SSL_CTX_SET_MSG_CALLBACK(3SSL) OpenSSL SSL_CTX_SET_MSG_CALLBACK(3SSL)
NAME
SSL_CTX_set_msg_callback, SSL_CTX_set_msg_callback_arg,
SSL_set_msg_callback, SSL_set_msg_callback_arg, SSL_trace - 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);
void SSL_trace(int write_p, int version, int content_type,
const void *buf, size_t len, 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/QUIC
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/QUIC 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,
OSSL_QUIC1_VERSION etc. For the SSL3_RT_HEADER pseudo content type
(see NOTES below) this value will be the decoded
version/legacy_version field of the record header.
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().
The SSL_trace() function can be used as a pre-written callback in a call
to SSL_CTX_set_msg_callback() or SSL_set_msg_callback(). It requires a
BIO to be set as the callback argument via
SSL_CTX_set_msg_callback_arg() or SSL_set_msg_callback_arg(). Setting
this callback will cause human readable diagostic tracing information
about an SSL/TLS/QUIC connection to be written to the BIO.
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 TLS 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.
SSL3_RT_QUIC_DATAGRAM
Used when a QUIC datagram is sent or received.
SSL3_RT_QUIC_PACKET
Used when a QUIC packet is sent or received.
SSL3_RT_QUIC_FRAME_FULL
Used when a QUIC frame is sent or received. This is only used for
non-crypto and stream data related frames. The full QUIC frame data
is supplied.
SSL3_RT_QUIC_FRAME_HEADER
Used when a QUIC stream data or crypto frame is sent or received.
Only the QUIC frame header data is supplied.
SSL3_RT_QUIC_FRAME_PADDING
Used when a sequence of one or more QUIC padding frames is sent or
received. A padding frame consists of a single byte and it is
common to have multiple such frames in a sequence. Rather than
supplying each frame individually the callback will supply all the
padding frames in one go via this pseudo content type.
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.
SEE ALSO
ssl(7), SSL_new(3)
HISTORY
The pseudo content type SSL3_RT_INNER_CONTENT_TYPE was added in OpenSSL
1.1.1.
The pseudo content types SSL3_RT_QUIC_DATAGRAM, SSL3_RT_QUIC_PACKET,
SSL3_RT_QUIC_FRAME_FULL, SSL3_RT_QUIC_FRAME_HEADER and
SSL3_RT_QUIC_FRAME_PADDING were added in OpenSSL 3.2.
In versions previous to OpenSSL 3.0 cb was called with 0 as version for
the pseudo content type SSL3_RT_HEADER for TLS records.
In versions previous to OpenSSL 3.2 cb was called with 0 as version for
the pseudo content type SSL3_RT_HEADER for DTLS records.
COPYRIGHT
Copyright 2001-2023 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (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>.
3.5.7 2026-06-09 SSL_CTX_SET_MSG_CALLBACK(3SSL)
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