TLS config: Enable selection of min and max TLS version

go1.18 changes the default minimum TLS version to 1.2.

In order not to break our users, let's make the default minimum version
configurable.

The allowed values (TLS10, ..) come from the exporter-toolkit:
https://github.com/prometheus/exporter-toolkit/blob/master/docs/web-configuration.md

TLSVersion is exported so the exporter toolkit can reuse them later.
DefaultTLSVersion is exposed so Prometheus can keep a conservative
default.

Signed-off-by: Julien Pivotto <roidelapluie@o11y.eu>

config/http_config.go

@@ -36,25 +36,63 @@ import (
 	"gopkg.in/yaml.v2"
 )
 
-// DefaultHTTPClientConfig is the default HTTP client configuration.
-var DefaultHTTPClientConfig = HTTPClientConfig{
-	FollowRedirects: true,
-	EnableHTTP2:     true,
-}
+var (
+	// DefaultHTTPClientConfig is the default HTTP client configuration.
+	DefaultHTTPClientConfig = HTTPClientConfig{
+		FollowRedirects: true,
+		EnableHTTP2:     true,
+	}
 
-// defaultHTTPClientOptions holds the default HTTP client options.
-var defaultHTTPClientOptions = httpClientOptions{
-	keepAlivesEnabled: true,
-	http2Enabled:      true,
-	// 5 minutes is typically above the maximum sane scrape interval. So we can
-	// use keepalive for all configurations.
-	idleConnTimeout: 5 * time.Minute,
-}
+	// defaultHTTPClientOptions holds the default HTTP client options.
+	defaultHTTPClientOptions = httpClientOptions{
+		keepAlivesEnabled: true,
+		http2Enabled:      true,
+		// 5 minutes is typically above the maximum sane scrape interval. So we can
+		// use keepalive for all configurations.
+		idleConnTimeout: 5 * time.Minute,
+	}
+
+	// DefaultTLSConfig holds the default TLS configuration.
+	DefaultTLSConfig = TLSConfig{
+		MinVersion: tls.VersionTLS12,
+	}
+)
 
 type closeIdler interface {
 	CloseIdleConnections()
 }
 
+type TLSVersion uint16
+
+var TLSVersions = map[string]TLSVersion{
+	"TLS13": (TLSVersion)(tls.VersionTLS13),
+	"TLS12": (TLSVersion)(tls.VersionTLS12),
+	"TLS11": (TLSVersion)(tls.VersionTLS11),
+	"TLS10": (TLSVersion)(tls.VersionTLS10),
+}
+
+func (tv *TLSVersion) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	var s string
+	err := unmarshal((*string)(&s))
+	if err != nil {
+		return err
+	}
+	if v, ok := TLSVersions[s]; ok {
+		*tv = v
+		return nil
+	}
+	return fmt.Errorf("unknown TLS version: %s", s)
+}
+
+func (tv *TLSVersion) MarshalYAML() (interface{}, error) {
+	for s, v := range TLSVersions {
+		if *tv == v {
+			return s, nil
+		}
+	}
+	return fmt.Sprintf("%v", tv), nil
+}
+
 // BasicAuth contains basic HTTP authentication credentials.
 type BasicAuth struct {
 	Username     string `yaml:"username" json:"username"`
@@ -669,7 +707,17 @@ func cloneRequest(r *http.Request) *http.Request {
 
 // NewTLSConfig creates a new tls.Config from the given TLSConfig.
 func NewTLSConfig(cfg *TLSConfig) (*tls.Config, error) {
-	tlsConfig := &tls.Config{InsecureSkipVerify: cfg.InsecureSkipVerify}
+	// Ensure that min and max versions are set even if TLSConfig is from its
+	// zero-value.
+	minv := uint16(cfg.MinVersion)
+	if minv == 0 {
+		minv = uint16(DefaultTLSConfig.MinVersion)
+	}
+
+	tlsConfig := &tls.Config{
+		InsecureSkipVerify: cfg.InsecureSkipVerify,
+		MinVersion:         minv,
+	}
 
 	// If a CA cert is provided then let's read it in so we can validate the
 	// scrape target's certificate properly.
@@ -714,6 +762,8 @@ type TLSConfig struct {
 	ServerName string `yaml:"server_name,omitempty" json:"server_name,omitempty"`
 	// Disable target certificate validation.
 	InsecureSkipVerify bool `yaml:"insecure_skip_verify" json:"insecure_skip_verify"`
+	// Minimum TLS version.
+	MinVersion TLSVersion `yaml:"min_version,omitempty"`
 }
 
 // SetDirectory joins any relative file paths with dir.
@@ -728,6 +778,7 @@ func (c *TLSConfig) SetDirectory(dir string) {
 
 // UnmarshalYAML implements the yaml.Unmarshaler interface.
 func (c *TLSConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	*c = DefaultTLSConfig
 	type plain TLSConfig
 	return unmarshal((*plain)(c))
 }

config/http_config_test.go

@@ -627,7 +627,9 @@ func TestTLSConfig(t *testing.T) {
 		CertFile:           ClientCertificatePath,
 		KeyFile:            ClientKeyNoPassPath,
 		ServerName:         "localhost",
-		InsecureSkipVerify: false}
+		InsecureSkipVerify: false,
+		MinVersion:         tls.VersionTLS12,
+	}
 
 	tlsCAChain, err := ioutil.ReadFile(TLSCAChainPath)
 	if err != nil {
@@ -640,7 +642,9 @@ func TestTLSConfig(t *testing.T) {
 	expectedTLSConfig := &tls.Config{
 		RootCAs:            rootCAs,
 		ServerName:         configTLSConfig.ServerName,
-		InsecureSkipVerify: configTLSConfig.InsecureSkipVerify}
+		InsecureSkipVerify: configTLSConfig.InsecureSkipVerify,
+		MinVersion:         tls.VersionTLS12,
+	}
 
 	tlsConfig, err := NewTLSConfig(&configTLSConfig)
 	if err != nil {
@@ -683,6 +687,7 @@ func TestTLSConfigEmpty(t *testing.T) {
 
 	expectedTLSConfig := &tls.Config{
 		InsecureSkipVerify: configTLSConfig.InsecureSkipVerify,
+		MinVersion:         tls.VersionTLS12,
 	}
 
 	tlsConfig, err := NewTLSConfig(&configTLSConfig)

config/testdata/tls_config.tlsversion.good.yml

@@ -0,0 +1 @@
+min_version: TLS11

config/tls_config_test.go

@@ -41,10 +41,13 @@ var expectedTLSConfigs = []struct {
 }{
 	{
 		filename: "tls_config.empty.good.yml",
-		config:   &tls.Config{},
+		config:   &tls.Config{MinVersion: tls.VersionTLS12},
 	}, {
 		filename: "tls_config.insecure.good.yml",
-		config:   &tls.Config{InsecureSkipVerify: true},
+		config:   &tls.Config{InsecureSkipVerify: true, MinVersion: tls.VersionTLS12},
+	}, {
+		filename: "tls_config.tlsversion.good.yml",
+		config:   &tls.Config{MinVersion: tls.VersionTLS11},
 	},
 }