1=pod
2
3=head1 NAME
4
5SSL_CTX_set_msg_callback,
6SSL_CTX_set_msg_callback_arg,
7SSL_set_msg_callback,
8SSL_set_msg_callback_arg,
9SSL_trace
10- install callback for observing protocol messages
11
12=head1 SYNOPSIS
13
14 #include <openssl/ssl.h>
15
16 void SSL_CTX_set_msg_callback(SSL_CTX *ctx,
17                               void (*cb)(int write_p, int version,
18                                          int content_type, const void *buf,
19                                          size_t len, SSL *ssl, void *arg));
20 void SSL_CTX_set_msg_callback_arg(SSL_CTX *ctx, void *arg);
21
22 void SSL_set_msg_callback(SSL *ssl,
23                           void (*cb)(int write_p, int version,
24                                      int content_type, const void *buf,
25                                      size_t len, SSL *ssl, void *arg));
26 void SSL_set_msg_callback_arg(SSL *ssl, void *arg);
27
28 void SSL_trace(int write_p, int version, int content_type,
29                const void *buf, size_t len, SSL *ssl, void *arg);
30
31=head1 DESCRIPTION
32
33SSL_CTX_set_msg_callback() or SSL_set_msg_callback() can be used to
34define a message callback function I<cb> for observing all SSL/TLS/QUIC
35protocol messages (such as handshake messages) that are received or
36sent, as well as other events that occur during processing.
37SSL_CTX_set_msg_callback_arg() and SSL_set_msg_callback_arg()
38can be used to set argument I<arg> to the callback function, which is
39available for arbitrary application use.
40
41SSL_CTX_set_msg_callback() and SSL_CTX_set_msg_callback_arg() specify
42default settings that will be copied to new B<SSL> objects by
43L<SSL_new(3)>. SSL_set_msg_callback() and
44SSL_set_msg_callback_arg() modify the actual settings of an B<SSL>
45object. Using a B<NULL> pointer for I<cb> disables the message callback.
46
47When I<cb> is called by the SSL/TLS/QUIC library the function arguments have the
48following meaning:
49
50=over 4
51
52=item I<write_p>
53
54This flag is B<0> when a protocol message has been received and B<1>
55when a protocol message has been sent.
56
57=item I<version>
58
59The protocol version according to which the protocol message is
60interpreted by the library such as B<TLS1_3_VERSION>, B<TLS1_2_VERSION>,
61B<OSSL_QUIC1_VERSION> etc. For the SSL3_RT_HEADER pseudo
62content type (see NOTES below) this value will be the decoded
63version/legacy_version field of the record header.
64
65=item I<content_type>
66
67This is one of the content type values defined in the protocol specification
68(B<SSL3_RT_CHANGE_CIPHER_SPEC>, B<SSL3_RT_ALERT>, B<SSL3_RT_HANDSHAKE>; but never
69B<SSL3_RT_APPLICATION_DATA> because the callback will only be called for protocol
70messages). Alternatively it may be a "pseudo" content type. These pseudo
71content types are used to signal some other event in the processing of data (see
72NOTES below).
73
74=item I<buf>, I<len>
75
76I<buf> points to a buffer containing the protocol message or other data (in the
77case of pseudo content types), which consists of I<len> bytes. The buffer is no
78longer valid after the callback function has returned.
79
80=item I<ssl>
81
82The B<SSL> object that received or sent the message.
83
84=item I<arg>
85
86The user-defined argument optionally defined by
87SSL_CTX_set_msg_callback_arg() or SSL_set_msg_callback_arg().
88
89=back
90
91The SSL_trace() function can be used as a pre-written callback in a call to
92SSL_CTX_set_msg_callback() or SSL_set_msg_callback(). It requires a BIO to be
93set as the callback argument via SSL_CTX_set_msg_callback_arg() or
94SSL_set_msg_callback_arg(). Setting this callback will cause human readable
95diagostic tracing information about an SSL/TLS/QUIC connection to be written to
96the BIO.
97
98=head1 NOTES
99
100Protocol messages are passed to the callback function after decryption
101and fragment collection where applicable. (Thus record boundaries are
102not visible.)
103
104If processing a received protocol message results in an error,
105the callback function may not be called.  For example, the callback
106function will never see messages that are considered too large to be
107processed.
108
109Due to automatic protocol version negotiation, I<version> is not
110necessarily the protocol version used by the sender of the message: If
111a TLS 1.0 ClientHello message is received by an SSL 3.0-only server,
112I<version> will be B<SSL3_VERSION>.
113
114Pseudo content type values may be sent at various points during the processing
115of data. The following pseudo content types are currently defined:
116
117=over 4
118
119=item B<SSL3_RT_HEADER>
120
121Used when a TLS record is sent or received. The B<buf> contains the record header
122bytes only.
123
124=item B<SSL3_RT_INNER_CONTENT_TYPE>
125
126Used when an encrypted TLSv1.3 record is sent or received. In encrypted TLSv1.3
127records the content type in the record header is always
128SSL3_RT_APPLICATION_DATA. The real content type for the record is contained in
129an "inner" content type. B<buf> contains the encoded "inner" content type byte.
130
131=item B<SSL3_RT_QUIC_DATAGRAM>
132
133Used when a QUIC datagram is sent or received.
134
135=item B<SSL3_RT_QUIC_PACKET>
136
137Used when a QUIC packet is sent or received.
138
139=item B<SSL3_RT_QUIC_FRAME_FULL>
140
141Used when a QUIC frame is sent or received. This is only used for non-crypto
142and stream data related frames. The full QUIC frame data is supplied.
143
144=item B<SSL3_RT_QUIC_FRAME_HEADER>
145
146Used when a QUIC stream data or crypto frame is sent or received. Only the QUIC
147frame header data is supplied.
148
149=item B<SSL3_RT_QUIC_FRAME_PADDING>
150
151Used when a sequence of one or more QUIC padding frames is sent or received.
152A padding frame consists of a single byte and it is common to have multiple
153such frames in a sequence. Rather than supplying each frame individually the
154callback will supply all the padding frames in one go via this pseudo content
155type.
156
157=back
158
159=head1 RETURN VALUES
160
161SSL_CTX_set_msg_callback(), SSL_CTX_set_msg_callback_arg(), SSL_set_msg_callback()
162and SSL_set_msg_callback_arg() do not return values.
163
164=head1 SEE ALSO
165
166L<ssl(7)>, L<SSL_new(3)>
167
168=head1 HISTORY
169
170The pseudo content type B<SSL3_RT_INNER_CONTENT_TYPE> was added in OpenSSL 1.1.1.
171
172The pseudo content types B<SSL3_RT_QUIC_DATAGRAM>, B<SSL3_RT_QUIC_PACKET>,
173B<SSL3_RT_QUIC_FRAME_FULL>, B<SSL3_RT_QUIC_FRAME_HEADER> and
174B<SSL3_RT_QUIC_FRAME_PADDING> were added in OpenSSL 3.2.
175
176In versions previous to OpenSSL 3.0 I<cb> was called with 0 as I<version> for
177the pseudo content type B<SSL3_RT_HEADER> for TLS records.
178
179In versions previous to OpenSSL 3.2 I<cb> was called with 0 as I<version> for
180the pseudo content type B<SSL3_RT_HEADER> for DTLS records.
181
182=head1 COPYRIGHT
183
184Copyright 2001-2023 The OpenSSL Project Authors. All Rights Reserved.
185
186Licensed under the Apache License 2.0 (the "License").  You may not use
187this file except in compliance with the License.  You can obtain a copy
188in the file LICENSE in the source distribution or at
189L<https://www.openssl.org/source/license.html>.
190
191=cut
192