TLS version negotiation

Updated the TLS negotiation logic to adaptively try to connect using the highest TLS version supported by both client and server. Previously, OpenVPN (when linked with OpenSSL) would always connect using TLS 1.0. Also added tls-version-min directive to force a higher TLS version than 1.0: tls-version-min <version> ['or-highest'] -- sets the minimum TLS version we will accept from the peer. Examples for version include "1.0" (default), "1.1", or "1.2". If 'or-highest' is specified and version is not recognized, we will only accept the highest TLS version supported by the local SSL implementation. Examples: tls-version-min 1.1 -- fail the connection unless peer can connect at TLS 1.1 or higher. tls-version-min 1.2 or-highest -- require that the peer connect at TLS 1.2 or higher, however if the local SSL implementation doesn't support TLS 1.2 (as it wouldn't if linked with an older version of OpenSSL), reduce the minimum required version to the highest version supported by the local SSL implementation (such as TLS 1.0). This is intended to allow client configurations to target higher TLS versions that are supported on the server, even if some older clients don't support these versions yet.

Author: jamesyonan

doc/openvpn.8

@@ -4235,6 +4235,15 @@ when you built your peer's certificate (see
 above).
 .\"*********************************************************
 .TP
+.B \-\-tls-version-min version ['or-highest']
+Sets the minimum
+TLS version we will accept from the peer (default is "1.0").
+Examples for version
+include "1.0", "1.1", or "1.2". If 'or-highest' is specified
+and version is not recognized, we will only accept the highest TLS
+version supported by the local SSL implementation.
+.\"*********************************************************
+.TP
 .B \-\-pkcs12 file
 Specify a PKCS #12 file containing local private key,
 local certificate, and root CA certificate.

src/openvpn/options.c

@@ -582,6 +582,9 @@ static const char usage_message[] =
  " by a Certificate Authority in --ca file.\n"
  "--extra-certs file : one or more PEM certs that complete the cert chain.\n"
  "--key file : Local private key in .pem format.\n"
+ "--tls-version-min <version> ['or-highest'] : sets the minimum TLS version we\n"
+ " will accept from the peer. If version is unrecognized and 'or-highest'\n"
+ " is specified, require max TLS version supported by SSL implementation.\n"
 #ifndef ENABLE_CRYPTO_POLARSSL
  "--pkcs12 file : PKCS#12 file containing local private key, local certificate\n"
  " and optionally the root CA certificate.\n"
@@ -6511,6 +6514,19 @@ add_option (struct options *options,
  options->priv_key_file_inline = p[2];
 }
  }
+ else if (streq (p[0], "tls-version-min") && p[1])
+ {
+ int ver;
+ VERIFY_PERMISSION (OPT_P_GENERAL);
+ ver = tls_version_min_parse(p[1], p[2]);
+ if (ver == TLS_VER_BAD)
+{
+ msg (msglevel, "unknown tls-version-min parameter: %s", p[1]);
+ goto err;
+}
+ options->ssl_flags &= ~(SSLF_TLS_VERSION_MASK << SSLF_TLS_VERSION_SHIFT);
+ options->ssl_flags |= (ver << SSLF_TLS_VERSION_SHIFT);
+ }
 #ifndef ENABLE_CRYPTO_POLARSSL
  else if (streq (p[0], "pkcs12") && p[1])
  {

src/openvpn/ssl.c

@@ -446,6 +446,27 @@ ssl_put_auth_challenge (const char *cr_str)
 
 #endif
 
+/*
+ * Parse a TLS version string, returning a TLS_VER_x constant.
+ * If version string is not recognized and extra == "or-highest",
+ * return tls_version_max().
+ */
+int
+tls_version_min_parse(const char *vstr, const char *extra)
+{
+ const int max_version = tls_version_max();
+ if (!strcmp(vstr, "1.0") && TLS_VER_1_0 <= max_version)
+ return TLS_VER_1_0;
+ else if (!strcmp(vstr, "1.1") && TLS_VER_1_1 <= max_version)
+ return TLS_VER_1_1;
+ else if (!strcmp(vstr, "1.2") && TLS_VER_1_2 <= max_version)
+ return TLS_VER_1_2;
+ else if (extra && !strcmp(extra, "or-highest"))
+ return max_version;
+ else
+ return TLS_VER_BAD;
+}
+
 /*
  * Initialize SSL context.
  * All files are in PEM format.

src/openvpn/ssl_backend.h

@@ -93,6 +93,29 @@ void tls_free_lib();
  */
 void tls_clear_error();
 
+/**
+ * Parse a TLS version specifier
+ *
+ * @param vstr	The TLS version string
+ * @param extra An optional extra parameter, may be NULL
+ *
+ * @return	One of the TLS_VER_x constants or TLS_VER_BAD
+ * if a parse error should be flagged.
+ */
+#define TLS_VER_BAD -1
+#define TLS_VER_1_0 0 /* default */
+#define TLS_VER_1_1 1
+#define TLS_VER_1_2 2
+int tls_version_min_parse(const char *vstr, const char *extra);
+
+/**
+ * Return the maximum TLS version (as a TLS_VER_x constant)
+ * supported by current SSL implementation
+ *
+ * @return	One of the TLS_VER_x constants (but not TLS_VER_BAD).
+ */
+int tls_version_max(void);
+
 /**
  * Initialise a library-specific TLS context for a server.
  *

src/openvpn/ssl_common.h

@@ -290,12 +290,14 @@ struct tls_options
  struct compress_options comp_options;
 #endif
 
- /* configuration file boolean options */
+ /* configuration file SSL-related boolean and low-permutation options */
 # define SSLF_CLIENT_CERT_NOT_REQUIRED (1<<0)
 # define SSLF_USERNAME_AS_COMMON_NAME (1<<1)
 # define SSLF_AUTH_USER_PASS_OPTIONAL (1<<2)
 # define SSLF_OPT_VERIFY (1<<4)
 # define SSLF_CRL_VERIFY_DIR (1<<5)
+# define SSLF_TLS_VERSION_SHIFT 6
+# define SSLF_TLS_VERSION_MASK 0xF /* (uses bit positions 6 to 9) */
  unsigned int ssl_flags;
 
 #ifdef MANAGEMENT_DEF_AUTH

src/openvpn/ssl_openssl.c

@@ -114,7 +114,7 @@ tls_ctx_server_new(struct tls_root_ctx *ctx)
 {
  ASSERT(NULL != ctx);
 
- ctx->ctx = SSL_CTX_new (TLSv1_server_method ());
+ ctx->ctx = SSL_CTX_new (SSLv23_server_method ());
 
  if (ctx->ctx == NULL)
  msg (M_SSLERR, "SSL_CTX_new TLSv1_server_method");
@@ -127,7 +127,7 @@ tls_ctx_client_new(struct tls_root_ctx *ctx)
 {
  ASSERT(NULL != ctx);
 
- ctx->ctx = SSL_CTX_new (TLSv1_client_method ());
+ ctx->ctx = SSL_CTX_new (SSLv23_client_method ());
 
  if (ctx->ctx == NULL)
  msg (M_SSLERR, "SSL_CTX_new TLSv1_client_method");
@@ -174,13 +174,48 @@ info_callback (INFO_CALLBACK_SSL_CONST SSL * s, int where, int ret)
  }
 }
 
+/*
+ * Return maximum TLS version supported by local OpenSSL library.
+ * Assume that presence of SSL_OP_NO_TLSvX macro indicates that
+ * TLSvX is supported.
+ */
+int
+tls_version_max(void)
+{
+#if defined(SSL_OP_NO_TLSv1_2)
+ return TLS_VER_1_2;
+#elif defined(SSL_OP_NO_TLSv1_1)
+ return TLS_VER_1_1;
+#else
+ return TLS_VER_1_0;
+#endif
+}
+
 void
 tls_ctx_set_options (struct tls_root_ctx *ctx, unsigned int ssl_flags)
 {
  ASSERT(NULL != ctx);
 
+ /* process SSL options including minimum TLS version we will accept from peer */
+ {
+ long sslopt = SSL_OP_SINGLE_DH_USE | SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3;
+ const int tls_version_min = (ssl_flags >> SSLF_TLS_VERSION_SHIFT) & SSLF_TLS_VERSION_MASK;
+#ifdef SSL_OP_NO_TLSv1
+ if (tls_version_min > TLS_VER_1_0)
+ sslopt |= SSL_OP_NO_TLSv1;
+#endif
+#ifdef SSL_OP_NO_TLSv1_1
+ if (tls_version_min > TLS_VER_1_1)
+ sslopt |= SSL_OP_NO_TLSv1_1;
+#endif
+#ifdef SSL_OP_NO_TLSv1_2
+ if (tls_version_min > TLS_VER_1_2)
+ sslopt |= SSL_OP_NO_TLSv1_2;
+#endif
+ SSL_CTX_set_options (ctx->ctx, sslopt);
+ }
+
  SSL_CTX_set_session_cache_mode (ctx->ctx, SSL_SESS_CACHE_OFF);
- SSL_CTX_set_options (ctx->ctx, SSL_OP_SINGLE_DH_USE);
  SSL_CTX_set_default_passwd_cb (ctx->ctx, pem_password_callback);
 
  /* Require peer certificate verification */

src/openvpn/ssl_polarssl.c

@@ -501,6 +501,18 @@ void tls_ctx_personalise_random(struct tls_root_ctx *ctx)
  }
 }
 
+int
+tls_version_max(void)
+{
+#if defined(SSL_MAJOR_VERSION_3) && defined(SSL_MINOR_VERSION_3)
+ return TLS_VER_1_2;
+#elif defined(SSL_MAJOR_VERSION_3) && defined(SSL_MINOR_VERSION_2)
+ return TLS_VER_1_1;
+#else
+ return TLS_VER_1_0;
+#endif
+}
+
 void key_state_ssl_init(struct key_state_ssl *ks_ssl,
  const struct tls_root_ctx *ssl_ctx, bool is_server, struct tls_session *session)
 {
@@ -550,6 +562,36 @@ void key_state_ssl_init(struct key_state_ssl *ks_ssl,
  /* TODO: PolarSSL does not currently support sending the CA chain to the client */
  ssl_set_ca_chain (ks_ssl->ctx, ssl_ctx->ca_chain, NULL, NULL );
 
+ /* Initialize minimum TLS version */
+ {
+const int tls_version_min = (session->opt->ssl_flags >> SSLF_TLS_VERSION_SHIFT) & SSLF_TLS_VERSION_MASK;
+int polar_major;
+int polar_minor;
+switch (tls_version_min)
+ {
+#if defined(SSL_MAJOR_VERSION_3) && defined(SSL_MINOR_VERSION_1)
+ case TLS_VER_1_0:
+ default:
+ polar_major = SSL_MAJOR_VERSION_3;
+ polar_minor = SSL_MINOR_VERSION_1;
+ break;
+#endif
+#if defined(SSL_MAJOR_VERSION_3) && defined(SSL_MINOR_VERSION_2)
+ case TLS_VER_1_1:
+ polar_major = SSL_MAJOR_VERSION_3;
+ polar_minor = SSL_MINOR_VERSION_2;
+ break;
+#endif
+#if defined(SSL_MAJOR_VERSION_3) && defined(SSL_MINOR_VERSION_3)
+ case TLS_VER_1_2:
+ polar_major = SSL_MAJOR_VERSION_3;
+ polar_minor = SSL_MINOR_VERSION_3;
+ break;
+#endif
+ }
+ssl_set_min_version(ks_ssl->ctx, polar_major, polar_minor);
+ }
+
  /* Initialise BIOs */
  ALLOC_OBJ_CLEAR (ks_ssl->ct_in, endless_buffer);
  ALLOC_OBJ_CLEAR (ks_ssl->ct_out, endless_buffer);