Perform RFC 2818 hostname verification in client SSL handshakes

If the verification mode is `system_ca`, clients will now do hostname
verification. Thus, only certificates from expected hostnames are
allowed when SSL is enabled. This can be overridden by forcible setting
the SSL mode to autodetect.

Clients will also send the hostname even when `system_ca` is not being
performed. This leaks possible metadata, but allows servers providing
multiple hostnames to respond with the correct certificate. One example
is cloudflare, which getmonero.org is currently using.

contrib/epee/include/net/net_helper.h

@@ -174,7 +174,7 @@ namespace net_utils
 					// SSL Options
 					if (ssl_support == epee::net_utils::ssl_support_t::e_ssl_support_enabled || ssl_support == epee::net_utils::ssl_support_t::e_ssl_support_autodetect)
 					{
-						if (!m_ssl_options.handshake(*m_ssl_socket, boost::asio::ssl::stream_base::client))
+						if (!m_ssl_options.handshake(*m_ssl_socket, boost::asio::ssl::stream_base::client, addr))
 						{
 							if (ssl_support == epee::net_utils::ssl_support_t::e_ssl_support_autodetect)
 							{

contrib/epee/include/net/net_ssl.h

@@ -108,9 +108,21 @@ namespace net_utils
           then the handshake will not fail when peer verification fails. The
           assumption is that a re-connect will be attempted, so a warning is
           logged instead of failure.
+
+        \note It is strongly encouraged that clients using `system_ca`
+          verification provide a non-empty `host` for rfc2818 verification.
+
+        \param socket Used in SSL handshake and verification
+        \param type Client or server
+        \param host This parameter is only used when
+          `type == client && !host.empty()`. The value is sent to the server for
+          situations where multiple hostnames are being handled by a server. If
+          `verification == system_ca` the client also does a rfc2818 check to
+          ensure that the server certificate is to the provided hostname.
+
         \return True if the SSL handshake completes with peer verification
           settings. */
-    bool handshake(boost::asio::ssl::stream<boost::asio::ip::tcp::socket> &socket, boost::asio::ssl::stream_base::handshake_type type) const;
+    bool handshake(boost::asio::ssl::stream<boost::asio::ip::tcp::socket> &socket, boost::asio::ssl::stream_base::handshake_type type, const std::string& host = {}) const;
   };
 
         // https://security.stackexchange.com/questions/34780/checking-client-hello-for-https-classification

contrib/epee/src/net_ssl.cpp

@@ -311,7 +311,7 @@ bool ssl_options_t::has_fingerprint(boost::asio::ssl::verify_context &ctx) const
   return false;
 }
 
-bool ssl_options_t::handshake(boost::asio::ssl::stream<boost::asio::ip::tcp::socket> &socket, boost::asio::ssl::stream_base::handshake_type type) const
+bool ssl_options_t::handshake(boost::asio::ssl::stream<boost::asio::ip::tcp::socket> &socket, boost::asio::ssl::stream_base::handshake_type type, const std::string& host) const
 {
   socket.next_layer().set_option(boost::asio::ip::tcp::no_delay(true));
 
@@ -330,11 +330,20 @@ bool ssl_options_t::handshake(boost::asio::ssl::stream<boost::asio::ip::tcp::soc
   else
   {
     socket.set_verify_mode(boost::asio::ssl::verify_peer | boost::asio::ssl::verify_fail_if_no_peer_cert);
-    socket.set_verify_callback([&](bool preverified, boost::asio::ssl::verify_context &ctx)
+
+    // in case server is doing "virtual" domains, set hostname
+    SSL* const ssl_ctx = socket.native_handle();
+    if (type == boost::asio::ssl::stream_base::client && !host.empty() && ssl_ctx)
+      SSL_set_tlsext_host_name(ssl_ctx, host.c_str());
+
+    socket.set_verify_callback([&](const bool preverified, boost::asio::ssl::verify_context &ctx)
     {
       // preverified means it passed system or user CA check. System CA is never loaded
       // when fingerprints are whitelisted.
-      if (!preverified && !has_fingerprint(ctx))
+      const bool verified = preverified &&
+        (verification != ssl_verification_t::system_ca || host.empty() || boost::asio::ssl::rfc2818_verification(host)(preverified, ctx));
+
+      if (!verified && !has_fingerprint(ctx))
       {
         // autodetect will reconnect without SSL - warn and keep connection encrypted
         if (support != ssl_support_t::e_ssl_support_autodetect)