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8313a787d7
RFC 7301 mandates that the server SHALL respond with a fatal
"no_application_protocol" alert when there is no overlap between
the client's supplied list and the server's list of supported protocols.
In commit 062178678f we changed from
ignoring non-success returns from the supplied alpn_select_cb() to
treating such non-success returns as indicative of non-overlap and
sending the fatal alert.
In effect, this is using the presence of an alpn_select_cb() as a proxy
to attempt to determine whether the application has configured a list
of supported protocols. However, there may be cases in which an
application's architecture leads it to supply an alpn_select_cb() but
have that callback be configured to take no action on connections that
do not have ALPN configured; returning SSL_TLSEXT_ERR_NOACK from
the callback would be the natural way to do so. Unfortunately, the
aforementioned behavior change also treated SSL_TLSEXT_ERR_NOACK as
indicative of no overlap and terminated the connection; this change
supplies special handling for SSL_TLSEXT_ERR_NOACK returns from the
callback. In effect, it provides a way for a callback to obtain the
behavior that would have occurred if no callback was registered at
all, which was not possible prior to this change.
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2570)
143 lines
5.2 KiB
Plaintext
143 lines
5.2 KiB
Plaintext
=pod
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=head1 NAME
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SSL_CTX_set_alpn_protos, SSL_set_alpn_protos, SSL_CTX_set_alpn_select_cb,
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SSL_select_next_proto, SSL_get0_alpn_selected - handle application layer
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protocol negotiation (ALPN)
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_CTX_set_alpn_protos(SSL_CTX *ctx, const unsigned char *protos,
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unsigned int protos_len);
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int SSL_set_alpn_protos(SSL *ssl, const unsigned char *protos,
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unsigned int protos_len);
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void SSL_CTX_set_alpn_select_cb(SSL_CTX *ctx,
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int (*cb) (SSL *ssl,
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const unsigned char **out,
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unsigned char *outlen,
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const unsigned char *in,
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unsigned int inlen,
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void *arg), void *arg);
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int SSL_select_next_proto(unsigned char **out, unsigned char *outlen,
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const unsigned char *server,
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unsigned int server_len,
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const unsigned char *client,
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unsigned int client_len)
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void SSL_get0_alpn_selected(const SSL *ssl, const unsigned char **data,
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unsigned int *len);
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=head1 DESCRIPTION
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SSL_CTX_set_alpn_protos() and SSL_set_alpn_protos() are used by the client to
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set the list of protocols available to be negotiated. The B<protos> must be in
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protocol-list format, described below. The length of B<protos> is specified in
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B<protos_len>.
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SSL_CTX_set_alpn_select_cb() sets the application callback B<cb> used by a
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server to select which protocol to use for the incoming connection. When B<cb>
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is NULL, ALPN is not used. The B<arg> value is a pointer which is passed to
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the application callback.
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B<cb> is the application defined callback. The B<in>, B<inlen> parameters are a
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vector in protocol-list format. The value of the B<out>, B<outlen> vector
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should be set to the value of a single protocol selected from the B<in>,
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B<inlen> vector. The B<out> buffer may point directly into B<in>, or to a
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buffer that outlives the handshake. The B<arg> parameter is the pointer set via
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SSL_CTX_set_alpn_select_cb().
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SSL_select_next_proto() is a helper function used to select protocols. It
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implements the standard protocol selection. It is expected that this function
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is called from the application callback B<cb>. The protocol data in B<server>,
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B<server_len> and B<client>, B<client_len> must be in the protocol-list format
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described below. The first item in the B<server>, B<server_len> list that
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matches an item in the B<client>, B<client_len> list is selected, and returned
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in B<out>, B<outlen>. The B<out> value will point into either B<server> or
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B<client>, so it should be copied immediately. If no match is found, the first
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item in B<client>, B<client_len> is returned in B<out>, B<outlen>. This
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function can also be used in the NPN callback.
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SSL_get0_alpn_selected() returns a pointer to the selected protocol in B<data>
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with length B<len>. It is not NUL-terminated. B<data> is set to NULL and B<len>
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is set to 0 if no protocol has been selected. B<data> must not be freed.
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=head1 NOTES
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The protocol-lists must be in wire-format, which is defined as a vector of
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non-empty, 8-bit length-prefixed, byte strings. The length-prefix byte is not
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included in the length. Each string is limited to 255 bytes. A byte-string
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length of 0 is invalid. A truncated byte-string is invalid. The length of the
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vector is not in the vector itself, but in a separate variable.
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Example:
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unsigned char vector[] = {
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6, 's', 'p', 'd', 'y', '/', '1',
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8, 'h', 't', 't', 'p', '/', '1', '.', '1'
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};
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unsigned int length = sizeof(vector);
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The ALPN callback is executed after the servername callback; as that servername
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callback may update the SSL_CTX, and subsequently, the ALPN callback.
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If there is no ALPN proposed in the ClientHello, the ALPN callback is not
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invoked.
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=head1 RETURN VALUES
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SSL_CTX_set_alpn_protos() and SSL_set_alpn_protos() return 0 on success, and
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non-0 on failure. WARNING: these functions reverse the return value convention.
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SSL_select_next_proto() returns one of the following:
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=over 4
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=item OPENSSL_NPN_NEGOTIATED
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A match was found and is returned in B<out>, B<outlen>.
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=item OPENSSL_NPN_NO_OVERLAP
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No match was found. The first item in B<client>, B<client_len> is returned in
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B<out>, B<outlen>.
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=back
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The ALPN select callback B<cb>, must return one of the following:
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=over 4
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=item SSL_TLSEXT_ERR_OK
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ALPN protocol selected.
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=item SSL_TLSEXT_ERR_ALERT_FATAL
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There was no overlap between the client's supplied list and the server
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configuration.
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=item SSL_TLSEXT_ERR_NOACK
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ALPN protocol not selected, e.g., because no ALPN protocols are configured for
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this connection.
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=back
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=head1 SEE ALSO
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L<ssl(7)>, L<SSL_CTX_set_tlsext_servername_callback(3)>,
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L<SSL_CTX_set_tlsext_servername_arg(3)>
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=head1 COPYRIGHT
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Copyright 2016 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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