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nbd_connect_uri - Man Page

connect to NBD URI


 #include <libnbd.h>

 int nbd_connect_uri (
       struct nbd_handle *h, const char *uri


Connect (synchronously) to an NBD server and export by specifying the NBD URI.  NBD URIs are a standard way to specify a network block device endpoint, using a syntax like "nbd://example.com" which is convenient, well defined and future proof.

This call works by parsing the URI parameter and calling nbd_set_export_name(3) and nbd_set_tls(3) and other calls as needed, followed by nbd_connect_tcp(3), nbd_connect_unix(3) or nbd_connect_vsock(3).

This call returns when the connection has been made.  By default, this proceeds all the way to transmission phase, but nbd_set_opt_mode(3) can be used for manual control over option negotiation performed before transmission phase.

Example URIs supported


Connect over TCP, unencrypted, to example.com port 10809.


Connect over TCP with TLS, to example.com port 10809.  If the server does not support TLS then this will fail.


Connect over the Unix domain socket /tmp/nbd.sock to an NBD server running locally.  The export name is set to foo (note without any leading / character).


Connect over a Unix domain socket, enabling TLS and setting the path to a directory containing certificates and keys.


In this scenario libnbd is running in a virtual machine.  Connect over AF_VSOCK to an NBD server running on the hypervisor.

Supported URI formats

The following schemes are supported in the current version of libnbd:


Connect over TCP without using TLS.


Connect over TCP.  TLS is required and the connection will fail if the server does not support TLS.


Connect over a Unix domain socket, without or with TLS respectively.  The socket parameter is required.


Connect over the AF_VSOCK transport, without or with TLS respectively. You can use nbd_supports_vsock(3) to see if this build of libnbd supports AF_VSOCK.

The authority part of the URI ([username@][servername][:port]) is parsed depending on the transport.  For TCP it specifies the server to connect to and optional port number.  For +unix it should not be present.  For +vsock the server name is the numeric CID (eg. 2 to connect to the host), and the optional port number may be present.  If the username is present it is used for TLS authentication.

For all transports, an export name may be present, parsed in accordance with the NBD URI specification.

Finally the query part of the URI can contain:


Specifies the Unix domain socket to connect on. Must be present for the +unix transport and must not be present for the other transports.


Set the certificates directory.  See nbd_set_tls_certificates(3). Note this is not allowed by default - see next section.


Set the PSK file.  See nbd_set_tls_psk_file(3).  Note this is not allowed by default - see next section.


Set the TLS hostname.  See nbd_set_tls_hostname(3).


Do not verify the server certificate.  See nbd_set_tls_verify_peer(3). The default is true.

Disable URI features

For security reasons you might want to disable certain URI features.  Pre-filtering URIs is error-prone and should not be attempted.  Instead use the libnbd APIs below to control what can appear in URIs.  Note you must call these functions on the same handle before calling nbd_connect_uri(3) or nbd_aio_connect_uri(3).

TCP, Unix domain socket or AF_VSOCK transports

Default: all allowed

To select which transports are allowed call nbd_set_uri_allow_transports(3).


Default: both non-TLS and TLS connections allowed

To force TLS off or on in URIs call nbd_set_uri_allow_tls(3).

Connect to Unix domain socket in the local filesystem

Default: allowed

To prevent this you must disable the +unix transport using nbd_set_uri_allow_transports(3).

Read from local files

Default: denied

To allow URIs to contain references to local files (eg. for parameters like tls-psk-file) call nbd_set_uri_allow_local_file(3).

Overriding the export name

It is possible to override the export name portion of a URI by using nbd_set_opt_mode(3) to enable option mode, then using nbd_set_export_name(3) and nbd_opt_go(3) as part of subsequent negotiation.

Optional features

This call will fail if libnbd was not compiled with libxml2; you can test whether this is the case with nbd_supports_uri(3).

Support for URIs that require TLS will fail if libnbd was not compiled with gnutls; you can test whether this is the case with nbd_supports_tls(3).

Constructing a URI from an existing connection

See nbd_get_uri(3).

Return Value

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

Handle State

nbd_connect_uri 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/connect-uri.c in the libnbd source code.

 /* This example shows how to connect to an NBD
  * server using the server's NBD URI.
  * To test this with a recent version of nbdkit
  * that supports the '$uri' syntax, do:
  * nbdkit -U - random 1M \
  *   --run './connect-uri $uri'
  * To test connecting to a remote NBD server
  * listening on port 10809, do:
  * ./connect-uri nbd://remote/

 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
 #include <string.h>
 #include <stdint.h>
 #include <inttypes.h>

 #include <libnbd.h>

 main (int argc, char *argv[])
   struct nbd_handle *nbd;
   char *s;
   int64_t size;

   if (argc != 2) {
     fprintf (stderr, "usage: %s URI\n",
     exit (EXIT_FAILURE);

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

   /* Request full information
    * (for nbd_get_canonical_export_name below)
   if (nbd_set_full_info (nbd, true) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);

   /* Connect to the NBD URI. */
   printf ("connecting to %s ...\n", argv[1]);
   fflush (stdout);
   if (nbd_connect_uri (nbd, argv[1]) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   printf ("connected\n");

   /* Print the URI, export name, size and other info. */
   printf ("requested URI: %s\n", argv[1]);
   s = nbd_get_uri (nbd);
   printf ("generated URI: %s\n", s ? s : "NULL");
   free (s);
   size = nbd_get_size (nbd);
   if (size == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   printf ("size: %" PRIi64 "\n", size);
   s = nbd_get_export_name (nbd);
   printf ("requested export name: %s\n", s ? s : "NULL");
   free (s);
   s = nbd_get_canonical_export_name (nbd);
   printf ("canonical export name: %s\n", s ? s : "NULL");
   free (s);
   s = nbd_get_export_description (nbd);
   printf ("export description: %s\n", s ? s : "NULL");
   free (s);

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

   exit (EXIT_SUCCESS);

See Also

nbd_aio_connect_uri(3), nbd_connect_tcp(3), nbd_connect_unix(3), nbd_connect_uri(3), nbd_connect_vsock(3), nbd_create(3), nbd_get_uri(3), nbd_opt_go(3), nbd_set_export_name(3), nbd_set_opt_mode(3), nbd_set_tls(3), nbd_set_tls_certificates(3), nbd_set_tls_hostname(3), nbd_set_tls_psk_file(3), nbd_set_tls_verify_peer(3), nbd_set_uri_allow_local_file(3), nbd_set_uri_allow_tls(3), nbd_set_uri_allow_transports(3), nbd_supports_tls(3), nbd_supports_uri(3), nbd_supports_vsock(3), libnbd(3), https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md.


Eric Blake

Richard W.M. Jones


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

Referenced By

libnbd(3), libnbd-release-notes-1.10(1), libnbd-release-notes-1.12(1), libnbd-release-notes-1.2(1), libnbd-release-notes-1.20(1), libnbd-release-notes-1.8(1), NBD(3), nbd_aio_connect_uri(3), nbd_aio_opt_go(3), nbd_aio_opt_info(3), nbd_aio_opt_list_meta_context(3), nbd_aio_opt_list_meta_context_queries(3), nbd_aio_opt_set_meta_context(3), nbd_aio_opt_set_meta_context_queries(3), nbdfuse(1), nbd_get_canonical_export_name(3), nbd_get_export_name(3), nbd_get_uri(3), nbdkit-tls(1), nbd_opt_go(3), nbd_opt_info(3), nbd_opt_list_meta_context(3), nbd_opt_list_meta_context_queries(3), nbd_opt_set_meta_context(3), nbd_opt_set_meta_context_queries(3), nbd_set_export_name(3), nbd_set_tls(3), nbd_set_uri_allow_local_file(3), nbd_set_uri_allow_tls(3), nbd_set_uri_allow_transports(3), nbd_supports_uri(3), nbd_supports_vsock(3), nbdublk(1).

2024-06-25 libnbd-1.21.1