---
name: "libressl-SSL_CTX_set_msg_callback"
section: "3"
platform: "linux"
package: "libressl"
version: "4.4.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/libressl-SSL_CTX_set_msg_callback"
---

# libressl-SSL\_CTX\_set\_msg\_callback(3)

install callback for observing protocol messages

## 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

SSL Library (libssl, -lssl) 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/3.md), 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.