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  1.\"	$OpenBSD: SSL_CTX_set_msg_callback.3,v 1.6 2025/06/08 22:52:00 schwarze Exp $
  2.\"	OpenSSL SSL_CTX_set_msg_callback.pod e9b77246 Jan 20 19:58:49 2017 +0100
  3.\"	OpenSSL SSL_CTX_set_msg_callback.pod b97fdb57 Nov 11 09:33:09 2016 +0100
  4.\"
  5.\" This file was written by Bodo Moeller <bodo@openssl.org>.
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 52.Dd $Mdocdate: June 8 2025 $
 53.Dt SSL_CTX_SET_MSG_CALLBACK 3
 54.Os
 55.Sh NAME
 56.Nm SSL_CTX_set_msg_callback ,
 57.Nm SSL_CTX_set_msg_callback_arg ,
 58.Nm SSL_set_msg_callback ,
 59.Nm SSL_set_msg_callback_arg
 60.Nd install callback for observing protocol messages
 61.Sh SYNOPSIS
 62.Lb libssl libcrypto
 63.In openssl/ssl.h
 64.Ft void
 65.Fo SSL_CTX_set_msg_callback
 66.Fa "SSL_CTX *ctx"
 67.Fa "void (*cb)(int write_p, int version, int content_type, const void *buf, size_t len, SSL *ssl, void *arg)"
 68.Fc
 69.Ft void
 70.Fn SSL_CTX_set_msg_callback_arg "SSL_CTX *ctx" "void *arg"
 71.Ft void
 72.Fo SSL_set_msg_callback
 73.Fa "SSL *ssl"
 74.Fa "void (*cb)(int write_p, int version, int content_type, const void *buf, size_t len, SSL *ssl, void *arg)"
 75.Fc
 76.Ft void
 77.Fn SSL_set_msg_callback_arg "SSL *ssl" "void *arg"
 78.Sh DESCRIPTION
 79.Fn SSL_CTX_set_msg_callback
 80or
 81.Fn SSL_set_msg_callback
 82can be used to define a message callback function
 83.Fa cb
 84for observing all SSL/TLS protocol messages (such as handshake messages)
 85that are received or sent.
 86.Fn SSL_CTX_set_msg_callback_arg
 87and
 88.Fn SSL_set_msg_callback_arg
 89can be used to set argument
 90.Fa arg
 91to the callback function, which is available for arbitrary application use.
 92.Pp
 93.Fn SSL_CTX_set_msg_callback
 94and
 95.Fn SSL_CTX_set_msg_callback_arg
 96specify default settings that will be copied to new
 97.Vt SSL
 98objects by
 99.Xr SSL_new 3 .
100.Fn SSL_set_msg_callback
101and
102.Fn SSL_set_msg_callback_arg
103modify the actual settings of an
104.Vt SSL
105object.
106Using a
107.Dv NULL
108pointer for
109.Fa cb
110disables the message callback.
111.Pp
112When
113.Fa cb
114is called by the SSL/TLS library for a protocol message,
115the function arguments have the following meaning:
116.Bl -tag -width Ds
117.It Fa write_p
118This flag is 0 when a protocol message has been received and 1 when a protocol
119message has been sent.
120.It Fa version
121The protocol version according to which the protocol message is
122interpreted by the library, such as
123.Dv TLS1_VERSION ,
124.Dv TLS1_1_VERSION ,
125.Dv TLS1_2_VERSION ,
126.Dv DTLS1_VERSION ,
127or
128.Dv DTLS1_2_VERSION .
129.It Fa content_type
130This is one of the
131.Em ContentType
132values defined in the protocol specification
133.Po
134.Dv SSL3_RT_CHANGE_CIPHER_SPEC ,
135.Dv SSL3_RT_ALERT ,
136.Dv SSL3_RT_HANDSHAKE ,
137but never
138.Dv SSL3_RT_APPLICATION_DATA
139because the callback will only be called for protocol messages.
140.Pc
141.It Fa buf , Fa len
142.Fa buf
143points to a buffer containing the protocol message, which consists of
144.Fa len
145bytes.
146The buffer is no longer valid after the callback function has returned.
147.It Fa ssl
148The
149.Vt SSL
150object that received or sent the message.
151.It Fa arg
152The user-defined argument optionally defined by
153.Fn SSL_CTX_set_msg_callback_arg
154or
155.Fn SSL_set_msg_callback_arg .
156.El
157.Pp
158Protocol messages are passed to the callback function after decryption
159and fragment collection where applicable.
160(Thus record boundaries are not visible.)
161.Pp
162If processing a received protocol message results in an error,
163the callback function may not be called.
164For example, the callback function will never see messages that are considered
165too large to be processed.
166.Pp
167Due to automatic protocol version negotiation,
168.Fa version
169is not necessarily the protocol version used by the sender of the message:
170If a TLS 1.0 ClientHello message is received by an SSL 3.0-only server,
171.Fa version
172will be
173.Dv SSL3_VERSION .
174.Sh SEE ALSO
175.Xr ssl 3 ,
176.Xr SSL_new 3
177.Sh HISTORY
178.Fn SSL_CTX_set_msg_callback ,
179.Fn SSL_CTX_set_msg_callback_arg ,
180.Fn SSL_set_msg_callback
181and
182.Fn SSL_set_msg_callback_arg
183first appeared in OpenSSL 0.9.7 and have been available since
184.Ox 3.2 .