tls: remove deprecated prefer_server_cipher_suites field

The TLS configuration object includes a deprecated `prefer_server_cipher_suites`
field. In version of Go prior to 1.17, this property controlled whether a TLS
connection would use the cipher suites preferred by the server or by the
client. This field is ignored as of 1.17 and, according to the `crypto/tls`
docs: "Servers now select the best mutually supported cipher suite based on
logic that takes into account inferred client hardware, server hardware, and
security."

This property has been long-deprecated and leaving it in place may lead to false
assumptions about how cipher suites are negotiated in connection to a server. So
we want to remove it in Nomad 1.9.0.

Fixes: hashicorp/nomad-enterprise#999
Ref: https://hashicorp.atlassian.net/browse/NET-10531

.changelog/23712.txt

@@ -0,0 +1,3 @@
+```release-note:breaking-change
+tls: Removed deprecated `tls.prefer_server_cipher_suites` field from agent configuration
+```

command/agent/config_parse_test.go

@@ -282,17 +282,16 @@ var basicConfig = &Config{
 		},
 	}},
 	TLSConfig: &config.TLSConfig{
-		EnableHTTP:                  true,
-		EnableRPC:                   true,
-		VerifyServerHostname:        true,
-		CAFile:                      "foo",
-		CertFile:                    "bar",
-		KeyFile:                     "pipe",
-		RPCUpgradeMode:              true,
-		VerifyHTTPSClient:           true,
-		TLSPreferServerCipherSuites: true,
-		TLSCipherSuites:             "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
-		TLSMinVersion:               "tls12",
+		EnableHTTP:           true,
+		EnableRPC:            true,
+		VerifyServerHostname: true,
+		CAFile:               "foo",
+		CertFile:             "bar",
+		KeyFile:              "pipe",
+		RPCUpgradeMode:       true,
+		VerifyHTTPSClient:    true,
+		TLSCipherSuites:      "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
+		TLSMinVersion:        "tls12",
 	},
 	HTTPAPIResponseHeaders: map[string]string{
 		"Access-Control-Allow-Origin": "*",

command/agent/testdata/basic.hcl

@@ -290,17 +290,16 @@ vault {
 }
 
 tls {
-  http                            = true
-  rpc                             = true
-  verify_server_hostname          = true
-  ca_file                         = "foo"
-  cert_file                       = "bar"
-  key_file                        = "pipe"
-  rpc_upgrade_mode                = true
-  verify_https_client             = true
-  tls_prefer_server_cipher_suites = true
-  tls_cipher_suites               = "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256"
-  tls_min_version                 = "tls12"
+  http                   = true
+  rpc                    = true
+  verify_server_hostname = true
+  ca_file                = "foo"
+  cert_file              = "bar"
+  key_file               = "pipe"
+  rpc_upgrade_mode       = true
+  verify_https_client    = true
+  tls_cipher_suites      = "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256"
+  tls_min_version        = "tls12"
 }
 
 sentinel {

command/agent/testdata/basic.json

@@ -389,7 +389,6 @@
       "rpc_upgrade_mode": true,
       "tls_cipher_suites": "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
       "tls_min_version": "tls12",
-      "tls_prefer_server_cipher_suites": true,
       "verify_https_client": true,
       "verify_server_hostname": true
     }

helper/tlsutil/config.go

@@ -142,12 +142,6 @@ type Config struct {
 	// these values for acceptable safe alternatives.
 	CipherSuites []uint16
 
-	// PreferServerCipherSuites controls whether the server selects the
-	// client's most preferred ciphersuite, or the server's most preferred
-	// ciphersuite. If true then the server's preference, as expressed in
-	// the order of elements in CipherSuites, is used.
-	PreferServerCipherSuites bool
-
 	// MinVersion contains the minimum SSL/TLS version that is accepted.
 	MinVersion uint16
 }
@@ -164,16 +158,15 @@ func NewTLSConfiguration(newConf *config.TLSConfig, verifyIncoming, verifyOutgoi
 	}
 
 	return &Config{
-		VerifyIncoming:           verifyIncoming,
-		VerifyOutgoing:           verifyOutgoing,
-		VerifyServerHostname:     newConf.VerifyServerHostname,
-		CAFile:                   newConf.CAFile,
-		CertFile:                 newConf.CertFile,
-		KeyFile:                  newConf.KeyFile,
-		KeyLoader:                newConf.GetKeyLoader(),
-		CipherSuites:             ciphers,
-		MinVersion:               minVersion,
-		PreferServerCipherSuites: newConf.TLSPreferServerCipherSuites,
+		VerifyIncoming:       verifyIncoming,
+		VerifyOutgoing:       verifyOutgoing,
+		VerifyServerHostname: newConf.VerifyServerHostname,
+		CAFile:               newConf.CAFile,
+		CertFile:             newConf.CertFile,
+		KeyFile:              newConf.KeyFile,
+		KeyLoader:            newConf.GetKeyLoader(),
+		CipherSuites:         ciphers,
+		MinVersion:           minVersion,
 	}, nil
 }
 
@@ -231,11 +224,10 @@ func (c *Config) OutgoingTLSConfig() (*tls.Config, error) {
 	}
 	// Create the tlsConfig
 	tlsConfig := &tls.Config{
-		RootCAs:                  x509.NewCertPool(),
-		InsecureSkipVerify:       true,
-		CipherSuites:             c.CipherSuites,
-		MinVersion:               c.MinVersion,
-		PreferServerCipherSuites: c.PreferServerCipherSuites,
+		RootCAs:            x509.NewCertPool(),
+		InsecureSkipVerify: true,
+		CipherSuites:       c.CipherSuites,
+		MinVersion:         c.MinVersion,
 	}
 	if c.VerifyServerHostname {
 		tlsConfig.InsecureSkipVerify = false
@@ -349,11 +341,10 @@ func WrapTLSClient(conn net.Conn, tlsConfig *tls.Config) (net.Conn, error) {
 func (c *Config) IncomingTLSConfig() (*tls.Config, error) {
 	// Create the tlsConfig
 	tlsConfig := &tls.Config{
-		ClientCAs:                x509.NewCertPool(),
-		ClientAuth:               tls.NoClientCert,
-		CipherSuites:             c.CipherSuites,
-		MinVersion:               c.MinVersion,
-		PreferServerCipherSuites: c.PreferServerCipherSuites,
+		ClientCAs:    x509.NewCertPool(),
+		ClientAuth:   tls.NoClientCert,
+		CipherSuites: c.CipherSuites,
+		MinVersion:   c.MinVersion,
 	}
 
 	// Parse the CA cert if any

helper/tlsutil/config_test.go

@@ -409,32 +409,6 @@ func TestConfig_OutgoingTLS_WithKeyPair(t *testing.T) {
 	assert.NotNil(cert)
 }
 
-func TestConfig_OutgoingTLS_PreferServerCipherSuites(t *testing.T) {
-	ci.Parallel(t)
-
-	require := require.New(t)
-
-	{
-		conf := &Config{
-			VerifyOutgoing: true,
-			CAFile:         cacert,
-		}
-		tlsConfig, err := conf.OutgoingTLSConfig()
-		require.Nil(err)
-		require.Equal(tlsConfig.PreferServerCipherSuites, false)
-	}
-	{
-		conf := &Config{
-			VerifyOutgoing:           true,
-			CAFile:                   cacert,
-			PreferServerCipherSuites: true,
-		}
-		tlsConfig, err := conf.OutgoingTLSConfig()
-		require.Nil(err)
-		require.Equal(tlsConfig.PreferServerCipherSuites, true)
-	}
-}
-
 func TestConfig_OutgoingTLS_TLSCipherSuites(t *testing.T) {
 	ci.Parallel(t)
 
@@ -731,27 +705,6 @@ func TestConfig_IncomingTLS_NoVerify(t *testing.T) {
 	}
 }
 
-func TestConfig_IncomingTLS_PreferServerCipherSuites(t *testing.T) {
-	ci.Parallel(t)
-
-	require := require.New(t)
-
-	{
-		conf := &Config{}
-		tlsConfig, err := conf.IncomingTLSConfig()
-		require.Nil(err)
-		require.Equal(tlsConfig.PreferServerCipherSuites, false)
-	}
-	{
-		conf := &Config{
-			PreferServerCipherSuites: true,
-		}
-		tlsConfig, err := conf.IncomingTLSConfig()
-		require.Nil(err)
-		require.Equal(tlsConfig.PreferServerCipherSuites, true)
-	}
-}
-
 func TestConfig_IncomingTLS_TLSCipherSuites(t *testing.T) {
 	ci.Parallel(t)
 

nomad/structs/config/tls.go

@@ -67,12 +67,6 @@ type TLSConfig struct {
 	// connections. Should be either "tls10", "tls11", or "tls12".
 	TLSMinVersion string `hcl:"tls_min_version"`
 
-	// TLSPreferServerCipherSuites controls whether the server selects the
-	// client's most preferred ciphersuite, or the server's most preferred
-	// ciphersuite. If true then the server's preference, as expressed in
-	// the order of elements in CipherSuites, is used.
-	TLSPreferServerCipherSuites bool `hcl:"tls_prefer_server_cipher_suites"`
-
 	// ExtraKeysHCL is used by hcl to surface unexpected keys
 	ExtraKeysHCL []string `hcl:",unusedKeys" json:"-"`
 }
@@ -170,8 +164,6 @@ func (t *TLSConfig) Copy() *TLSConfig {
 	new.TLSCipherSuites = t.TLSCipherSuites
 	new.TLSMinVersion = t.TLSMinVersion
 
-	new.TLSPreferServerCipherSuites = t.TLSPreferServerCipherSuites
-
 	new.SetChecksum()
 
 	return new
@@ -225,9 +217,6 @@ func (t *TLSConfig) Merge(b *TLSConfig) *TLSConfig {
 	if b.TLSMinVersion != "" {
 		result.TLSMinVersion = b.TLSMinVersion
 	}
-	if b.TLSPreferServerCipherSuites {
-		result.TLSPreferServerCipherSuites = true
-	}
 	return result
 }
 

nomad/structs/config/tls_test.go

@@ -21,15 +21,14 @@ func TestTLSConfig_Merge(t *testing.T) {
 	}
 
 	b := &TLSConfig{
-		EnableHTTP:                  true,
-		EnableRPC:                   true,
-		VerifyServerHostname:        true,
-		CAFile:                      "test-ca-file-2",
-		CertFile:                    "test-cert-file-2",
-		RPCUpgradeMode:              true,
-		TLSCipherSuites:             "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256",
-		TLSMinVersion:               "tls12",
-		TLSPreferServerCipherSuites: true,
+		EnableHTTP:           true,
+		EnableRPC:            true,
+		VerifyServerHostname: true,
+		CAFile:               "test-ca-file-2",
+		CertFile:             "test-cert-file-2",
+		RPCUpgradeMode:       true,
+		TLSCipherSuites:      "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256",
+		TLSMinVersion:        "tls12",
 	}
 
 	new := a.Merge(b)
@@ -188,12 +187,11 @@ func TestTLS_Copy(t *testing.T) {
 		fookey  = "../../../helper/tlsutil/testdata/regionFoo-client-nomad-key.pem"
 	)
 	a := &TLSConfig{
-		CAFile:                      cafile,
-		CertFile:                    foocert,
-		KeyFile:                     fookey,
-		TLSCipherSuites:             "TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305",
-		TLSMinVersion:               "tls12",
-		TLSPreferServerCipherSuites: true,
+		CAFile:          cafile,
+		CertFile:        foocert,
+		KeyFile:         fookey,
+		TLSCipherSuites: "TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305",
+		TLSMinVersion:   "tls12",
 	}
 	a.SetChecksum()
 

website/content/docs/configuration/tls.mdx

@@ -78,9 +78,6 @@ the [Enable TLS Encryption for Nomad Tutorial](/nomad/tutorials/transport-securi
 - `tls_min_version` `(string: "tls12")`- Specifies the minimum supported version
   of TLS. Accepted values are "tls10", "tls11", "tls12".
 
-- `tls_prefer_server_cipher_suites` `(bool: false)` - Specifies whether
-  TLS connections should prefer the server's ciphersuites over the client's.
-
 - `verify_https_client` `(bool: false)` - Specifies agents should require client
   certificates for all incoming HTTPS requests, effectively upgrading
   [`tls.http=true`](#http) to mTLS. The client certificates must be signed by