SSL_CTX_SET_MSG_CALLBACK(3SSL)

📦

version 4.0.3 · 14 versions

💖 SSL_CTX_SET_MSG_CALLBACK 💖

okay so this is the 3SSL page from the OpenSSL family, and bestie, it's literally about installing a callback so you can watch protocol messages go by. nosy in the best way. let's get into it 🫶

🌸 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 🦋

so SSL_CTX_set_msg_callback() and SSL_set_msg_callback() are how you define a message callback function cb that gets to observe all SSL/TLS/QUIC protocol messages (handshake messages, the works) that are received or sent — plus other events that happen while processing goes down. and SSL_CTX_set_msg_callback_arg() and SSL_set_msg_callback_arg() set the argument arg for that callback function, which is there for your arbitrary application use. do whatever you want with it, it's yours 💅

now the ✨context✨ difference, because it matters:

SSL_CTX_set_msg_callback() and SSL_CTX_set_msg_callback_arg() set up default settings that get copied onto 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.

and if you pass a NULL pointer for cb? that disables the message callback. bye bestie 👋

🌈 when `cb` gets called, the arguments mean this 🌈

write_p — this flag is 0 when a protocol message has been received and 1 when a protocol message has been sent. that's it, that's the whole flag.
version — the protocol version according to which the protocol message is interpreted by the library, like TLS1_3_VERSION, TLS1_2_VERSION, OSSL_QUIC1_VERSION etc. and heads up: for the SSL3_RT_HEADER pseudo content type (more on pseudo content types down in NOTES, iykyk) 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 only gets called for protocol messages. alternatively it may be a "pseudo" content type. these pseudo content types signal some other event in the processing of data (check NOTES below, bestie).
buf, len — buf points to a buffer containing the protocol message or other data (this is the pseudo content type situation), and it consists of len bytes. ⚠️ lowkey important: the buffer is no longer valid after the callback function has returned. do NOT hold onto it, do not pass it around, do not make it your whole personality. it's gone.
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().

💫 and then there's `SSL_trace()` 💫

SSL_trace() is a pre-written callback you can just drop into a call to SSL_CTX_set_msg_callback() or SSL_set_msg_callback(). iconic, honestly, someone already did the work. it needs a BIO set as the callback argument via SSL_CTX_set_msg_callback_arg() or SSL_set_msg_callback_arg(). set this callback and you get human readable diagnostic tracing information about an SSL/TLS/QUIC connection written straight to that BIO. free lore 🎀

🔥 NOTES 🔥

protocol messages are passed to the callback function after decryption and fragment collection where applicable. (so record boundaries are not visible. sad but true.)
if processing a received protocol message results in an error, the callback function might not get called. for example, the callback function will never see messages that are considered too large to be processed. not ghosting you, they just never show up.
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. plot twist 🫠

🧚 pseudo content type values 🧚

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 lives in an "inner" content type. buf contains the encoded "inner" content type byte. it's giving … layers 💅
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 one 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 supplies all the padding frames in one go via this pseudo content type. bundling queen 👑

⭐ 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. nothing. zero. void era. ✨

💜 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 👑

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>.

and that's the whole page bestie, no cap. this was version 4.0.3, dated 2026-09-29 of the SSL_CTX_SET_MSG_CALLBACK(3SSL) manual — go install a callback, watch some protocol messages, slay 💖✨🎀🦋

📦 where this page comes from

ubuntu stonking: libssl-doc 4.0.3-1ubuntu1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms

SSL_CTX_SET_MSG_CALLBACK(3SSL)