SSL_CTX_set_alpn_select_cb.3ssl man page

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)


#include <openssl/ssl.h>
int SSL_CTX_set_alpn_protos(SSL_CTX *ctx, const unsigned char *protos,
                            unsigned int protos_len);
int SSL_set_alpn_protos(SSL *ssl, const unsigned char *protos,
                        unsigned int 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 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 out buffer may point directly into in, or to a buffer that outlives the handshake. 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.


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 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:

A match was found and is returned in out, outlen.
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:

ALPN protocol selected.
ALPN protocol not selected.

See Also

ssl(3), SSL_CTX_set_tlsext_servername_callback(3), SSL_CTX_set_tlsext_servername_arg(3)