Cryptography and SSL/TLS Toolkit



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


 #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. 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:


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, respectively).


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 The buffer is no longer valid after the callback function has returned.


The SSL object that received or sent the message.


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 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.


ssl(3), SSL_new(3)


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.