NAME

       SSL_CTX_set_tlsext_servername_callback, SSL_CTX_set_tlsext_servername_arg,
       SSL_get_servername_type, SSL_get_servername, SSL_set_tlsext_host_name - handle server name
       indication (SNI)

SYNOPSIS

        #include <openssl/ssl.h>

        long SSL_CTX_set_tlsext_servername_callback(SSL_CTX *ctx,
                                          int (*cb)(SSL *s, int *al, void *arg));
        long SSL_CTX_set_tlsext_servername_arg(SSL_CTX *ctx, void *arg);

        const char *SSL_get_servername(const SSL *s, const int type);
        int SSL_get_servername_type(const SSL *s);

        int SSL_set_tlsext_host_name(const SSL *s, const char *name);

DESCRIPTION

       The functionality provided by the servername callback is mostly superseded by the
       ClientHello callback, which can be set using SSL_CTX_set_client_hello_cb().  However, even
       where the ClientHello callback is used, the servername callback is still necessary in
       order to acknowledge the servername requested by the client.

       SSL_CTX_set_tlsext_servername_callback() sets the application callback cb used by a server
       to perform any actions or configuration required based on the servername extension
       received in the incoming connection. When cb is NULL, SNI is not used.

       The servername callback should return one of the following values:

       SSL_TLSEXT_ERR_OK
           This is used to indicate that the servername requested by the client has been
           accepted. Typically a server will call SSL_set_SSL_CTX() in the callback to set up a
           different configuration for the selected servername in this case.

       SSL_TLSEXT_ERR_ALERT_FATAL
           In this case the servername requested by the client is not accepted and the handshake
           will be aborted. The value of the alert to be used should be stored in the location
           pointed to by the al parameter to the callback. By default this value is initialised
           to SSL_AD_UNRECOGNIZED_NAME.

       SSL_TLSEXT_ERR_ALERT_WARNING
           If this value is returned then the servername is not accepted by the server.  However
           the handshake will continue and send a warning alert instead. The value of the alert
           should be stored in the location pointed to by the al parameter as for
           SSL_TLSEXT_ERR_ALERT_FATAL above. Note that TLSv1.3 does not support warning alerts,
           so if TLSv1.3 has been negotiated then this return value is treated the same way as
           SSL_TLSEXT_ERR_NOACK.

       SSL_TLSEXT_ERR_NOACK
           This return value indicates that the servername is not accepted by the server.  No
           alerts are sent and the server will not acknowledge the requested servername.

       SSL_CTX_set_tlsext_servername_arg() sets a context-specific argument to be passed into the
       callback (via the arg parameter) for this SSL_CTX.

       The behaviour of SSL_get_servername() depends on a number of different factors.  In
       particular note that in TLSv1.3 the servername is negotiated in every handshake. In
       TLSv1.2 the servername is only negotiated on initial handshakes and not on resumption
       handshakes.

       On the client, before the handshake
           If a servername has been set via a call to SSL_set_tlsext_host_name() then it will
           return that servername.

           If one has not been set, but a TLSv1.2 resumption is being attempted and the session
           from the original handshake had a servername accepted by the server then it will
           return that servername.

           Otherwise it returns NULL.

       On the client, during or after the handshake and a TLSv1.2 (or below) resumption occurred
           If the session from the orignal handshake had a servername accepted by the server then
           it will return that servername.

           Otherwise it returns the servername set via SSL_set_tlsext_host_name() or NULL if it
           was not called.

       On the client, during or after the handshake and a TLSv1.2 (or below) resumption did not
       occur
           It will return the servername set via SSL_set_tlsext_host_name() or NULL if it was not
           called.

       On the server, before the handshake
           The function will always return NULL before the handshake

       On the server, after the servername extension has been processed and a TLSv1.2 (or below)
       resumption occurred
           If a servername was accepted by the server in the original handshake then it will
           return that servername, or NULL otherwise.

       On the server, after the servername extension has been processed and a TLSv1.2 (or below)
       resumption did not occur
           The function will return the servername requested by the client in this handshake or
           NULL if none was requested.

       Note that the ClientHello callback occurs before a servername extension from the client is
       processed. The servername, certificate and ALPN callbacks occur after a servername
       extension from the client is processed.

       SSL_get_servername_type() returns the servername type or -1 if no servername is present.
       Currently the only supported type (defined in RFC3546) is TLSEXT_NAMETYPE_host_name.

       SSL_set_tlsext_host_name() sets the server name indication ClientHello extension to
       contain the value name. The type of server name indication extension is set to
       TLSEXT_NAMETYPE_host_name (defined in RFC3546).

NOTES

       Several callbacks are executed during ClientHello processing, including the ClientHello,
       ALPN, and servername callbacks.  The ClientHello callback is executed first, then the
       servername callback, followed by the ALPN callback.

       The SSL_set_tlsext_host_name() function should only be called on SSL objects that will act
       as clients; otherwise the configured name will be ignored.

RETURN VALUES

       SSL_CTX_set_tlsext_servername_callback() and SSL_CTX_set_tlsext_servername_arg() both
       always return 1 indicating success.  SSL_set_tlsext_host_name() returns 1 on success, 0 in
       case of error.

SEE ALSO

       ssl(7), SSL_CTX_set_alpn_select_cb(3), SSL_get0_alpn_selected(3),
       SSL_CTX_set_client_hello_cb(3)

HISTORY

       SSL_get_servername() historically provided some unexpected results in certain corner
       cases. This has been fixed from OpenSSL 1.1.1e.

       Prior to 1.1.1e, when the client requested a servername in an initial TLSv1.2 handshake,
       the server accepted it, and then the client successfully resumed but set a different
       explict servername in the second handshake then when called by the client it returned the
       servername from the second handshake. This has now been changed to return the servername
       requested in the original handshake.

       Also prior to 1.1.1e, if the client sent a servername in the first handshake but the
       server did not accept it, and then a second handshake occured where TLSv1.2 resumption was
       successful then when called by the server it returned the servername requested in the
       original handshake. This has now been changed to NULL.

COPYRIGHT

       Copyright 2017-2020 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>.

1.1.1f                                      2024-02-1SSL_CTX_SET_TLSEXT_SERVERNAME_CALLBACK(3SSL)