nbd_set_tls_psk_file - set the TLS Pre-Shared Keys (PSK) filename


 #include <libnbd.h>

 int nbd_set_tls_psk_file (
       struct nbd_handle *h, const char *filename


Set the TLS Pre-Shared Keys (PSK) filename. This is used if trying to authenticate to the server using with a pre-shared key. There is no default so if this is not set then PSK authentication cannot be used to connect to the server.

This function may be called regardless of whether TLS is supported, but will have no effect unless nbd_set_tls(3) is also used to request or require TLS.


If the call is successful the function returns 0.


On error -1 is returned.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

The following parameters must not be NULL: h, filename. For more information see "Non-NULL parameters" in libnbd(3).


nbd_set_tls_psk_file can be called when the handle is in the following state:

 │ Handle created, before connecting   │ ✅ allowed              │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ❌ error                │
 │ Connected to the server             │ ❌ error                │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │


This function first appeared in libnbd 1.0.

If you need to test if this function is available at compile time check if the following macro is defined:



This example is also available as examples/encryption.c in the libnbd source code.

 /* An example showing how to connect to a server which is
  * using TLS encryption.
  * This requires nbdkit, and psktool from gnutls.
  * Both libnbd and nbdkit support TLS-PSK which is a
  * simpler-to-deploy form of encryption.  (Of course
  * certificate-based encryption is also supported, but
  * it’s harder to make a self-contained example).

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>

 #include <libnbd.h>

 #define TMPDIR "/tmp/XXXXXX"
 #define KEYS "keys.psk"
 #define USERNAME "alice"

 static char dir[] = TMPDIR;
 static char keys[] = TMPDIR "/" KEYS;
 static char cmd[] =
   "psktool -u " USERNAME " -p " TMPDIR "/" KEYS;

 /* Remove the temporary keys file when the program
  * exits.
 static void
 cleanup_keys (void)
   unlink (keys);
   rmdir (dir);

 /* Create the temporary keys file to share with the
  * server.
 static void
 create_keys (void)
   size_t i;

   if (mkdtemp (dir) == NULL) {
     perror ("mkdtemp");
     exit (EXIT_FAILURE);
   i = strlen (cmd) - strlen (TMPDIR) - strlen (KEYS) - 1;
   memcpy (&cmd[i], dir, strlen (TMPDIR));
   memcpy (keys, dir, strlen (TMPDIR));

   if (system (cmd) != 0) {
     fprintf (stderr, "psktool command failed\n");
     exit (EXIT_FAILURE);

   atexit (cleanup_keys);

 main (int argc, char *argv[])
   struct nbd_handle *nbd;
   char buf[512];

   create_keys ();

   /* Create the libnbd handle. */
   nbd = nbd_create ();
   if (nbd == NULL) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Enable TLS in the client. */
   if (nbd_set_tls (nbd, LIBNBD_TLS_REQUIRE) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Enable TLS-PSK and pass the keys filename. */
   if (nbd_set_tls_psk_file (nbd, keys) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Set the local username for authentication. */
   if (nbd_set_tls_username (nbd, USERNAME) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Run nbdkit as a subprocess, enabling and requiring
    * TLS-PSK encryption.
   char *args[] = {
     "nbdkit", "-s", "--exit-with-parent",
     "--tls", "require", "--tls-psk", keys,
     "pattern", "size=1M", NULL
   if (nbd_connect_command (nbd, args) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Read the first sector. */
   if (nbd_pread (nbd, buf, sizeof buf, 0, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* TLS connections must be shut down. */
   if (nbd_shutdown (nbd, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Close the libnbd handle. */
   nbd_close (nbd);

   exit (EXIT_SUCCESS);


nbd_create(3), nbd_set_tls(3), libnbd(3).


Eric Blake

Richard W.M. Jones


Copyright Red Hat


This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA