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_CTX_set_next_proto_select_cb,
SSL_CTX_set_next_protos_advertised_cb, SSL_select_next_proto,
SSL_get0_alpn_selected, SSL_get0_next_proto_negotiated - 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);
void SSL_get0_alpn_selected(const SSL *ssl, const unsigned char **data,
unsigned int *len);
void SSL_CTX_set_next_protos_advertised_cb(SSL_CTX *ctx,
int (*cb)(SSL *ssl,
const unsigned char **out,
unsigned int *outlen,
void *arg),
void *arg);
void SSL_CTX_set_next_proto_select_cb(SSL_CTX *ctx,
int (*cb)(SSL *s,
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_next_proto_negotiated(const SSL *s, const unsigned char **data,
unsigned *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. Setting protos_len to 0 clears any
existing list of ALPN protocols and no ALPN extension will be sent to
the server.
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. The client list
must include at least one valid (nonempty) protocol entry in the list.
The SSL_select_next_proto() helper function can be useful from either
the ALPN callback or the NPN callback (described below). If no match is
found, the first item in client, client_len is returned in out, outlen
and OPENSSL_NPN_NO_OVERLAP is returned. This can be useful when
implementating the NPN callback. In the ALPN case, the value returned
in out and outlen must be ignored if OPENSSL_NPN_NO_OVERLAP has been
returned from SSL_select_next_proto().
SSL_CTX_set_next_proto_select_cb() sets a callback cb that is called
when a client needs to select a protocol from the server's provided
list, and a user-defined pointer argument arg which will be passed to
this callback. For the callback itself, out must be set to point to
the selected protocol (which may be within in). The length of the
protocol name must be written into outlen. The server's advertised
protocols are provided in in and inlen. The callback can assume that in
is syntactically valid. The client must select a protocol (although it
may be an empty, zero length protocol). It is fatal to the connection
if this callback returns a value other than SSL_TLSEXT_ERR_OK or if the
zero length protocol is selected. The arg parameter is the pointer set
via SSL_CTX_set_next_proto_select_cb().
SSL_CTX_set_next_protos_advertised_cb() sets a callback cb that is
called when a TLS server needs a list of supported protocols for Next
Protocol Negotiation. The returned list must be in protocol-list
format, described below. The list is returned by setting out to point
to it and outlen to its length. This memory will not be modified, but
the SSL does keep a reference to it. The callback should return
SSL_TLSEXT_ERR_OK if it wishes to advertise. Otherwise, no such
extension will be included in the ServerHello.
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.
SSL_get0_next_proto_negotiated() sets data and len to point to the
client's requested protocol for this connection. If the client did not
request any protocol or NPN is not enabled, then data is set to NULL
and len to 0. Note that the client can request any protocol it chooses.
The value returned from this function need not be a member of the list
of supported protocols provided by the callback.
NOTES
The protocol-lists must be in wire-format, which is defined as a vector
of nonempty, 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 (or NULL and 0 in the case where the first
entry in client is invalid).
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.
The callback set using SSL_CTX_set_next_proto_select_cb() should return
SSL_TLSEXT_ERR_OK if successful. Any other value is fatal to the
connection.
The callback set using SSL_CTX_set_next_protos_advertised_cb() should
return SSL_TLSEXT_ERR_OK if it wishes to advertise. Otherwise, no such
extension will be included in the ServerHello.
SEE ALSO
ssl(7), SSL_CTX_set_tlsext_servername_callback(3),
SSL_CTX_set_tlsext_servername_arg(3)
COPYRIGHT
Copyright 2016-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>;.
1.1.1k 2024-10-09 SSL_CTX_SET_ALPN_SELECT_CB(3)
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