[Help-gnutls] gnutls_record_send() problem

[Laurent Birtz]

I've encountered a weird problem with the stock code examples (anonymous
server and client). By itself the example works fine, but if I replace
the string sent ("GET / HTTP/1.0\r\n\r\n") by a zeroed buffer, then the
client hangs. This does not occur if I send a buffer containing bytes
with the value 1.

The sample code is shown below. I am using GnuTLS 2.2.1-1, kernel
2.6.22-3-686, Debian unstable, Debian package libgnutls26. Can someone
tell me what is going on?


***** Makefile *****
all:
    gcc -Wall -g ex-serv-anon.c -o serv /usr/lib/libgnutls-openssl.so.26.1.2
    gcc -Wall -g ex-client1.c tcp.c -o client 
/usr/lib/libgnutls-openssl.so.26.1.2



***** tcp.c *****:

#if HAVE_CONFIG_H
# include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <unistd.h>

#define SA struct sockaddr

/* Connects to the peer and returns a socket
 * descriptor.
 */
extern int
tcp_connect (void)
{
  const char *PORT = "5556";
  const char *SERVER = "127.0.0.1";
  int err, sd;
  struct sockaddr_in sa;

  /* connects to server
   */
  sd = socket (AF_INET, SOCK_STREAM, 0);

  memset (&sa, '\0', sizeof (sa));
  sa.sin_family = AF_INET;
  sa.sin_port = htons (atoi (PORT));
  inet_pton (AF_INET, SERVER, &sa.sin_addr);

  err = connect (sd, (SA *) & sa, sizeof (sa));
  if (err < 0)
    {
      fprintf (stderr, "Connect error\n");
      exit (1);
    }

  return sd;
}

/* closes the given socket descriptor.
 */
extern void
tcp_close (int sd)
{
  shutdown (sd, SHUT_RDWR);    /* no more receptions */
  close (sd);
}

***** ex-client1.c *****:

#if HAVE_CONFIG_H
# include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <gnutls/gnutls.h>

/* A very basic TLS client, with anonymous authentication.
 */

#define MAX_BUF 1024
#define SA struct sockaddr
#define MSG "GET / HTTP/1.0\r\n\r\n"

extern int tcp_connect (void);
extern void tcp_close (int sd);

int
main (void)
{
  int ret, sd, ii;
  gnutls_session_t session;
  char buffer[MAX_BUF + 1];
  gnutls_anon_client_credentials_t anoncred;
  /* Need to enable anonymous KX specifically. */
  const int kx_prio[] = { GNUTLS_KX_ANON_DH, 0 };

  gnutls_global_init ();

  gnutls_anon_allocate_client_credentials (&anoncred);

  /* Initialize TLS session
   */
  gnutls_init (&session, GNUTLS_CLIENT);

  /* Use default priorities */
  gnutls_set_default_priority (session);
  gnutls_kx_set_priority (session, kx_prio);

  /* put the anonymous credentials to the current session
   */
  gnutls_credentials_set (session, GNUTLS_CRD_ANON, anoncred);

  /* connect to the peer
   */
  sd = tcp_connect ();

  gnutls_transport_set_ptr (session, (gnutls_transport_ptr_t) sd);

  /* Perform the TLS handshake
   */
  ret = gnutls_handshake (session);

  if (ret < 0)
    {
      fprintf (stderr, "*** Handshake failed\n");
      gnutls_perror (ret);
      goto end;
    }
  else
    {
      printf ("- Handshake was completed\n");
    }

/* CHANGE HERE */
  #if 1
  char s[100]; memset(s, 0, 100);
  gnutls_record_send (session, s, 100);
  #else
  gnutls_record_send (session, MSG, strlen (MSG));
  #endif
    
  ret = gnutls_record_recv (session, buffer, MAX_BUF);
  if (ret == 0)
    {
      printf ("- Peer has closed the TLS connection\n");
      goto end;
    }
  else if (ret < 0)
    {
      fprintf (stderr, "*** Error: %s\n", gnutls_strerror (ret));
      goto end;
    }

  printf ("- Received %d bytes: ", ret);
  for (ii = 0; ii < ret; ii++)
    {
      fputc (buffer[ii], stdout);
    }
  fputs ("\n", stdout);

  gnutls_bye (session, GNUTLS_SHUT_RDWR);

end:

  tcp_close (sd);

  gnutls_deinit (session);

  gnutls_anon_free_client_credentials (anoncred);

  gnutls_global_deinit ();

  return 0;
}


***** ex-serv-anon.c: *****

#if HAVE_CONFIG_H
# include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>

/* This is a sample TLS 1.0 echo server, for anonymous authentication only.
 */


#define SA struct sockaddr
#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556        /* listen to 5556 port */
#define DH_BITS 1024

/* These are global */
gnutls_anon_server_credentials_t anoncred;

gnutls_session_t
initialize_tls_session (void)
{
  gnutls_session_t session;
  const int kx_prio[] = { GNUTLS_KX_ANON_DH, 0 };

  gnutls_init (&session, GNUTLS_SERVER);

  /* avoid calling all the priority functions, since the defaults
   * are adequate.
   */
  gnutls_set_default_priority (session);
  gnutls_kx_set_priority (session, kx_prio);

  gnutls_credentials_set (session, GNUTLS_CRD_ANON, anoncred);

  gnutls_dh_set_prime_bits (session, DH_BITS);

  return session;
}

static gnutls_dh_params_t dh_params;

static int
generate_dh_params (void)
{

  /* Generate Diffie Hellman parameters - for use with DHE
   * kx algorithms. These should be discarded and regenerated
   * once a day, once a week or once a month. Depending on the
   * security requirements.
   */
  gnutls_dh_params_init (&dh_params);
  gnutls_dh_params_generate2 (dh_params, DH_BITS);

  return 0;
}

int
main (void)
{
  int err, listen_sd, i;
  int sd, ret;
  struct sockaddr_in sa_serv;
  struct sockaddr_in sa_cli;
  int client_len;
  char topbuf[512];
  gnutls_session_t session;
  char buffer[MAX_BUF + 1];
  int optval = 1;

  /* this must be called once in the program
   */
  gnutls_global_init ();

  gnutls_anon_allocate_server_credentials (&anoncred);

  generate_dh_params ();

  gnutls_anon_set_server_dh_params (anoncred, dh_params);

  /* Socket operations
   */
  listen_sd = socket (AF_INET, SOCK_STREAM, 0);
  SOCKET_ERR (listen_sd, "socket");

  memset (&sa_serv, '\0', sizeof (sa_serv));
  sa_serv.sin_family = AF_INET;
  sa_serv.sin_addr.s_addr = INADDR_ANY;
  sa_serv.sin_port = htons (PORT);    /* Server Port number */

  setsockopt (listen_sd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof (int));

  err = bind (listen_sd, (SA *) & sa_serv, sizeof (sa_serv));
  SOCKET_ERR (err, "bind");
  err = listen (listen_sd, 1024);
  SOCKET_ERR (err, "listen");

  printf ("Server ready. Listening to port '%d'.\n\n", PORT);

  client_len = sizeof (sa_cli);
  for (;;)
    {
      session = initialize_tls_session ();

      sd = accept (listen_sd, (SA *) & sa_cli, &client_len);

      printf ("- connection from %s, port %d\n",
          inet_ntop (AF_INET, &sa_cli.sin_addr, topbuf,
             sizeof (topbuf)), ntohs (sa_cli.sin_port));

      gnutls_transport_set_ptr (session, (gnutls_transport_ptr_t) sd);
      ret = gnutls_handshake (session);
      if (ret < 0)
    {
      close (sd);
      gnutls_deinit (session);
      fprintf (stderr, "*** Handshake has failed (%s)\n\n",
           gnutls_strerror (ret));
      continue;
    }
      printf ("- Handshake was completed\n");

      /* see the Getting peer's information example */
      /* print_info(session); */

      i = 0;
      for (;;)
    {
      memset (buffer, 0, MAX_BUF + 1);
      ret = gnutls_record_recv (session, buffer, MAX_BUF);

      if (ret == 0)
        {
          printf ("\n- Peer has closed the GNUTLS connection\n");
          break;
        }
      else if (ret < 0)
        {
          fprintf (stderr, "\n*** Received corrupted "
               "data(%d). Closing the connection.\n\n", ret);
          break;
        }
      else if (ret > 0)
        {
          /* echo data back to the client
           */
          gnutls_record_send (session, buffer, strlen (buffer));
        }
    }
      printf ("\n");
      /* do not wait for the peer to close the connection.
       */
      gnutls_bye (session, GNUTLS_SHUT_WR);

      close (sd);
      gnutls_deinit (session);

    }
  close (listen_sd);

  gnutls_anon_free_server_credentials (anoncred);

  gnutls_global_deinit ();

  return 0;

}


The 'ldd' dump of the server binary is:
 > ldd serv
linux-gate.so.1 =>  (0xffffe000)
libgnutls-openssl.so.26 => /usr/lib/libgnutls-openssl.so.26 (0xb7f22000)
libc.so.6 => /lib/i686/cmov/libc.so.6 (0xb7dd5000)
libgnutls.so.26 => /usr/lib/libgnutls.so.26 (0xb7d5b000)
libtasn1.so.3 => /usr/lib/libtasn1.so.3 (0xb7d4b000)
libgcrypt.so.11 => /usr/lib/libgcrypt.so.11 (0xb7ce3000)
libgpg-error.so.0 => /usr/lib/libgpg-error.so.0 (0xb7cdf000)
/lib/ld-linux.so.2 (0xb7f45000)


Thanks,
Laurent Birtz