nbd_set_handshake_flags - 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.


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 handle must be newly created, otherwise this call will return an 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:



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


Eric Blake

Richard W.M. Jones


Copyright (C) 2019-2021 Red Hat Inc.


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