SSL_CTX_set_msg_callback(3) OpenSSL SSL_CTX_set_msg_callback(3)
SSL_set_msg_callback, SSL_get_msg_callback_arg - install callback for
observing protocol messages
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);
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 proto-
col 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 mes-
When cb is called by the SSL/TLS library for a protocol message, 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. Currently, this is one of SSL2_VERSION,
SSL3_VERSION and TLS1_VERSION (for SSL 2.0, SSL 3.0 and TLS 1.0,
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), hand-
shake(22); but never application_data(23) because the callback will
only be called for protocol messages).
buf points to a buffer containing the protocol message, which con-
sists of len bytes. The buffer is no longer valid after the call-
back 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
If processing a received protocol message results in an error, the
callback function may not be called. For example, the callback func-
tion will never see messages that are considered too large to be pro-
Due to automatic protocol version negotiation, version is not necessar-
ily 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.
SSL_set_msg_callback() and SSL_get_msg_callback_arg() were added in
3rd Berkeley Distribution 0.9.7d SSL_CTX_set_msg_callback(3)