Custom extension documentation.

=pod

=head1 NAME

SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext - custom TLS extension handling

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 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);

 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);

=head1 DESCRIPTION

SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS client 

with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and

B<parse_cb>.

SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS server 

with extension type B<ext_type> and callbacks B<add_cb>, B<free_cb> and

B<parse_cb>.

In both cases the extension type must not be handled by OpenSSL internally

or an error occurs.

SSL_extension_supported() returns 1 if the extension B<ext_type> is handled

internally by OpenSSL and 0 otherwise.

=head1 EXTENSION CALLBACKS

The callback B<add_cb> is called to send custom extension data to be 

included in ClientHello for TLS clients or ServerHello for servers. The

B<ext_type> parameter is set to the extension type which will be added and

B<add_arg> to the value set when the extension handler was added.

If the application wishes to include the extension B<ext_type> it should

set B<*out> to the extension data, set B<*outlen> to the length of the

extension data and return 1.

If the B<add_cb> does not wish to include the extension it must return 0.

If B<add_cb> returns -1 a fatal handshake error occurs using the TLS

alert value specified in B<*al>.

For clients (but not servers) if B<add_cb> is set to NULL a zero length

extension is added for B<ext_type>.

For clients every registered B<add_cb> is always called to see if the

application wishes to add an extension to ClientHello.

For servers every registered B<add_cb> is called once if and only if the

corresponding extension was received in ClientHello to see if the application

wishes to add the extension to ServerHello. That is, if no corresponding extension

was received in ClientHello then B<add_cb> will not be called.

If an extension is added (that is B<add_cb> returns 1) B<free_cb> is called

(if it is set) with the value of B<out> set by the add callback. It can be

used to free up any dynamic extension data set by B<add_cb>. Since B<out> is

constant (to permit use of constant data in B<add_cb>) applications may need to

cast away const to free the data.

The callback B<parse_cb> receives data for TLS extensions. For TLS clients

the extension data will come from ServerHello and for TLS servers it will

come from ClientHello.

The extension data consists of B<inlen> bytes in the buffer B<in> for the

extension B<extension_type>.

If the B<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 B<*al>.

The buffer B<in> is a temporary internal buffer which will not be valid after

the callback returns.

=head1 NOTES

The B<add_arg> and B<parse_arg> parameters can be set to arbitrary values

which will be passed to the corresponding callbacks. They can, for example,

be used to store the extension data received in a convenient structure or

pass the extension data to be added or freed when adding extensions.

The B<ext_type> parameter corresponds to the B<extension_type> field of

RFC5246 et al. It is B<not> a NID.

If the same custom extension type is received multiple times a fatal

B<decode_error> alert is sent and the handshake aborts. If a custom extension

is received in ServerHello which was not sent in ClientHello a fatal

B<unsupported_extension> alert is sent and the handshake is aborted. The

ServerHello B<add_cb> callback is only called if the corresponding extension

was received in ClientHello. This is compliant with the TLS specifications.

This behaviour ensures that each callback is called at most once and that

an application can never send unsolicited extensions.

=head1 RETURN VALUES

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 B<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).

SSL_extension_supported() returns 1 if the extension B<ext_type> is handled

internally by OpenSSL and 0 otherwise.

=cut