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.IX Title "nbdkit-tls-fallback-filter 1"
.TH nbdkit-tls-fallback-filter 1 2026-07-14 nbdkit-1.48.0 NBDKIT
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.SH NAME
nbdkit\-tls\-fallback\-filter \- nbdkit TLS protection filter
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 2
\& nbdkit \-\-tls=on \-\-filter=tls\-fallback plugin [plugin\-args...]
\&        [tlsreadme=MESSAGE]
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-tls\-fallback\-filter\*(C'\fR is designed to be used when offering a
connection that allows but does not require TLS from clients, in order
to offer safe alternative content to plaintext clients, only exposing
the underlying plugin to authenticated users.  This may provide a
nicer failure mode for plaintext clients than the harsher \f(CW\*(C`nbdkit
\&\-\-tls=require\*(C'\fR.
.PP
When this filter detects a plaintext connection, it ignores the
client\*(Aqs export name, and provides a single read\-only export with 512
bytes of data and content that defaults to the message "This NBD
server requires TLS authentication before it will serve useful data."
.PP
When using this filter, it is recommended to place this filter first
in the command line, to reduce the chance that any work done by
\&\f(CW\*(C`.open\*(C'\fR in earlier filters can be exploited by plaintext connections
as a denial of service attack to starve further authenticated
connections.  Note that this filter will fail to load if the plugin
requests the \f(CW\*(C`SERIALIZE_CONNECTIONS\*(C'\fR thread model, as a plaintext
client holding its connection open indefinitely would be such a
starvation.
.SH PARAMETERS
.IX Header "PARAMETERS"
.IP \fBtlsreadme=\fRMESSAGE 4
.IX Item "tlsreadme=MESSAGE"
This optional parameter can be used to use \f(CW\*(C`MESSAGE\*(C'\fR as the contents
of the dummy export exposed to plaintext clients, using trailing NUL
bytes to round the size up to 512 bytes.
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$filterdir\fR\fI/nbdkit\-tls\-fallback\-filter.so\fR 4
.el .IP \fR\f(CI$filterdir\fR\fI/nbdkit\-tls\-fallback\-filter.so\fR 4
.IX Item "$filterdir/nbdkit-tls-fallback-filter.so"
The filter.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$filterdir\fR.
.SH VERSION
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-tls\-fallback\-filter\*(C'\fR first appeared in nbdkit 1.22.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-tls\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3).
.SH AUTHORS
.IX Header "AUTHORS"
Eric Blake
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "LICENSE"
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
.IP \(bu 4
Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
.IP \(bu 4
Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
.IP \(bu 4
Neither the name of Red Hat nor the names of its contributors may be
used to endorse or promote products derived from this software without
specific prior written permission.
.PP
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS \*(Aq\*(AqAS IS\*(Aq\*(Aq AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
