SSL_EXTENSION_SUPPORTED(3ossl)

📦
version 3.5.9 · 8 versions
, in fedora 43, fedora 44

okay bestie, welcome to SSL_extension_supported(3ossl) — the OpenSSL page that's literally all about custom TLS extension handling, and yes it's a whole vibe. 💅✨

🌸 NAME 🌸

So we're talking about a whole little friend group here: 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, and custom_ext_parse_cb — and together they handle custom TLS extensions. iconic behavior, honestly. 🫶

✨ SYNOPSIS ✨

#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 (peep the "EXTENSION CALLBACKS" section below, bestie). the context value is what tells OpenSSL which messages and under what conditions the extension gets added or parsed — more on that in "EXTENSION CONTEXTS" below. 💖

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. it's lowkey 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. same energy as 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 thing as SSL_CTX_add_client_custom_ext() up there. ✨

the ext_type parameter corresponds to the extension_type field of RFC5246 et al. and no, it is not a NID. in all cases the extension type must not be handled by OpenSSL internally or an error occurs. so don't try to double-book it, periodt. 💀

SSL_extension_supported() returns 1 if the extension ext_type is handled internally by OpenSSL and 0 otherwise. that's it, that's the whole function. 🎀

🎀 EXTENSION CALLBACKS 🎀

the callback add_cb gets called to send custom extension data that'll be included in various TLS messages. ext_type is set to the extension type that's gonna be added, and add_arg is set to the value that was set when the extension handler was added. when you're using the new style callbacks, the context parameter will indicate which message is currently being constructed — e.g. for the ClientHello it'll 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. slay. 💅

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. so maybe don't do that unless you mean it. 😭

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. noted. 📝

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. so like, keep track bestie. 🧚

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. no invite, no entrance. 💅

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 way that const-correctness is fighting you here… 😭

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. same rules as before, bestie. 📜

if the 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. dramatic? yes. accurate? also yes. 💀

the buffer in is a temporary internal buffer which will not be valid after the callback returns. so don't get attached, it's giving temporary. 🦋

🌸 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). so like, don't forget the message value, bestie, or your extension is just gonna sit there with no plans. 💖

and that's the first half, there's more coming so don't go anywhere. 🎀

💅 NOTES 💅

Okay bestie, this is the part where we talk about vibes and consequences, so lock in 🔒✨

The add_arg and parse_arg parameters can be set to arbitrary values — like, literally whatever you want — and those values get passed straight to the corresponding callbacks. Think of them as lil friendship bracelets you hand to your callback so it remembers who it is 🫶 They can, for example, be used to store the extension data received in a convenient structure, or to pass the extension data to be added or freed when adding extensions. Very demure, very useful.

Now the ✨rules✨, because TLS does not play:

  • If the same custom extension type is received multiple times, a fatal decode_error alert is sent and the handshake aborts. Not a vibe. Not negotiable. 💀
  • If a custom extension is received in a ServerHello/EncryptedExtensions message which was not sent in the ClientHello, a fatal unsupported_extension alert is sent and the handshake is aborted. No sneaky surprise extensions allowed 🙅‍♀️
  • The ServerHello/EncryptedExtensions add_cb callback is only called if the corresponding extension was received in the ClientHello. That's the rule, and honestly it's giving protective bestie energy.

This is compliant with the TLS specifications, so no, we're not being extra — this is just how it's supposed to work, periodt. This behaviour ensures that each callback is called at most once and that an application can never send unsolicited extensions. Iconic behaviour, honestly. 👑

🎀 RETURN VALUES 🎀

Here's the scoreboard, bestie:

  • 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.
    • A failure can occur if an attempt is made to add the same ext_type more than once, if an attempt is made to use an extension type handled internally by OpenSSL, or if an internal error occurs (for example a memory allocation failure). Yeah — double-dipping on extension types is a no. 🙈
  • SSL_extension_supported() returns 1 if the extension ext_type is handled internally by OpenSSL and 0 otherwise. Simple as. ✨

🌈 SEE ALSO 🌈

That's the whole squad. Go read it, it's good for you. 📚

🦋 HISTORY 🦋

The SSL_CTX_add_custom_ext() function was added in OpenSSL 1.1.1. A classic. A legend. A founding mother of custom extensions. 💜

Copyright 2014-2020 The OpenSSL Project Authors. All Rights Reserved.

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>.

Yes bestie, read the license. It's the law, and also it's right there. 📜✨

And that's a wrap on this page, angel 💖 This was OpenSSL 3.5.9, dated 2026-09-29, filed under SSL_EXTENSION_SUPPORTED(3ossl) — go forth, add your custom extensions, and remember: at most once, no unsolicited vibes. Slay. 💅🌸

📦 where this page comes from