NAME

       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

SYNOPSIS

        #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);

DESCRIPTION

       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:

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

       version
           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).

       content_type
           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().

NOTES

       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 processed.

       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 defined:

       SSL3_RT_HEADER
           Used when a record is sent or received. The buf contains the record header bytes only.

       SSL3_RT_INNER_CONTENT_TYPE
           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.

RETURN VALUES

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

SEE ALSO

       ssl(7), SSL_new(3)

HISTORY

       The pseudo content type SSL3_RT_INNER_CONTENT_TYPE was added in OpenSSL 1.1.1.

COPYRIGHT

       Copyright 2001-2018 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.