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nbd_set_handshake_flags - Man Page

control use of handshake flags


 #include <libnbd.h>

 int nbd_set_handshake_flags (
       struct nbd_handle *h, uint32_t flags


By default, libnbd tries to negotiate all possible handshake flags that are also supported by the server, since omitting a handshake flag can prevent the use of other functionality such as TLS encryption or structured replies.  However, for integration testing, it can be useful to reduce the set of flags supported by the client to test that a particular server can handle various clients that were compliant to older versions of the NBD specification.

The flags argument is a bitmask, including zero or more of the following handshake flags:


The server gracefully handles unknown option requests from the client, rather than disconnecting.  Without this flag, a client cannot safely request to use extensions such as TLS encryption or structured replies, as the request may cause an older server to drop the connection.


If the client is forced to use NBD_OPT_EXPORT_NAME instead of the preferred NBD_OPT_GO, this flag allows the server to send fewer all-zero padding bytes over the connection.

For convenience, the constant LIBNBD_HANDSHAKE_FLAG_MASK is available to describe all flags supported by this build of libnbd. Future NBD extensions may add further flags, which in turn may be enabled by default in newer libnbd.  As such, when attempting to disable only one specific bit, it is wiser to first call nbd_get_handshake_flags(3) and modify that value, rather than blindly setting a constant value.

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

Handle State

nbd_set_handshake_flags 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 1.2.

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


See Also

nbd_create(3), nbd_get_handshake_flags(3), nbd_set_request_structured_replies(3), libnbd(3).


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-1.2(1), nbd_get_handshake_flags(3), nbd_set_request_extended_headers(3), nbd_set_request_structured_replies(3), nbd_set_strict_mode(3).

2024-06-25 libnbd-1.21.1