.\" $OpenBSD: SSL_CTX_set_alpn_select_cb.3,v 1.10 2024/07/11 13:50:44 tb Exp $ .\" OpenSSL 87b81496 Apr 19 12:38:27 2017 -0400 .\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 .\" .\" This file was written by Todd Short . .\" Copyright (c) 2016 The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. All advertising materials mentioning features or use of this .\" software must display the following acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" .\" .\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to .\" endorse or promote products derived from this software without .\" prior written permission. For written permission, please contact .\" openssl-core@openssl.org. .\" .\" 5. Products derived from this software may not be called "OpenSSL" .\" nor may "OpenSSL" appear in their names without prior written .\" permission of the OpenSSL Project. .\" .\" 6. Redistributions of any form whatsoever must retain the following .\" acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" .\" .\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY .\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: July 11 2024 $ .Dt SSL_CTX_SET_ALPN_SELECT_CB 3 .Os .Sh NAME .Nm SSL_CTX_set_alpn_protos , .Nm SSL_set_alpn_protos , .Nm SSL_CTX_set_alpn_select_cb , .Nm SSL_select_next_proto , .Nm SSL_get0_alpn_selected .Nd handle application layer protocol negotiation (ALPN) .Sh SYNOPSIS .In openssl/ssl.h .Ft int .Fo SSL_CTX_set_alpn_protos .Fa "SSL_CTX *ctx" .Fa "const unsigned char *protos" .Fa "unsigned int protos_len" .Fc .Ft int .Fo SSL_set_alpn_protos .Fa "SSL *ssl" .Fa "const unsigned char *protos" .Fa "unsigned int protos_len" .Fc .Ft void .Fo SSL_CTX_set_alpn_select_cb .Fa "SSL_CTX *ctx" .Fa "int (*cb)(SSL *ssl, const unsigned char **out,\ unsigned char *outlen, const unsigned char *in,\ unsigned int inlen, void *arg)" .Fa "void *arg" .Fc .Ft int .Fo SSL_select_next_proto .Fa "unsigned char **out" .Fa "unsigned char *outlen" .Fa "const unsigned char *peer_list" .Fa "unsigned int peer_list_len" .Fa "const unsigned char *supported_list" .Fa "unsigned int supported_list_len" .Fc .Ft void .Fo SSL_get0_alpn_selected .Fa "const SSL *ssl" .Fa "const unsigned char **data" .Fa "unsigned int *len" .Fc .Sh DESCRIPTION .Fn SSL_CTX_set_alpn_protos and .Fn SSL_set_alpn_protos are used by the client to set the list of protocols available to be negotiated. The .Fa protos must be in protocol-list format, described below. The length of .Fa protos is specified in .Fa protos_len . .Pp .Fn SSL_CTX_set_alpn_select_cb sets the application callback .Fa cb used by a server to select which protocol to use for the incoming connection. When .Fa cb is .Dv NULL , ALPN is not used. The .Fa arg value is a pointer which is passed to the application callback. .Pp .Fa cb is the application defined callback. The .Fa in , .Fa inlen parameters are a vector in protocol-list format. The value of the .Fa out , .Fa outlen vector should be set to the value of a single protocol selected from the .Fa in , .Fa inlen vector. The .Fa out buffer may point directly into .Fa in , or to a buffer that outlives the handshake. The .Fa arg parameter is the pointer set via .Fn SSL_CTX_set_alpn_select_cb . .Pp .Fn SSL_select_next_proto is a helper function used to select protocols. It is expected that this function is called from the application callback .Fa cb . If .Fn SSL_select_next_proto returns .Dv OPENSSL_NPN_NO_OVERLAP , .Fa cb should ignore .Fa out and fail by returning .Dv SSL_TLSEXT_ERR_ALERT_FATAL . The protocol data in .Fa peer_list , .Fa peer_list_len and .Fa supported_list , .Fa supported_list_len must be two non-empty lists, validly encoded in the protocol-list format described below. The first item in the .Fa peer_list that matches an item in the .Fa supported_list is selected, and returned in .Fa out , .Fa outlen . The .Fa out value will point into either .Fa peer_list or .Fa supported_list , so it must not be modified and should be copied immediately. If no match is found, the first item in .Fa supported_list is returned in .Fa out , .Fa outlen . .Pp .Fn SSL_get0_alpn_selected returns a pointer to the selected protocol in .Fa data with length .Fa len . It is not NUL-terminated. .Fa data is set to .Dv NULL and .Fa len is set to 0 if no protocol has been selected. .Fa data must not be freed. .Pp 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. .Pp For example: .Bd -literal const unsigned char *vector = "\e6" "spdy/1" "\e8" "http/1.1"; unsigned int length = strlen(vector); .Ed .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 .Fn SSL_CTX_set_alpn_protos and .Fn SSL_set_alpn_protos return 0 on success or non-zero on failure. WARNING: these functions reverse the return value convention. .Pp .Fn SSL_select_next_proto returns one of the following: .Bl -tag -width Ds .It OPENSSL_NPN_NEGOTIATED A match was found and is returned in .Fa out , .Fa outlen . .It OPENSSL_NPN_NO_OVERLAP No match was found. The first item in .Fa supported_list , .Fa supported_list_len is returned in .Fa out , .Fa outlen . .El .Pp The ALPN select callback .Fa cb must return one of the following: .Bl -tag -width Ds .It SSL_TLSEXT_ERR_OK ALPN protocol selected. .It SSL_TLSEXT_ERR_ALERT_FATAL There was no overlap between the client's supplied list and the server configuration. .It SSL_TLSEXT_ERR_NOACK ALPN protocol not selected, e.g., because no ALPN protocols are configured for this connection. .El .Sh SEE ALSO .Xr ssl 3 , .Xr SSL_CTX_set_tlsext_servername_arg 3 , .Xr SSL_CTX_set_tlsext_servername_callback 3 .Sh STANDARDS .Rs .%T TLS Application-Layer Protocol Negotiation Extension .%R RFC 7301 .Re .Pp .Rs .%T TLS Next Protocol Negotiation Extension .%U https://datatracker.ietf.org/doc/html/draft-agl-tls-nextprotoneg .Re .Sh HISTORY .Fn SSL_select_next_proto first appeared in OpenSSL 1.0.1 and has been available since .Ox 5.3 . .Pp .Fn SSL_CTX_set_alpn_protos , .Fn SSL_set_alpn_protos , .Fn SSL_CTX_set_alpn_select_cb , and .Fn SSL_get0_alpn_selected first appeared in OpenSSL 1.0.2 and have been available since .Ox 5.7 . .Sh CAVEATS The fallback to the first supported protocol in .Fn 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. .Sh BUGS The .Fa out argument of .Fn SSL_select_next_proto should have been const.