Pfthb

I'm using Ubuntu 18.04/gcc 7.3/OpenSSL 1.1.0g to make C++ app performing TLS/SSL connection with non-blocking BIO API.

When BIO_do_connect() fails connecting, e.g. if using wrong host name or port, there is no errors reported by OpenSSL. ERR_get_error() returns zero and ERR_print_errors_xx() doesn't print anything.

So the question is - how to get actual connection failure reason, e.g. 'Connection refused' or 'Host not resolved' etc?

Used code snippet below:

#include <cstdio>
#include <cstring>
#include <iostream>

#include "openssl/bio.h"
#include "openssl/err.h"
#include "openssl/ssl.h"

int main(int argc, char *argv)

 OPENSSL_init_ssl(OPENSSL_INIT_SSL_DEFAULT, nullptr);

 std::cout << OpenSSL_version(OPENSSL_VERSION) << std::endl;

 SSL_CTX* ctx = SSL_CTX_new(TLS_client_method());
 if (!ctx)
 
 std::cerr << "Error creating SSL context:" << std::endl;
 ERR_print_errors_fp(stderr);
 return 1;
 

 SSL_CTX_set_options(ctx, SSL_OP_NO_SSLv2);
 SSL_CTX_set_options(ctx, SSL_OP_NO_SSLv3);

 if(!SSL_CTX_load_verify_locations(ctx,
 "/etc/ssl/certs/ca-certificates.crt",
 nullptr))
 
 std::cerr << "Error loading trust store into SSL context" << std::endl;
 ERR_print_errors_fp(stderr);
 return 1;
 

 BIO* cbio = BIO_new_ssl_connect(ctx);

 SSL* ssl = nullptr;
 BIO_get_ssl(cbio, &ssl);
 SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);

 BIO_set_conn_hostname(cbio, "not_actually_existing_host.com:https");
 BIO_set_nbio(cbio, 1);

 std::cout << "Start connecting" << std::endl;

 next:
 if (BIO_do_connect(cbio) <= 0)
 
 if (!BIO_should_retry(cbio))
 
 std::cerr << "Error attempting to connect:" << std::endl;
 ERR_print_errors_fp(stderr); // <---- PRINTS NOTHING!!!
 BIO_free_all(cbio);
 SSL_CTX_free(ctx);
 return 1;
 
 else goto next;
 

 std::cout << "Connected OK" << std::endl;

 BIO_free_all(cbio);
 SSL_CTX_free(ctx);
 return 0;

This approach finally works for me:

 const auto sysErrorCode = errno;
 const auto sslErrorCode = ERR_get_error();

 std::string errorDescription;
 if (sslErrorCode != 0) errorDescription = ERR_error_string(sslErrorCode, nullptr);
 if (sysErrorCode != 0)
 
 if (!errorDescription.empty()) errorDescription += 'n';
 errorDescription += "System error, code=" + std::to_string(sysErrorCode);
 errorDescription += ", ";
 errorDescription += strerror(sysErrorCode);