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.IX Title "SSL_CTX_set_alpn_select_cb 3"
.TH SSL_CTX_set_alpn_select_cb 3 2019-12-20 1.0.2u OpenSSL
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH 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)
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_CTX_set_alpn_protos(SSL_CTX *ctx, const unsigned char *protos,
\&                             unsigned protos_len);
\& int SSL_set_alpn_protos(SSL *ssl, const unsigned char *protos,
\&                         unsigned 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);
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\fBSSL_CTX_set_alpn_protos()\fR and \fBSSL_set_alpn_protos()\fR are used by the client to
set the list of protocols available to be negotiated. The \fBprotos\fR must be in
protocol-list format, described below. The length of \fBprotos\fR is specified in
\&\fBprotos_len\fR.
.PP
\&\fBSSL_CTX_set_alpn_select_cb()\fR sets the application callback \fBcb\fR used by a
server to select which protocol to use for the incoming connection. When \fBcb\fR
is NULL, ALPN is not used. The \fBarg\fR value is a pointer which is passed to
the application callback.
.PP
\&\fBcb\fR is the application defined callback. The \fBin\fR, \fBinlen\fR parameters are a
vector in protocol-list format. The value of the \fBout\fR, \fBoutlen\fR vector
should be set to the value of a single protocol selected from the \fBin\fR,
\&\fBinlen\fR vector. The \fBarg\fR parameter is the pointer set via
\&\fBSSL_CTX_set_alpn_select_cb()\fR.
.PP
\&\fBSSL_select_next_proto()\fR 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 \fBcb\fR. The protocol data in \fBserver\fR,
\&\fBserver_len\fR and \fBclient\fR, \fBclient_len\fR must be in the protocol-list format
described below. The first item in the \fBserver\fR, \fBserver_len\fR list that
matches an item in the \fBclient\fR, \fBclient_len\fR list is selected, and returned
in \fBout\fR, \fBoutlen\fR. The \fBout\fR value will point into either \fBserver\fR or
\&\fBclient\fR, so it should be copied immediately. If no match is found, the first
item in \fBclient\fR, \fBclient_len\fR is returned in \fBout\fR, \fBoutlen\fR. This
function can also be used in the NPN callback.
.PP
\&\fBSSL_get0_alpn_selected()\fR returns a pointer to the selected protocol in \fBdata\fR
with length \fBlen\fR. It is not NUL-terminated. \fBdata\fR is set to NULL and \fBlen\fR
is set to 0 if no protocol has been selected. \fBdata\fR must not be freed.
.SH NOTES
.IX Header "NOTES"
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.
.PP
Example:
.PP
.Vb 5
\& unsigned char vector[] = {
\&     6, \*(Aqs\*(Aq, \*(Aqp\*(Aq, \*(Aqd\*(Aq, \*(Aqy\*(Aq, \*(Aq/\*(Aq, \*(Aq1\*(Aq,
\&     8, \*(Aqh\*(Aq, \*(Aqt\*(Aq, \*(Aqt\*(Aq, \*(Aqp\*(Aq, \*(Aq/\*(Aq, \*(Aq1\*(Aq, \*(Aq.\*(Aq, \*(Aq1\*(Aq
\& };
\& unsigned int length = sizeof(vector);
.Ve
.PP
The ALPN callback is executed after the servername callback; as that servername
callback may update the SSL_CTX, and subsequently, the ALPN callback.
.PP
If there is no ALPN proposed in the ClientHello, the ALPN callback is not
invoked.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBSSL_CTX_set_alpn_protos()\fR and \fBSSL_set_alpn_protos()\fR return 0 on success, and
non\-0 on failure. WARNING: these functions reverse the return value convention.
.PP
\&\fBSSL_select_next_proto()\fR returns one of the following:
.IP OPENSSL_NPN_NEGOTIATED 4
.IX Item "OPENSSL_NPN_NEGOTIATED"
A match was found and is returned in \fBout\fR, \fBoutlen\fR.
.IP OPENSSL_NPN_NO_OVERLAP 4
.IX Item "OPENSSL_NPN_NO_OVERLAP"
No match was found. The first item in \fBclient\fR, \fBclient_len\fR is returned in
\&\fBout\fR, \fBoutlen\fR.
.PP
The ALPN select callback \fBcb\fR, must return one of the following:
.IP SSL_TLSEXT_ERR_OK 4
.IX Item "SSL_TLSEXT_ERR_OK"
ALPN protocol selected.
.IP SSL_TLSEXT_ERR_NOACK 4
.IX Item "SSL_TLSEXT_ERR_NOACK"
ALPN protocol not selected.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBssl\fR\|(3), \fBSSL_CTX_set_tlsext_servername_callback\fR\|(3),
\&\fBSSL_CTX_set_tlsext_servername_arg\fR\|(3)