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 The buffer is no longer valid after the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.
