SSL_CTX_SET_T...NAME_CALLBACK(3SSL) OpenSSL SSL_CTX_SET_T...NAME_CALLBACK(3SSL)
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superseded by the ClientHello callback, which can be set using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. Note that this callback occurs late in
the processing of the ClientHello message. In particular it happens
after session resumption has occurred, and so typically this callback
should not call functions such as SSL_set_session_id_context(3) since it
is too late to affect the session resumption for the current handshake.
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 original 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 explicit 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
occurred 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 2017-2026 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (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>.
3.5.7 2026-06-0SSL_CTX_SET_T...NAME_CALLBACK(3SSL)
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