Cryptography and SSL/TLS Toolkit



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)


 #include <openssl/ssl.h>

 long SSL_CTX_set_tlsext_servername_callback(SSL_CTX *ctx,
                                   int (*cb)(SSL *, int *, void *));
 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);


The functionality provided by the servername callback is superseded by the ClientHello callback, which can be set using SSL_CTX_set_client_hello_cb(). The servername callback is retained for historical compatibility.

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 arg value is a pointer which is passed to the application callback.

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

SSL_get_servername() returns a servername extension value of the specified type if provided in the Client Hello or NULL.

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).


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.


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.


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

Copyright 2017 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