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=pod

=head1 NAME

SSL_CTX_set_alpn_protos, SSL_set_alpn_protos, SSL_CTX_set_alpn_select_cb,
SSL_select_next_proto, SSL_get0_alpn_selected - handle application layer
protocol negotiation (ALPN)

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_CTX_set_alpn_protos(SSL_CTX *ctx, const unsigned char *protos,
                             unsigned protos_len);
 int SSL_set_alpn_protos(SSL *ssl, const unsigned char *protos,
                         unsigned protos_len);
 void SSL_CTX_set_alpn_select_cb(SSL_CTX *ctx,
                                 int (*cb) (SSL *ssl,
                                            const unsigned char **out,
                                            unsigned char *outlen,
                                            const unsigned char *in,
                                            unsigned int inlen,
                                            void *arg), void *arg);
 int SSL_select_next_proto(unsigned char **out, unsigned char *outlen,
                           const unsigned char *server,
                           unsigned int server_len,
                           const unsigned char *client,
                           unsigned int client_len)
 void SSL_get0_alpn_selected(const SSL *ssl, const unsigned char **data,
                             unsigned int *len);

=head1 DESCRIPTION

SSL_CTX_set_alpn_protos() and SSL_set_alpn_protos() are used by the client to
set the list of protocols available to be negotiated. The B<protos> must be in
protocol-list format, described below. The length of B<protos> is specified in
B<protos_len>.

SSL_CTX_set_alpn_select_cb() sets the application callback B<cb> used by a
server to select which protocol to use for the incoming connection. When B<cb>
is NULL, ALPN is not used. The B<arg> value is a pointer which is passed to
the application callback.

B<cb> is the application defined callback. The B<in>, B<inlen> parameters are a
vector in protocol-list format. The value of the B<out>, B<outlen> vector
should be set to the value of a single protocol selected from the B<in>,
B<inlen> vector. The B<arg> parameter is the pointer set via
SSL_CTX_set_alpn_select_cb().

SSL_select_next_proto() is a helper function used to select protocols. It
implements the standard protocol selection. It is expected that this function
is called from the application callback B<cb>. The protocol data in B<server>,
B<server_len> and B<client>, B<client_len> must be in the protocol-list format
described below. The first item in the B<server>, B<server_len> list that
matches an item in the B<client>, B<client_len> list is selected, and returned
in B<out>, B<outlen>. The B<out> value will point into either B<server> or
B<client>, so it should be copied immediately. If no match is found, the first
item in B<client>, B<client_len> is returned in B<out>, B<outlen>. This
function can also be used in the NPN callback.

SSL_get0_alpn_selected() returns a pointer to the selected protocol in B<data>
with length B<len>. It is not NUL-terminated. B<data> is set to NULL and B<len>
is set to 0 if no protocol has been selected. B<data> must not be freed.

=head1 NOTES

The protocol-lists must be in wire-format, which is defined as a vector of
non-empty, 8-bit length-prefixed, byte strings. The length-prefix byte is not
included in the length. Each string is limited to 255 bytes. A byte-string
length of 0 is invalid. A truncated byte-string is invalid. The length of the
vector is not in the vector itself, but in a separate variable.

Example:

 unsigned char vector[] = {
     6, 's', 'p', 'd', 'y', '/', '1',
     8, 'h', 't', 't', 'p', '/', '1', '.', '1'
 };
 unsigned int length = sizeof(vector);

The ALPN callback is executed after the servername callback; as that servername
callback may update the SSL_CTX, and subsequently, the ALPN callback.

If there is no ALPN proposed in the ClientHello, the ALPN callback is not
invoked.

=head1 RETURN VALUES

SSL_CTX_set_alpn_protos() and SSL_set_alpn_protos() return 0 on success, and
non-0 on failure. WARNING: these functions reverse the return value convention.

SSL_select_next_proto() returns one of the following:

=over 4

=item OPENSSL_NPN_NEGOTIATED

A match was found and is returned in B<out>, B<outlen>.

=item OPENSSL_NPN_NO_OVERLAP

No match was found. The first item in B<client>, B<client_len> is returned in
B<out>, B<outlen>.

=back

The ALPN select callback B<cb>, must return one of the following:

=over 4

=item SSL_TLSEXT_ERR_OK

ALPN protocol selected.

=item SSL_TLSEXT_ERR_NOACK

ALPN protocol not selected.

=back

=head1 SEE ALSO

L<ssl(3)>, L<SSL_CTX_set_tlsext_servername_callback(3)>,
L<SSL_CTX_set_tlsext_servername_arg(3)>

=cut