SSL_EXTENSION_SUPPORTED(3SSL)

📦

version 4.0.3 · 13 versions

okay bestie, this is the SSL_EXTENSION_SUPPORTED(3SSL) page from OpenSSL 💅 it's all about custom TLS extension handling, and we are gonna go through every single bit, no skimming, no cap ✨

✨ NAME ✨

Meet the whole girl squad: SSL_extension_supported, SSL_custom_ext_add_cb_ex, SSL_custom_ext_free_cb_ex, SSL_custom_ext_parse_cb_ex, SSL_CTX_add_custom_ext, SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext, custom_ext_add_cb, custom_ext_free_cb, custom_ext_parse_cb — and they are collectively serving custom TLS extension handling 👑

🌸 SYNOPSIS 🌸

first, include this header, bestie:

#include <openssl/ssl.h>

typedef int (*SSL_custom_ext_add_cb_ex)(SSL *s, unsigned int ext_type,
                                        unsigned int context,
                                        const unsigned char **out,
                                        size_t *outlen, X509 *x,
                                        size_t chainidx, int *al,
                                        void *add_arg);

typedef void (*SSL_custom_ext_free_cb_ex)(SSL *s, unsigned int ext_type,
                                          unsigned int context,
                                          const unsigned char *out,
                                          void *add_arg);

typedef int (*SSL_custom_ext_parse_cb_ex)(SSL *s, unsigned int ext_type,
                                          unsigned int context,
                                          const unsigned char *in,
                                          size_t inlen, X509 *x,
                                          size_t chainidx, int *al,
                                          void *parse_arg);

int SSL_CTX_add_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
                           unsigned int context,
                           SSL_custom_ext_add_cb_ex add_cb,
                           SSL_custom_ext_free_cb_ex free_cb,
                           void *add_arg,
                           SSL_custom_ext_parse_cb_ex parse_cb,
                           void *parse_arg);

typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
                                 const unsigned char **out,
                                 size_t *outlen, int *al,
                                 void *add_arg);

typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type,
                                   const unsigned char *out,
                                   void *add_arg);

typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type,
                                   const unsigned char *in,
                                   size_t inlen, int *al,
                                   void *parse_arg);

int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
                                  custom_ext_add_cb add_cb,
                                  custom_ext_free_cb free_cb, void *add_arg,
                                  custom_ext_parse_cb parse_cb,
                                  void *parse_arg);

int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
                                  custom_ext_add_cb add_cb,
                                  custom_ext_free_cb free_cb, void *add_arg,
                                  custom_ext_parse_cb parse_cb,
                                  void *parse_arg);

int SSL_extension_supported(unsigned int ext_type);

💖 DESCRIPTION 💖

okay so SSL_CTX_add_custom_ext() is the main character here 💅 it adds a custom extension for a TLS/DTLS client or server for all supported protocol versions, with extension type ext_type and callbacks add_cb, free_cb and parse_cb (the EXTENSION CALLBACKS section below is the full lore). The context value determines which messages and under what conditions the extension will be added/parsed (the EXTENSION CONTEXTS section below has the map).

SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS/DTLS client with extension type ext_type and callbacks add_cb, free_cb and parse_cb. This function is similar to SSL_CTX_add_custom_ext() except:

it only applies to clients,
it uses the older style of callbacks,
and it implicitly sets the context value to:

SSL_EXT_TLS1_2_AND_BELOW_ONLY | SSL_EXT_CLIENT_HELLO
| SSL_EXT_TLS1_2_SERVER_HELLO | SSL_EXT_IGNORE_ON_RESUMPTION

SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS/DTLS server with extension type ext_type and callbacks add_cb, free_cb and parse_cb. This function is similar to SSL_CTX_add_custom_ext() except it only applies to servers, uses the older style of callbacks, and implicitly sets the context value to the same as for SSL_CTX_add_client_custom_ext() above.

The ext_type parameter corresponds to the extension_type field of RFC5246 et al. It is not a NID. In all cases the extension type must not be handled by OpenSSL internally or an error occurs 😭 so do not try to re-home an extension OpenSSL already claims.

SSL_extension_supported() returns 1 if the extension ext_type is handled internally by OpenSSL and 0 otherwise 🫶

🦋 EXTENSION CALLBACKS 🦋

The callback add_cb is called to send custom extension data to be included in various TLS messages. The ext_type parameter is set to the extension type which will be added, and add_arg to the value set when the extension handler was added. When using the new style callbacks, the context parameter will indicate which message is currently being constructed — e.g. for the ClientHello it will be set to SSL_EXT_CLIENT_HELLO ✨

If the application wishes to include the extension ext_type, it should set *out to the extension data, set *outlen to the length of the extension data, and return 1.
If add_cb does not wish to include the extension, it must return 0.
If add_cb returns -1, a fatal handshake error occurs using the TLS alert value specified in *al 💀
When constructing the ClientHello, if add_cb is set to NULL, a zero length extension is added for ext_type. For all other messages, if add_cb is set to NULL, then no extension is added.
When constructing a Certificate message, the callback will be called for each certificate in the message. The x parameter will indicate the current certificate, and the chainidx parameter will indicate the position of the certificate in the message. The first certificate is always the end entity certificate and has a chainidx value of 0. The certificates are in the order that they were received in the Certificate message.
For all messages except the ServerHello and EncryptedExtensions, every registered add_cb is always called to see if the application wishes to add an extension (as long as all requirements of the specified context are met).
For the ServerHello and EncryptedExtension messages, every registered add_cb is called once if and only if the requirements of the specified context are met and the corresponding extension was received in the ClientHello. That is, if no corresponding extension was received in the ClientHello, then add_cb will not be called.

If an extension is added (that is, add_cb returns 1), free_cb is called (if it is set) with the value of out set by the add callback. It can be used to free up any dynamic extension data set by add_cb. Since out is constant (to permit use of constant data in add_cb), applications may need to cast away const to free the data 🎀

The callback parse_cb receives data for TLS extensions. The callback is only called if the extension is present and relevant for the context (see EXTENSION CONTEXTS below).

The extension data consists of inlen bytes in the buffer in for the extension ext_type.
If the message being parsed is a TLSv1.3 compatible Certificate message, then parse_cb will be called for each certificate contained within the message. The x parameter will indicate the current certificate and the chainidx parameter will indicate the position of the certificate in the message. The first certificate is always the end entity certificate and has a chainidx value of 0.
If parse_cb considers the extension data acceptable, it must return 1. If it returns 0 or a negative value, a fatal handshake error occurs using the TLS alert value specified in *al.
The buffer in is a temporary internal buffer which will not be valid after the callback returns ⚠️

🌈 EXTENSION CONTEXTS 🌈

An extension context defines which messages and under which conditions an extension should be added or expected. The context is built up by performing a bitwise OR of multiple pre-defined values together. The valid context values are:

SSL_EXT_TLS_ONLY — The extension is only allowed in TLS
SSL_EXT_DTLS_ONLY — The extension is only allowed in DTLS
SSL_EXT_TLS_IMPLEMENTATION_ONLY — The extension is allowed in DTLS, but there is only a TLS implementation available (so it is ignored in DTLS).
SSL_EXT_SSL3_ALLOWED — Extensions are not typically defined for SSLv3. Setting this value will allow the extension in SSLv3. Applications will not typically need to use this.
SSL_EXT_TLS1_2_AND_BELOW_ONLY — The extension is only defined for TLSv1.2/DTLSv1.2 and below. Servers will ignore this extension if it is present in the ClientHello and TLSv1.3 is negotiated.
SSL_EXT_TLS1_3_ONLY — The extension is only defined for TLS1.3 and above. Servers will ignore this extension if it is present in the ClientHello and TLSv1.2 or below is negotiated.
SSL_EXT_IGNORE_ON_RESUMPTION — The extension will be ignored during parsing if a previous session is being successfully resumed.
SSL_EXT_CLIENT_HELLO — The extension may be present in the ClientHello message.
SSL_EXT_TLS1_2_SERVER_HELLO — The extension may be present in a TLSv1.2 or below compatible ServerHello message.
SSL_EXT_TLS1_3_SERVER_HELLO — The extension may be present in a TLSv1.3 compatible ServerHello message.
SSL_EXT_TLS1_3_ENCRYPTED_EXTENSIONS — The extension may be present in an EncryptedExtensions message.
SSL_EXT_TLS1_3_HELLO_RETRY_REQUEST — The extension may be present in a HelloRetryRequest message.
SSL_EXT_TLS1_3_CERTIFICATE — The extension may be present in a TLSv1.3 compatible Certificate message.
SSL_EXT_TLS1_3_NEW_SESSION_TICKET — The extension may be present in a TLSv1.3 compatible NewSessionTicket message.
SSL_EXT_TLS1_3_CERTIFICATE_REQUEST — The extension may be present in a TLSv1.3 compatible CertificateRequest message.

The context must include at least one message value (otherwise the extension will never be used).

📝 NOTES 📝

okay so real talk bestie, let's talk about add_arg and parse_arg 👀

the add_arg and parse_arg parameters can literally be set to arbitrary values, and those values get passed straight to the corresponding callbacks. no cap.
they're giving "flexible little messenger bags" energy — you can use them, for example, to stash the extension data you received into a convenient structure, or to pass along the extension data that needs to be added or freed when you're adding extensions. 💅

now the ✨drama✨ — what happens when things go sideways:

if the same custom extension type gets received multiple times, a fatal decode_error alert is sent and the handshake aborts. periodt. 💀
if a custom extension is received in a ServerHello/EncryptedExtensions message that was not sent in the ClientHello, a fatal unsupported_extension alert is sent and the handshake is aborted. not me catching you trying to be sneaky 🙈
the ServerHello/EncryptedExtensions add_cb callback is only called if the corresponding extension was received in the ClientHello.
this is fully compliant with the TLS specifications, bestie — we don't make the rules, we just slay by them 👑

so what does all this behavior actually guarantee? two iconic things:

each callback is called at most once
an application can never send unsolicited extensions. it's giving "consent-based extension exchange" and honestly? we love that for us 🫶

💖 RETURN VALUES 💖

here's the tea on what you get back:

SSL_CTX_add_custom_ext(), SSL_CTX_add_client_custom_ext() and SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure. simple as that ✨
a failure can occur if:
- you make an attempt to add the same ext_type more than once 😭
- you make an attempt to use an extension type that's handled internally by OpenSSL 🚫
- or an internal error occurs (for example, a memory allocation failure — the classic 💀)
SSL_extension_supported() returns 1 if the extension ext_type is handled internally by OpenSSL, and 0 otherwise. lowkey a vibe check for extensions 👑

👀 SEE ALSO 👀

ssl(7) — go read it, bestie, it's the main character fr fr

🕰️ HISTORY 🕰️

the SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1. so it's been serving looks since then 💅

⚖️ 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 🎀 this was OpenSSL 4.0.3, dated 2026-09-29, from the SSL_EXTENSION_SUPPORTED(3SSL) manual page. now go forth and customize those extensions responsibly 💖✨🦋

📦 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