string) (int, string, error) {
+	id, err := tcon.Cmd("%s", text)
+	if err != nil {
+		return 0, "", err
+	}
+	tcon.StartResponse(id)
+	defer tcon.EndResponse(id)
+	return tcon.ReadResponse(expectCode)
+}
+
+func (f smtpGetter) getCertificate(tlsConfig *tls.Config, address string) (*x509.Certificate, error) {
+	conn, err := net.Dial("tcp", address)
+	if err != nil {
+		return nil, err
+	}
+	text := textproto.NewConn(conn)
+	defer text.Close()
+	if _, _, err := text.ReadResponse(220); err != nil {
+		return nil, err
+	}
+	if _, _, err := f.cmd(text, 250, "HELO localhost"); err != nil {
+		return nil, err
+	}
+	if _, _, err := f.cmd(text, 220, "STARTTLS"); err != nil {
+		return nil, err
+	}
+	t := tls.Client(conn, tlsConfig)
+	if err := t.Handshake(); err != nil {
+		return nil, err
+	}
+	return t.ConnectionState().PeerCertificates[0], nil
+}
+
+// Supported StartTLS protocols
+var certGetters map[string]certGetter = map[string]certGetter{
+	"":     fullTLSGetter{},
+	"smtp": &smtpGetter{},
+}
+
+//--------------------------------------------------------------------------------------------------------
+
 // Command line flags that have been parsed.
 type programFlags struct {
 	hostname     string   // Main host name to connect to
@@ -22,12 +90,14 @@ type programFlags struct {
 	crit         int      // Threshold for critical state (days)
 	ignoreCnOnly bool     // Do not warn about SAN-less certificates
 	extraNames   []string // Extra names the certificate should include
+	startTLS     string   // Protocol to use before requesting a switch to TLS.
 }
 
 // Program data including configuration and runtime data.
 type checkProgram struct {
 	programFlags                   // Flags from the command line
 	plugin       *plugin.Plugin    // Plugin output state
+	getter       certGetter        // Certificate getter
 	certificate  *x509.Certificate // X.509 certificate from the server
 }
 
@@ -49,6 +119,8 @@ func (flags *programFlags) parseArguments() {
 		"Do not issue warnings regarding certificates that do not use SANs at all.")
 	golf.StringVarP(&names, 'a', "additional-names", "",
 		"A comma-separated list of names that the certificate should also provide.")
+	golf.StringVarP(&flags.startTLS, 's', "start-tls", "",
+		"Protocol to use before requesting a switch to TLS. Supported protocols: smtp.")
 	golf.Parse()
 	if help {
 		golf.Usage()
@@ -90,6 +162,11 @@ func (program *checkProgram) checkFlags() bool {
 		program.plugin.SetState(plugin.UNKNOWN, "nonsensical thresholds")
 		return false
 	}
+	if _, ok := certGetters[program.startTLS]; !ok {
+		errstr := fmt.Sprintf("unsupported StartTLS protocol %s", program.startTLS)
+		program.plugin.SetState(plugin.UNKNOWN, errstr)
+		return false
+	}
 	program.hostname = strings.ToLower(program.hostname)
 	return true
 }
@@ -102,18 +179,13 @@ func (program *checkProgram) getCertificate() error {
 		MinVersion:         tls.VersionTLS10,
 	}
 	connString := fmt.Sprintf("%s:%d", program.hostname, program.port)
-	conn, err := tls.Dial("tcp", connString, tlsConfig)
-	if err != nil {
-		return fmt.Errorf("connection failed: %s", err.Error())
-	}
-	defer conn.Close()
-	if err := conn.Handshake(); err != nil {
-		return fmt.Errorf("handshake failed: %s", err.Error())
-	}
-	program.certificate = conn.ConnectionState().PeerCertificates[0]
-	return nil
+	certificate, err := certGetters[program.startTLS].getCertificate(tlsConfig, connString)
+	program.certificate = certificate
+	return err
 }
 
+// Check that the CN of a certificate that doesn't contain a SAN actually
+// matches the requested host name.
 func (program *checkProgram) checkSANlessCertificate() bool {
 	if !program.ignoreCnOnly || len(program.extraNames) != 0 {
 		program.plugin.SetState(plugin.WARNING,