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SSL_CTX_set_alpn_select_cb(3)       OpenSSL      SSL_CTX_set_alpn_select_cb(3)

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)
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);
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
       protos must be in protocol-list format, described below. The length of
       protos is specified in protos_len.
       SSL_CTX_set_alpn_select_cb() sets the application callback cb used by a
       server to select which protocol to use for the incoming connection.
       When cb is NULL, ALPN is not used. The arg value is a pointer which is
       passed to the application callback.
       cb is the application defined callback. The in, inlen parameters are a
       vector in protocol-list format. The value of the out, outlen vector
       should be set to the value of a single protocol selected from the in,
       inlen vector. The 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 cb. The protocol data
       in server, server_len and client, client_len must be in the protocol-
       list format described below. The first item in the server, server_len
       list that matches an item in the client, client_len list is selected,
       and returned in out, outlen. The out value will point into either
       server or client, so it should be copied immediately. If no match is
       found, the first item in client, client_len is returned in out, outlen.
       This function can also be used in the NPN callback.
       SSL_get0_alpn_selected() returns a pointer to the selected protocol in
       data with length len. It is not NUL-terminated. data is set to NULL and
       len is set to 0 if no protocol has been selected. data must not be
       freed.
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.
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:
       OPENSSL_NPN_NEGOTIATED
           A match was found and is returned in out, outlen.
       OPENSSL_NPN_NO_OVERLAP
           No match was found. The first item in client, client_len is
           returned in out, outlen.
       The ALPN select callback cb, must return one of the following:
       SSL_TLSEXT_ERR_OK
           ALPN protocol selected.
       SSL_TLSEXT_ERR_NOACK
           ALPN protocol not selected.
SEE ALSO
       ssl(3), SSL_CTX_set_tlsext_servername_callback(3),
       SSL_CTX_set_tlsext_servername_arg(3)

1.0.2k                            2017-01-26     SSL_CTX_set_alpn_select_cb(3)