Cryptography and SSL/TLS Toolkit



SSL_CTX_set_tlsext_servername_callback, SSL_CTX_set_tlsext_servername_arg, SSL_get_servername_type, SSL_get_servername - 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);


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.


The ALPN and SNI callbacks are both executed during Client Hello processing. The servername callback is executed first, followed by the ALPN callback.


SSL_CTX_set_tlsext_servername_callback() and SSL_CTX_set_tlsext_servername_arg() both always return 1 indicating success.


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