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SSL_CTX_SET_MSG_CALLBACK(3) Library Functions Manual SSL_CTX_SET_MSG_CALLBACK(3)

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

#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 NULL 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, such as TLS1_VERSION, TLS1_1_VERSION, TLS1_2_VERSION, or DTLS1_VERSION.
 
 
content_type
This is one of the ContentType 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.)
 
 
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 The buffer is no longer valid after the callback function has returned.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().
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_set_msg_callback_arg() first appeared in OpenSSL 0.9.7 and have been available since OpenBSD 3.2.
March 22, 2018 OpenBSD-current