client.c

/* compilation: gcc -o client client.c -lssl -lcrypto */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <openssl/bio.h>   /* BasicInput/Output streams */
#include <openssl/err.h>   /* errors */
#include <openssl/ssl.h>   /* core library */

#define BuffSize 1024

void report_and_exit(const char* msg) {
  perror(msg);
  ERR_print_errors_fp(stderr);
  exit(-1);
}

void init_ssl() {
  SSL_load_error_strings();
  SSL_library_init();  
}

void cleanup(SSL_CTX* ctx, BIO* bio) {
  SSL_CTX_free(ctx);
  BIO_free_all(bio);
}

void secure_connect(const char* hostname) {
  char request[BuffSize];
  char response[BuffSize];

  const SSL_METHOD* method = TLS_client_method();
  if (NULL == method) report_and_exit("TLS_client_method...");
  
  SSL_CTX* ctx = SSL_CTX_new(method);
  if (NULL == ctx) report_and_exit("SSL_CTX_new...");

  BIO* bio = BIO_new_ssl_connect(ctx);
  if (NULL == bio) report_and_exit("BIO_new_ssl_connect...");

  SSL* ssl = NULL;

  /* link bio channel, SSL session, and server endpoint */
  BIO_get_ssl(bio, &ssl);                   /* session */
  SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY);   /* robustness */
  BIO_set_conn_hostname(bio, hostname);         /* prepare to connect */

  /* try to connect */
  if (BIO_do_connect(bio) <= 0) {
    cleanup(ctx, bio);
    report_and_exit("BIO_do_connect...");
  }

  /* verify truststore, check cert */
  if (!SSL_CTX_load_verify_locations(ctx,
				     "/etc/ssl/certs/ca-certificates.crt", /* truststore */
				     "/etc/ssl/certs/")) /* more truststore */
    report_and_exit("SSL_CTX_load_verify_locations...");

  long verify_flag = SSL_get_verify_result(ssl);
  if (verify_flag != X509_V_OK) 
    fprintf(stderr,
	    "##### Certificate verification error (%i) but continuing...\n",
	    (int) verify_flag);

  /* now fetch the homepage as sample data */
  sprintf(request,
	  "GET / HTTP/1.1\x0D\x0AHost: %s\x0D\x0A\x43onnection: Close\x0D\x0A\x0D\x0A",
	  hostname);
  BIO_puts(bio, request);

  /* read HTTP response from server and print to stdout */
  while (1) {
    memset(response, '\0', sizeof(response));
    int n = BIO_read(bio, response, BuffSize);
    if (n <= 0) break; /* 0 is end-of-stream, < 0 is an error */
    puts(response);
  }
  
  cleanup(ctx, bio);
}

int main() {
  init_ssl();
  
  const char* hostname = "www.google.com:443";
  fprintf(stderr, "Trying an HTTPS connection to %s...\n", hostname);
  secure_connect(hostname);
  
  return 0;
}