1SSL_CTX_SET_ALPN_SELECT_CB(3) OpenSSL SSL_CTX_SET_ALPN_SELECT_CB(3)
2
6 SSL_CTX_set_alpn_protos, SSL_set_alpn_protos,
7 SSL_CTX_set_alpn_select_cb, SSL_CTX_set_next_proto_select_cb,
8 SSL_CTX_set_next_protos_advertised_cb, SSL_select_next_proto,
9 SSL_get0_alpn_selected, SSL_get0_next_proto_negotiated - handle
10 application layer protocol negotiation (ALPN)
11
13 #include <openssl/ssl.h>
14 int SSL_CTX_set_alpn_protos(SSL_CTX *ctx, const unsigned char *protos,
16 unsigned int protos_len);
17 int SSL_set_alpn_protos(SSL *ssl, const unsigned char *protos,
18 unsigned int protos_len);
19 void SSL_CTX_set_alpn_select_cb(SSL_CTX *ctx,
20 int (*cb) (SSL *ssl,
21 const unsigned char **out,
22 unsigned char *outlen,
23 const unsigned char *in,
24 unsigned int inlen,
25 void *arg), void *arg);
26 void SSL_get0_alpn_selected(const SSL *ssl, const unsigned char **data,
27 unsigned int *len);
28 void SSL_CTX_set_next_protos_advertised_cb(SSL_CTX *ctx,
30 int (*cb)(SSL *ssl,
31 const unsigned char **out,
32 unsigned int *outlen,
33 void *arg),
34 void *arg);
35 void SSL_CTX_set_next_proto_select_cb(SSL_CTX *ctx,
36 int (*cb)(SSL *s,
37 unsigned char **out,
38 unsigned char *outlen,
39 const unsigned char *in,
40 unsigned int inlen,
41 void *arg),
42 void *arg);
43 int SSL_select_next_proto(unsigned char **out, unsigned char *outlen,
44 const unsigned char *server,
45 unsigned int server_len,
46 const unsigned char *client,
47 unsigned int client_len)
48 void SSL_get0_next_proto_negotiated(const SSL *s, const unsigned char **data,
49 unsigned *len);
50
52 SSL_CTX_set_alpn_protos() and SSL_set_alpn_protos() are used by the
53 client to set the list of protocols available to be negotiated. The
54 protos must be in protocol-list format, described below. The length of
55 protos is specified in protos_len.
56 SSL_CTX_set_alpn_select_cb() sets the application callback cb used by a
58 server to select which protocol to use for the incoming connection.
59 When cb is NULL, ALPN is not used. The arg value is a pointer which is
60 passed to the application callback.
61 cb is the application defined callback. The in, inlen parameters are a
63 vector in protocol-list format. The value of the out, outlen vector
64 should be set to the value of a single protocol selected from the in,
65 inlen vector. The out buffer may point directly into in, or to a buffer
66 that outlives the handshake. The arg parameter is the pointer set via
67 SSL_CTX_set_alpn_select_cb().
68 SSL_select_next_proto() is a helper function used to select protocols.
70 It implements the standard protocol selection. It is expected that this
71 function is called from the application callback cb. The protocol data
72 in server, server_len and client, client_len must be in the protocol-
73 list format described below. The first item in the server, server_len
74 list that matches an item in the client, client_len list is selected,
75 and returned in out, outlen. The out value will point into either
76 server or client, so it should be copied immediately. If no match is
77 found, the first item in client, client_len is returned in out, outlen.
78 This function can also be used in the NPN callback.
79 SSL_CTX_set_next_proto_select_cb() sets a callback cb that is called
81 when a client needs to select a protocol from the server's provided
82 list, and a user-defined pointer argument arg which will be passed to
83 this callback. For the callback itself, out must be set to point to
84 the selected protocol (which may be within in). The length of the
85 protocol name must be written into outlen. The server's advertised
86 protocols are provided in in and inlen. The callback can assume that in
87 is syntactically valid. The client must select a protocol. It is fatal
88 to the connection if this callback returns a value other than
89 SSL_TLSEXT_ERR_OK. The arg parameter is the pointer set via
90 SSL_CTX_set_next_proto_select_cb().
91 SSL_CTX_set_next_protos_advertised_cb() sets a callback cb that is
93 called when a TLS server needs a list of supported protocols for Next
94 Protocol Negotiation. The returned list must be in protocol-list
95 format, described below. The list is returned by setting out to point
96 to it and outlen to its length. This memory will not be modified, but
97 the SSL does keep a reference to it. The callback should return
98 SSL_TLSEXT_ERR_OK if it wishes to advertise. Otherwise, no such
99 extension will be included in the ServerHello.
100 SSL_get0_alpn_selected() returns a pointer to the selected protocol in
102 data with length len. It is not NUL-terminated. data is set to NULL and
103 len is set to 0 if no protocol has been selected. data must not be
104 freed.
105 SSL_get0_next_proto_negotiated() sets data and len to point to the
107 client's requested protocol for this connection. If the client did not
108 request any protocol or NPN is not enabled, then data is set to NULL
109 and len to 0. Note that the client can request any protocol it chooses.
110 The value returned from this function need not be a member of the list
111 of supported protocols provided by the callback.
112
114 The protocol-lists must be in wire-format, which is defined as a vector
115 of nonempty, 8-bit length-prefixed, byte strings. The length-prefix
116 byte is not included in the length. Each string is limited to 255
117 bytes. A byte-string length of 0 is invalid. A truncated byte-string is
118 invalid. The length of the vector is not in the vector itself, but in a
119 separate variable.
120 Example:
122 unsigned char vector[] = {
124 6, 's', 'p', 'd', 'y', '/', '1',
125 8, 'h', 't', 't', 'p', '/', '1', '.', '1'
126 };
127 unsigned int length = sizeof(vector);
128 The ALPN callback is executed after the servername callback; as that
130 servername callback may update the SSL_CTX, and subsequently, the ALPN
131 callback.
132 If there is no ALPN proposed in the ClientHello, the ALPN callback is
134 not invoked.
135
137 SSL_CTX_set_alpn_protos() and SSL_set_alpn_protos() return 0 on
138 success, and non-0 on failure. WARNING: these functions reverse the
139 return value convention.
140 SSL_select_next_proto() returns one of the following:
142 OPENSSL_NPN_NEGOTIATED
144 A match was found and is returned in out, outlen.
145 OPENSSL_NPN_NO_OVERLAP
147 No match was found. The first item in client, client_len is
148 returned in out, outlen.
149 The ALPN select callback cb, must return one of the following:
151 SSL_TLSEXT_ERR_OK
153 ALPN protocol selected.
154 SSL_TLSEXT_ERR_ALERT_FATAL
156 There was no overlap between the client's supplied list and the
157 server configuration.
158 SSL_TLSEXT_ERR_NOACK
160 ALPN protocol not selected, e.g., because no ALPN protocols are
161 configured for this connection.
162 The callback set using SSL_CTX_set_next_proto_select_cb() should return
164 SSL_TLSEXT_ERR_OK if successful. Any other value is fatal to the
165 connection.
166 The callback set using SSL_CTX_set_next_protos_advertised_cb() should
168 return SSL_TLSEXT_ERR_OK if it wishes to advertise. Otherwise, no such
169 extension will be included in the ServerHello.
170
172 ssl(7), SSL_CTX_set_tlsext_servername_callback(3),
173 SSL_CTX_set_tlsext_servername_arg(3)
174
176 Copyright 2016-2020 The OpenSSL Project Authors. All Rights Reserved.
177 Licensed under the OpenSSL license (the "License"). You may not use
179 this file except in compliance with the License. You can obtain a copy
180 in the file LICENSE in the source distribution or at
181 <https://www.openssl.org/source/license.html>.
1821.1.1q 2022-07-07 SSL_CTX_SET_ALPN_SELECT_CB(3)