path: root/doc/ssl/SSL_CTX_set_alpn_select_cb.pod
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+=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);
+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
+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_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.
+ 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
+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
+A match was found and is returned in B<out>, B<outlen>.
+No match was found. The first item in B<client>, B<client_len> is returned in
+B<out>, B<outlen>.
+The ALPN select callback B<cb>, must return one of the following:
+=over 4
+ALPN protocol selected.
+ALPN protocol not selected.
+=head1 SEE ALSO
+L<ssl(3)>, L<SSL_CTX_set_tlsext_servername_callback(3)>,