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       The functionality provided by the servername callback is mostly superseded by the ClientHello callback,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                                             2025-02-05       SSL_CTX_SET_TLSEXT_SERVERNAME_CALLBACK(3SSL)