nbd_opt_starttls - request the server to initiate TLS


 #include <libnbd.h>

 int nbd_opt_starttls (
       struct nbd_handle *h


Request that the server initiate a secure TLS connection, by sending NBD_OPT_STARTTLS. This can only be used if nbd_set_opt_mode(3) enabled option mode; furthermore, if you use nbd_set_tls(3) to request anything other than the default of LIBNBD_TLS_DISABLE, then libnbd will have already attempted a TLS connection prior to allowing you control over option negotiation. This command is disabled if nbd_supports_tls(3) reports false.

This function is mainly useful for integration testing of corner cases in server handling; in particular, misuse of this function when coupled with a server that is not careful about resetting stateful commands such as nbd_opt_structured_reply(3) could result in a security hole (see CVE-2021-3716 against nbdkit, for example). Thus, when security is a concern, you should instead prefer to use nbd_set_tls(3) with LIBNBD_TLS_REQUIRE and let libnbd negotiate TLS automatically.

This function returns true if the server replies with success, false if the server replies with an error, and fails only if the server does not reply (such as for a loss of connection, which can include when the server rejects credentials supplied during the TLS handshake). Note that the NBD protocol documents that requesting TLS after it is already enabled is a client error; most servers will gracefully fail a second request, but that does not downgrade a TLS session that has already been established, as reported by nbd_get_tls_negotiated(3).


This call returns a boolean value.


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

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


This function first appeared in libnbd 1.16.

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



nbd_aio_opt_starttls(3), nbd_create(3), nbd_get_tls_negotiated(3), nbd_opt_structured_reply(3), nbd_set_opt_mode(3), nbd_set_tls(3), nbd_supports_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