SSL_CTX_SET_MSG_CALLBACK(3) | Library Functions Manual | SSL_CTX_SET_MSG_CALLBACK(3)

# 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

library “libssl” libcrypto
`#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. `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`, `DTLS1_VERSION`, or `DTLS1_2_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 The buffer is no longer valid after the callback function has returned.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`.

# SEE ALSO

ssl(3), SSL_new(3)

# HISTORY

`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.

June 8, 2025 | Debian