SSL_CTX_SET_MSG_CALLBACK

Section: OpenSSL (3)
Updated: 2020-07-28
Page Index

NAME

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

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 *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);

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, as well as other events that occur during processing. 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 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_3_VERSION, TLS1_2_VERSION etc. This is set to 0 for the SSL3_RT_HEADER pseudo content type (see NOTES below).
content_type: This is one of the content type 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). Alternatively it may be a ``pseudo'' content type. These pseudo content types are used to signal some other event in the processing of data (see NOTES below).
buf, len: buf points to a buffer containing the protocol message or other data (in the case of pseudo content types), 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.

Pseudo content type values may be sent at various points during the processing of data. The following pseudo content types are currently defined:

SSL3_RT_HEADER: Used when a record is sent or received. The buf contains the record header bytes only.
SSL3_RT_INNER_CONTENT_TYPE: Used when an encrypted TLSv1.3 record is sent or received. In encrypted TLSv1.3 records the content type in the record header is always SSL3_RT_APPLICATION_DATA. The real content type for the record is contained in an ``inner'' content type. buf contains the encoded ``inner'' content type byte.

RETURN VALUES

SSL_CTX_set_msg_callback(), SSL_CTX_set_msg_callback_arg(), SSL_set_msg_callback() and SSL_set_msg_callback_arg() do not return values.

HISTORY

The pseudo content type SSL3_RT_INNER_CONTENT_TYPE was added in OpenSSL 1.1.1.

COPYRIGHT

Licensed under the OpenSSL license (the ``License''). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.