NAME

       nbd_opt_starttls - request the server to initiate TLS

SYNOPSIS

        #include <libnbd.h>

        int nbd_opt_starttls (
              struct nbd_handle *h
            );

DESCRIPTION

       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).

RETURN VALUE

       This call returns a boolean value.

ERRORS

       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).

HANDLE STATE

       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                │
        └─────────────────────────────────────┴─────────────────────────┘

VERSION

       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:

        #define LIBNBD_HAVE_NBD_OPT_STARTTLS 1

SEE ALSO

       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).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

       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