nbd_set_tls - enable or require TLS (authentication and encryption)


 #include <libnbd.h>

 int nbd_set_tls (
       struct nbd_handle *h, int tls


Enable or require TLS (authenticated and encrypted connections) to the NBD server. The possible settings are:


Disable TLS. (The default setting, unless using nbd_connect_uri(3) with a URI that requires TLS).

This setting is also necessary if you use nbd_set_opt_mode(3) and want to interact in plaintext with a server that implements the NBD protocol's SELECTIVETLS mode, prior to enabling TLS with nbd_opt_starttls(3). Most NBD servers with TLS support prefer the NBD protocol's FORCEDTLS mode, so this sort of manual interaction tends to be useful mainly during integration testing.


Enable TLS if possible.

This option is insecure (or best effort) in that in some cases it will fall back to an unencrypted and/or unauthenticated connection if TLS could not be established. Use LIBNBD_TLS_REQUIRE below if the connection must be encrypted.

Some servers will drop the connection if TLS fails so fallback may not be possible.


Require an encrypted and authenticated TLS connection. Always fail to connect if the connection is not encrypted and authenticated.

As well as calling this you may also need to supply the path to the certificates directory (nbd_set_tls_certificates(3)), the username (nbd_set_tls_username(3)) and/or the Pre-Shared Keys (PSK) file (nbd_set_tls_psk_file(3)). For now, when using nbd_connect_uri(3), any URI query parameters related to TLS are not handled automatically. Setting the level higher than zero will fail if libnbd was not compiled against gnutls; you can test whether this is the case with nbd_supports_tls(3).


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. For more information see "Non-NULL parameters" in libnbd(3).


nbd_set_tls 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_connect_uri(3), nbd_create(3), nbd_get_tls(3), nbd_get_tls_negotiated(3), nbd_opt_starttls(3), nbd_set_opt_mode(3), nbd_set_tls_certificates(3), nbd_set_tls_psk_file(3), nbd_set_tls_username(3), nbd_supports_tls(3), "ENCRYPTION AND AUTHENTICATION" in libnbd(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.

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