Updated: 2021/Apr/14

SSL_CTX_set_options(3)              OpenSSL             SSL_CTX_set_options(3)

       SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options,
       SSL_clear_options, SSL_CTX_get_options, SSL_get_options,
       SSL_get_secure_renegotiation_support - manipulate SSL options

       libcrypto, -lcrypto

        #include <openssl/ssl.h>

        long SSL_CTX_set_options(SSL_CTX *ctx, long options);
        long SSL_set_options(SSL *ssl, long options);

        long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
        long SSL_clear_options(SSL *ssl, long options);

        long SSL_CTX_get_options(SSL_CTX *ctx);
        long SSL_get_options(SSL *ssl);

        long SSL_get_secure_renegotiation_support(SSL *ssl);

       SSL_CTX_set_options() adds the options set via bit mask in options to
       ctx.  Options already set before are not cleared!

       SSL_set_options() adds the options set via bit mask in options to ssl.
       Options already set before are not cleared!

       SSL_CTX_clear_options() clears the options set via bit mask in options
       to ctx.

       SSL_clear_options() clears the options set via bit mask in options to

       SSL_CTX_get_options() returns the options set for ctx.

       SSL_get_options() returns the options set for ssl.

       SSL_get_secure_renegotiation_support() indicates whether the peer
       supports secure renegotiation.  Note, this is implemented via a macro.

       The behaviour of the SSL library can be changed by setting several
       options.  The options are coded as bit masks and can be combined by a
       bitwise or operation (|).

       SSL_CTX_set_options() and SSL_set_options() affect the (external)
       protocol behaviour of the SSL library. The (internal) behaviour of the
       API can be changed by using the similar SSL_CTX_set_mode(3) and
       SSL_set_mode() functions.

       During a handshake, the option settings of the SSL object are used.
       When a new SSL object is created from a context using SSL_new(), the
       current option setting is copied. Changes to ctx do not affect already
       created SSL objects. SSL_clear() does not affect the settings.

       The following bug workaround options are available:

           Don't prefer ECDHE-ECDSA ciphers when the client appears to be
           Safari on OS X.  OS X 10.8..10.8.3 has broken support for ECDHE-
           ECDSA ciphers.

           Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
           vulnerability affecting CBC ciphers, which cannot be handled by
           some broken SSL implementations.  This option has no effect for
           connections using other ciphers.

           Adds a padding extension to ensure the ClientHello size is never
           between 256 and 511 bytes in length. This is needed as a workaround
           for some implementations.

           All of the above bug workarounds plus SSL_OP_LEGACY_SERVER_CONNECT
           as mentioned below.

       It is usually safe to use SSL_OP_ALL to enable the bug workaround
       options if compatibility with somewhat broken implementations is

       The following modifying options are available:

           Disable version rollback attack detection.

           During the client key exchange, the client must send the same
           information about acceptable SSL/TLS protocol levels as during the
           first hello. Some clients violate this rule by adapting to the
           server's answer. (Example: the client sends a SSLv2 hello and
           accepts up to SSLv3.1=TLSv1, the server only understands up to
           SSLv3. In this case the client must still use the same
           SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with
           respect to the server's answer and violate the version rollback

           When choosing a cipher, use the server's preferences instead of the
           client preferences. When not set, the SSL server will always follow
           the clients preferences. When set, the SSL/TLS server will choose
           following its own preferences.

           These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or
           TLSv1.3 protocol versions with TLS or the DTLSv1, DTLSv1.2 versions
           with DTLS, respectively.  As of OpenSSL 1.1.0, these options are
           deprecated, use SSL_CTX_set_min_proto_version(3) and
           SSL_CTX_set_max_proto_version(3) instead.

           When performing renegotiation as a server, always start a new
           session (i.e., session resumption requests are only accepted in the
           initial handshake). This option is not needed for clients.

           Do not use compression even if it is supported.

           Do not query the MTU. Only affects DTLS connections.

           Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only
           affects DTLS connections.

           SSL/TLS supports two mechanisms for resuming sessions: session ids
           and stateless session tickets.

           When using session ids a copy of the session information is cached
           on the server and a unique id is sent to the client. When the
           client wishes to resume it provides the unique id so that the
           server can retrieve the session information from its cache.

           When using stateless session tickets the server uses a session
           ticket encryption key to encrypt the session information. This
           encrypted data is sent to the client as a "ticket". When the client
           wishes to resume it sends the encrypted data back to the server.
           The server uses its key to decrypt the data and resume the session.
           In this way the server can operate statelessly - no session
           information needs to be cached locally.

           The TLSv1.3 protocol only supports tickets and does not directly
           support session ids. However, OpenSSL allows two modes of ticket
           operation in TLSv1.3: stateful and stateless. Stateless tickets
           work the same way as in TLSv1.2 and below.  Stateful tickets mimic
           the session id behaviour available in TLSv1.2 and below.  The
           session information is cached on the server and the session id is
           wrapped up in a ticket and sent back to the client. When the client
           wishes to resume, it presents a ticket in the same way as for
           stateless tickets. The server can then extract the session id from
           the ticket and retrieve the session information from its cache.

           By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET
           option will cause stateless tickets to not be issued. In TLSv1.2
           and below this means no ticket gets sent to the client at all. In
           TLSv1.3 a stateful ticket will be sent. This is a server-side
           option only.

           In TLSv1.3 it is possible to suppress all tickets (stateful and
           stateless) from being sent by calling SSL_CTX_set_num_tickets(3) or

           Allow legacy insecure renegotiation between OpenSSL and unpatched
           clients or servers. See the SECURE RENEGOTIATION section for more

           Allow legacy insecure renegotiation between OpenSSL and unpatched
           servers only: this option is currently set by default. See the
           SECURE RENEGOTIATION section for more details.

           Normally clients and servers will transparently attempt to
           negotiate the RFC7366 Encrypt-then-MAC option on TLS and DTLS

           If this option is set, Encrypt-then-MAC is disabled. Clients will
           not propose, and servers will not accept the extension.

           Disable all renegotiation in TLSv1.2 and earlier. Do not send
           HelloRequest messages, and ignore renegotiation requests via

           In TLSv1.3 allow a non-(ec)dhe based key exchange mode on
           resumption. This means that there will be no forward secrecy for
           the resumed session.

           When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily
           reprioritize ChaCha20-Poly1305 ciphers to the top of the server
           cipher list if a ChaCha20-Poly1305 cipher is at the top of the
           client cipher list. This helps those clients (e.g. mobile) use
           ChaCha20-Poly1305 if that cipher is anywhere in the server cipher
           list; but still allows other clients to use AES and other ciphers.

           If set then dummy Change Cipher Spec (CCS) messages are sent in
           TLSv1.3. This has the effect of making TLSv1.3 look more like
           TLSv1.2 so that middleboxes that do not understand TLSv1.3 will not
           drop the connection. Regardless of whether this option is set or
           not CCS messages received from the peer will always be ignored in
           TLSv1.3. This option is set by default. To switch it off use
           SSL_clear_options(). A future version of OpenSSL may not set this
           by default.

           By default, when a server is configured for early data (i.e.,
           max_early_data > 0), OpenSSL will switch on replay protection. See
           SSL_read_early_data(3) for a description of the replay protection
           feature. Anti-replay measures are required to comply with the
           TLSv1.3 specification. Some applications may be able to mitigate
           the replay risks in other ways and in such cases the built in
           OpenSSL functionality is not required. Those applications can turn
           this feature off by setting this option. This is a server-side
           opton only. It is ignored by clients.

       The following options no longer have any effect but their identifiers
       are retained for compatibility purposes:


       OpenSSL always attempts to use secure renegotiation as described in
       RFC5746. This counters the prefix attack described in CVE-2009-3555 and

       This attack has far reaching consequences which application writers
       should be aware of. In the description below an implementation
       supporting secure renegotiation is referred to as patched. A server not
       supporting secure renegotiation is referred to as unpatched.

       The following sections describe the operations permitted by OpenSSL's
       secure renegotiation implementation.

   Patched client and server
       Connections and renegotiation are always permitted by OpenSSL

   Unpatched client and patched OpenSSL server
       The initial connection succeeds but client renegotiation is denied by
       the server with a no_renegotiation warning alert if TLS v1.0 is used or
       a fatal handshake_failure alert in SSL v3.0.

       If the patched OpenSSL server attempts to renegotiate a fatal
       handshake_failure alert is sent. This is because the server code may be
       unaware of the unpatched nature of the client.

       If the option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set then
       renegotiation always succeeds.

   Patched OpenSSL client and unpatched server.
       If the option SSL_OP_LEGACY_SERVER_CONNECT or
       connections and renegotiation between patched OpenSSL clients and
       unpatched servers succeeds. If neither option is set then initial
       connections to unpatched servers will fail.

       The option SSL_OP_LEGACY_SERVER_CONNECT is currently set by default
       even though it has security implications: otherwise it would be
       impossible to connect to unpatched servers (i.e. all of them initially)
       and this is clearly not acceptable. Renegotiation is permitted because
       this does not add any additional security issues: during an attack
       clients do not see any renegotiations anyway.

       As more servers become patched the option SSL_OP_LEGACY_SERVER_CONNECT
       will not be set by default in a future version of OpenSSL.

       OpenSSL client applications wishing to ensure they can connect to
       unpatched servers should always set SSL_OP_LEGACY_SERVER_CONNECT

       OpenSSL client applications that want to ensure they can not connect to
       unpatched servers (and thus avoid any security issues) should always
       clear SSL_OP_LEGACY_SERVER_CONNECT using SSL_CTX_clear_options() or

       The difference between the SSL_OP_LEGACY_SERVER_CONNECT and
       SSL_OP_LEGACY_SERVER_CONNECT enables initial connections and secure
       renegotiation between OpenSSL clients and unpatched servers only, while
       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION allows initial connections and
       renegotiation between OpenSSL and unpatched clients or servers.

       SSL_CTX_set_options() and SSL_set_options() return the new options bit
       mask after adding options.

       SSL_CTX_clear_options() and SSL_clear_options() return the new options
       bit mask after clearing options.

       SSL_CTX_get_options() and SSL_get_options() return the current bit

       SSL_get_secure_renegotiation_support() returns 1 is the peer supports
       secure renegotiation and 0 if it does not.

       ssl(7), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3),
       SSL_CTX_set_min_proto_version(3), dhparam(1)

       The attempt to always try to use secure renegotiation was added in
       OpenSSL 0.9.8m.

       added in OpenSSL 1.1.1.

       Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1i                            2020-12-10            SSL_CTX_set_options(3)