SSL_CTX_use_serverinfo.3ssl man page
SSL_CTX_use_serverinfo, SSL_CTX_use_serverinfo_file — use serverinfo extension
#include <openssl/ssl.h> int SSL_CTX_use_serverinfo(SSL_CTX *ctx, const unsigned char *serverinfo, size_t serverinfo_length); int SSL_CTX_use_serverinfo_file(SSL_CTX *ctx, const char *file);
These functions load "serverinfo" TLS ServerHello Extensions into the SSL_CTX. A "serverinfo" extension is returned in response to an empty ClientHello Extension.
SSL_CTX_use_serverinfo() loads one or more serverinfo extensions from a byte array into ctx. The extensions must be concatenated into a sequence of bytes. Each extension must consist of a 2-byte Extension Type, a 2-byte length, and then length bytes of extension_data.
SSL_CTX_use_serverinfo_file() loads one or more serverinfo extensions from file into ctx. The extensions must be in PEM format. Each extension must consist of a 2-byte Extension Type, a 2-byte length, and then length bytes of extension_data. Each PEM extension name must begin with the phrase "BEGIN SERVERINFO FOR ".
If more than one certificate (RSA/DSA) is installed using SSL_CTX_use_certificate(), the serverinfo extension will be loaded into the last certificate installed. If e.g. the last item was a RSA certificate, the loaded serverinfo extension data will be loaded for that certificate. To use the serverinfo extension for multiple certificates, SSL_CTX_use_serverinfo() needs to be called multiple times, once after each time a certificate is loaded.
On success, the functions return 1. On failure, the functions return 0. Check out the error stack to find out the reason.