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 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 *peer_list, unsigned int peer_list_len, const unsigned char *supported_list, unsigned int supported_list_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 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 is expected that this function is called from the application callback cb. If SSL_select_next_proto() returns OPENSSL_NPN_NO_OVERLAP, cb should ignore out and fail by returning SSL_TLSEXT_ERR_ALERT_FATAL. The protocol data in peer_list, peer_list_len and supported_list, supported_list_len must be two non-empty lists, validly encoded in the protocol-list format described below. The first item in the peer_list that matches an item in the supported_list is selected, and returned in out, outlen. The out value will point into either peer_list or supported_list, so it must not be modified and should be copied immediately. If no match is found, the first item in supported_list is returned in out, outlen.

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. The length of the vector is not in the vector itself, but in a separate variable.

For example:

    const unsigned char *vector = "\x06" "spdy/1" "\x08" "http/1.1";
    unsigned int length = strlen(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 or non-zero 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 supported_list, supported_list_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_ALERT_FATAL
    There was no overlap between the client's supplied list and the server configuration.

SSL_TLSEXT_ERR_NOACK
    ALPN protocol not selected, e.g., because no ALPN protocols are configured for this connection.

SEE ALSO
ssl(3), SSL_CTX_set_tlsext_servername_arg(3), SSL_CTX_set_tlsext_servername_callback(3)

STANDARDS
TLS Application-Layer Protocol Negotiation Extension, RFC 7301.

TLS Next Protocol Negotiation Extension, https://datatracker.ietf.org/doc/html/draft-agl-tls-nextprotoneg.

HISTORY
SSL_select_next_proto() first appeared in OpenSSL 1.0.1 and has been available since OpenBSD 5.3.

SSL_CTX_set_alpn_protos(), SSL_set_alpn_protos(), SSL_CTX_set_alpn_select_cb(), and SSL_get0_alpn_selected() first appeared in OpenSSL 1.0.2 and have been available since OpenBSD 5.7.

CAVEATS
The fallback to the first supported protocol in SSL_select_next_proto() comes from the opportunistic fallback mechanism in the NPN extension. This behavior does not make sense for ALPN, where missing protocol overlap should result in a handshake failure. To avoid accidental selection of a protocol that the server does not support, it is recommended to pass the locally configured protocols as second pair of protocols in the ALPN callback.

BUGS
The out argument of SSL_select_next_proto() should have been const.