Updated: 2021/Apr/14

SSL_CTX_set_msg_callback(3)         OpenSSL        SSL_CTX_set_msg_callback(3)

       SSL_CTX_set_msg_callback, SSL_CTX_set_msg_callback_arg,
       SSL_set_msg_callback, SSL_set_msg_callback_arg - install callback for
       observing protocol messages

       libcrypto, -lcrypto

        #include <openssl/ssl.h>

        void SSL_CTX_set_msg_callback(SSL_CTX *ctx,
                                      void (*cb)(int write_p, int version,
                                                 int content_type, const void *buf,
                                                 size_t len, SSL *ssl, void *arg));
        void SSL_CTX_set_msg_callback_arg(SSL_CTX *ctx, void *arg);

        void SSL_set_msg_callback(SSL *ssl,
                                  void (*cb)(int write_p, int version,
                                             int content_type, const void *buf,
                                             size_t len, SSL *ssl, void *arg));
        void SSL_set_msg_callback_arg(SSL *ssl, void *arg);

       SSL_CTX_set_msg_callback() or SSL_set_msg_callback() can be used to
       define a message callback function cb for observing all SSL/TLS
       protocol messages (such as handshake messages) that are received or
       sent, as well as other events that occur during processing.
       SSL_CTX_set_msg_callback_arg() and SSL_set_msg_callback_arg() can be
       used to set argument arg to the callback function, which is available
       for arbitrary application use.

       SSL_CTX_set_msg_callback() and SSL_CTX_set_msg_callback_arg() specify
       default settings that will be copied to new SSL objects by SSL_new(3).
       SSL_set_msg_callback() and SSL_set_msg_callback_arg() modify the actual
       settings of an SSL object. Using a NULL pointer for cb disables the
       message callback.

       When cb is called by the SSL/TLS library the function arguments have
       the following meaning:

           This flag is 0 when a protocol message has been received and 1 when
           a protocol message has been sent.

           The protocol version according to which the protocol message is
           interpreted by the library such as TLS1_3_VERSION, TLS1_2_VERSION
           etc.  This is set to 0 for the SSL3_RT_HEADER pseudo content type
           (see NOTES below).

           This is one of the content type values defined in the protocol
           specification (SSL3_RT_CHANGE_CIPHER_SPEC, SSL3_RT_ALERT,
           SSL3_RT_HANDSHAKE; but never SSL3_RT_APPLICATION_DATA because the
           callback will only be called for protocol messages). Alternatively
           it may be a "pseudo" content type. These pseudo content types are
           used to signal some other event in the processing of data (see
           NOTES below).

       buf, len
           buf points to a buffer containing the protocol message or other
           data (in the case of pseudo content types), which consists of len
           bytes. The buffer is no longer valid after the callback function
           has returned.

       ssl The SSL object that received or sent the message.

       arg The user-defined argument optionally defined by
           SSL_CTX_set_msg_callback_arg() or SSL_set_msg_callback_arg().

       Protocol messages are passed to the callback function after decryption
       and fragment collection where applicable. (Thus record boundaries are
       not visible.)

       If processing a received protocol message results in an error, the
       callback function may not be called.  For example, the callback
       function will never see messages that are considered too large to be

       Due to automatic protocol version negotiation, version is not
       necessarily the protocol version used by the sender of the message: If
       a TLS 1.0 ClientHello message is received by an SSL 3.0-only server,
       version will be SSL3_VERSION.

       Pseudo content type values may be sent at various points during the
       processing of data. The following pseudo content types are currently

           Used when a record is sent or received. The buf contains the record
           header bytes only.

           Used when an encrypted TLSv1.3 record is sent or received. In
           encrypted TLSv1.3 records the content type in the record header is
           always SSL3_RT_APPLICATION_DATA. The real content type for the
           record is contained in an "inner" content type. buf contains the
           encoded "inner" content type byte.

       SSL_CTX_set_msg_callback(), SSL_CTX_set_msg_callback_arg(),
       SSL_set_msg_callback() and SSL_set_msg_callback_arg() do not return

       ssl(7), SSL_new(3)

       The pseudo content type SSL3_RT_INNER_CONTENT_TYPE was added in OpenSSL

       Copyright 2001-2018 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                            2019-03-12       SSL_CTX_set_msg_callback(3)