SSL_CTX_SET_MSG_CALLBACK(3ossl)

💅 SSL_CTX_SET_MSG_CALLBACK 💅

okay bestie, so this is from the 3ossl manual — OpenSSL's own little corner of the docs — and it's literally about installing a callback so you can eavesdrop on every protocol message. not in a creepy way. in a debugging way. 🦋

✨ 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. that's the whole vibe: you get a front-row seat to the group chat between client and server. 👑

🌸 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() or SSL_set_msg_callback() is how you define a message callback function cb for observing all SSL/TLS/QUIC protocol messages (think handshake messages, bestie) that are received or sent — plus other events that happen while processing. it's giving backstage pass. 🎀

and then SSL_CTX_set_msg_callback_arg() and SSL_set_msg_callback_arg() let you set the argument arg that gets handed to the callback function, and that one is available for arbitrary application use — like, whatever you want, it's yours. 🍓

okay so the CTX ones vs the non-CTX ones, iykyk:

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.

and if you pass a NULL pointer for cb, that disables the message callback. periodt. ✨

💜 what the arguments mean when `cb` gets called

when cb is called by the SSL/TLS/QUIC library, 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. so like, in or out. 🙈
version — the protocol version according to which the protocol message is interpreted by the library, such as TLS1_3_VERSION, TLS1_2_VERSION, OSSL_QUIC1_VERSION etc. for the SSL3_RT_HEADER pseudo content type (see NOTES below) this value will be the decoded version/legacy_version field of the record header. it's giving receipts. 🔥
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 will only be called for protocol messages). alternatively it may be a "pseudo" content type. these pseudo content types are used to signal some other event in the processing of data (see NOTES below).
buf, len — buf points to a buffer containing the protocol message or other data (in the case of pseudo content types), which consists of len bytes. ⚠️ the buffer is no longer valid after the callback function has returned — so don't go saving that pointer for later, bestie, copy what you need. 😭
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().

🧚 the pre-written one: `SSL_trace()`

the SSL_trace() function can be used as a pre-written callback in a call to SSL_CTX_set_msg_callback() or SSL_set_msg_callback(). it requires a BIO to be set as the callback argument via SSL_CTX_set_msg_callback_arg() or SSL_set_msg_callback_arg(). setting this callback will cause human readable diagostic tracing information about an SSL/TLS/QUIC connection to be written to the BIO. slay, free logging. 💅

🦋 NOTES 🦋

protocol messages are passed to the callback function after decryption and fragment collection where applicable. (thus record boundaries are not visible.) so like, the seams are gone. 🪡

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. so sad, but that's the rules. 💀

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, honestly. 🌈

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 is contained in an "inner" content type. buf contains the encoded "inner" content type byte. it's giving disguise. 🎭
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 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 will supply all the padding frames in one go via this pseudo content type. bundling, iconic. 🎁

⭐ 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 to check, nothing to unwrap. that's the whole answer. 🎀

🔍 SEE ALSO 🔍

ssl(7), SSL_new(3) — go read those too, bestie. 👑

📜 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.

so yeah, legacy behavior is a whole lore arc. 🧚

💖 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 page, bestie!! 💅✨ this one's from OpenSSL 3.5.0, dated 2026-09-25 — go forth and trace those messages, you absolute icon. 🦋🌸👑

📦 where this page comes from

opensuse-leap 16.0: openssl-3-doc 3.5.0-160000.11.1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms

SSL_CTX_SET_MSG_CALLBACK(3ossl)