NAME

       SSL_CTX_set_msg_callback, SSL_CTX_set_msg_callback_arg, SSL_set_msg_callback,
       SSL_get_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_CTX *ctx, 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_CTX *ctx, 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.  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 0 pointer
       for cb disables the message callback.

       When cb is called by the SSL/TLS library for a protocol message, 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. Currently, this is one of SSL2_VERSION, SSL3_VERSION and TLS1_VERSION (for
           SSL 2.0, SSL 3.0 and TLS 1.0, respectively).

       content_type
           In the case of SSL 2.0, this is always 0.  In the case of SSL 3.0 or TLS 1.0, this is
           one of the ContentType values defined in the protocol specification
           (change_cipher_spec(20), alert(21), handshake(22); but never application_data(23)
           because the callback will only be called for protocol messages).

       buf, len
           buf points to a buffer containing the protocol message, 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           The buffer is no longer valid after the callback function has returned.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.

SEE ALSO

       ssl(3), SSL_new(3)

HISTORY

       SSL_CTX_set_msg_callback(), SSL_CTX_set_msg_callback_arg(), SSL_set_msg_callback() and
       SSL_get_msg_callback_arg() were added in OpenSSL 0.9.7.