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.IX Title "nbdkit-evil-filter 1"
.TH nbdkit-evil-filter 1 2026-04-25 nbdkit-1.47.8 NBDKIT
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.SH NAME
nbdkit\-evil\-filter \- add random data corruption to reads
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 4
\& nbdkit \-\-filter=evil PLUGIN [PLUGIN\-ARGS...]
\&        evil=[cosmic\-rays|stuck\-bits|stuck\-wires]
\&        [evil\-probability=PROB] [evil\-stuck\-probability=PROB]
\&        [evil\-seed=SEED]
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
nbdkit\-evil\-filter is a Byzantine filter for \fBnbdkit\fR\|(1) that
randomly corrupts data when reading from the underlying plugin.  This
can be used for testing filesystem checksums.  Note that it does not
change write operations, so the underlying plugin contains the correct
data.
.PP
\&\fBnbdkit\-error\-filter\fR\|(1) is a related filter that injects hard errors
into the NBD protocol.
.PP
This filter has several modes, controlled using the \f(CW\*(C`evil=...\*(C'\fR
parameter.  These are:
.ie n .IP """evil=cosmic\-rays""" 4
.el .IP \f(CWevil=cosmic\-rays\fR 4
.IX Item "evil=cosmic-rays"
Bits are flipped at random when reading data.  The probability that a
bit is flipped is controlled using the \f(CW\*(C`evil\-probability\*(C'\fR parameter,
defaulting to 1e\-8 (on average 1 in every 100 million bits read is
flipped).
.ie n .IP """evil=stuck\-bits""" 4
.el .IP \f(CWevil=stuck\-bits\fR 4
.IX Item "evil=stuck-bits"
This is the default mode.
.Sp
Fixed bits in the backing file are stuck randomly high or low.  The
\&\f(CW\*(C`evil\-probability\*(C'\fR parameter controls the expected probability that a
particular bit is stuck, defaulting in this mode to 1e\-8 (1 in 100
million).  \f(CW\*(C`evil\-stuck\-probability\*(C'\fR controls the probability that a
stuck bit is read as its stuck value or its correct value, defaulting
to 100% (always read as a stuck bit).
.ie n .IP """evil=stuck\-wires""" 4
.el .IP \f(CWevil=stuck\-wires\fR 4
.IX Item "evil=stuck-wires"
This is similar to \f(CW\*(C`stuck\-bits\*(C'\fR but instead of simulating bad backing
data, it simulates stuck wires along the data path (eg. in a
register).  The difference is that when reading, the stuck bit always
happens at the same position in the packet of data being read,
regardless of where on the underlying disk it is being read from.
\&\f(CW\*(C`evil\-probability\*(C'\fR controls the probability of a stuck wire,
defaulting in this mode to 1e\-6 (1 in 1 million).
\&\f(CW\*(C`evil\-stuck\-probability\*(C'\fR controls the probability that a stuck bit is
read as its stuck value or its correct value, defaulting to 100%
(always read as a stuck bit).
.SH EXAMPLES
.IX Header "EXAMPLES"
Add some stuck bits to the backing file at random:
.PP
.Vb 1
\& nbdkit \-\-filter=evil file disk.img
.Ve
.PP
Cosmic rays will flip (on average) one in every 100 million bits
copied from the backing file over NBD:
.PP
.Vb 2
\& nbdkit \-\-filter=evil file disk.img evil=cosmic\-rays \e
\&        \-\-run \*(Aqnbdcopy $uri output.img\*(Aq
.Ve
.SH NOTES
.IX Header "NOTES"
.SS Extents
.IX Subsection "Extents"
Plugins can be sparse.  This filter only corrupts bits in non\-sparse
parts of the backing disk and it leaves sparse regions unchanged
(which is realistic behaviour).  If you wish to use this filter to
corrupt sparse regions, then combine this filter with
\&\fBnbdkit\-noextents\-filter\fR\|(1).  For example:
.PP
.Vb 1
\& nbdkit \-\-filter=evil \-\-filter=noextents memory 1G
.Ve
.SS "Probability limited to [ 1e\-12 .. 1/8 ]"
.IX Subsection "Probability limited to [ 1e-12 .. 1/8 ]"
The current implementation limits probabilities to the range
[\ 1e\-12\ ..\ 1/8\ ].  Values below this range are treated the same as
0%.  Values above this range are treated the same as 100%.
.SH PARAMETERS
.IX Header "PARAMETERS"
.IP \fBevil=cosmic\-rays\fR 4
.IX Item "evil=cosmic-rays"
.PD 0
.IP \fBevil=stuck\-bits\fR 4
.IX Item "evil=stuck-bits"
.IP \fBevil=stuck\-wires\fR 4
.IX Item "evil=stuck-wires"
.PD
Select the mode of evil.  See the "DESCRIPTION" above.  The default
is \f(CW\*(C`stuck\-bits\*(C'\fR.
.IP \fBevil\-probability=\fRN 4
.IX Item "evil-probability=N"
.PD 0
.IP \fBevil\-probability=\fRN\fB:\fRM 4
.IX Item "evil-probability=N:M"
.IP \fBevil\-probability=\fRN\fB%\fR 4
.IX Item "evil-probability=N%"
.PD
Set the probability for the mode.  You can either use a floating point
number between 0 and 1, eg. \f(CW\*(C`evil\-probability=0.001\*(C'\fR or
\&\f(CW\*(C`evil\-probability=1e\-6\*(C'\fR.  Or you can write it as N in M, eg.
\&\f(CW\*(C`evil\-probability=1:1000000\*(C'\fR or \f(CW\*(C`evil\-probability=3.33:100000\*(C'\fR.  Or
you can write this as a percentage, eg. \f(CW\*(C`evil\-probability=1%\*(C'\fR.
.Sp
The default probability depends on the mode.
.IP \fBevil\-seed=\fRSEED 4
.IX Item "evil-seed=SEED"
To make runs repeatable, use this to set a seed for the random number
generator.  Note that runs are repeatable only if you use the same
version of nbdkit, since in future we may change the algorithm.
.Sp
The default is to choose a seed at random.
.IP \fBevil\-stuck\-probability=\fRN 4
.IX Item "evil-stuck-probability=N"
.PD 0
.IP \fBevil\-stuck\-probability=\fRN\fB:\fRM 4
.IX Item "evil-stuck-probability=N:M"
.IP \fBevil\-stuck\-probability=\fRN\fB%\fR 4
.IX Item "evil-stuck-probability=N%"
.PD
For the "stuck\-*" modes, the probability that when reading a stuck bit
you will read the stuck bit or the correct value.  This defaults to 1
(ie. 100%) which means the bit is always stuck.  Setting it to 0.5 for
example will mean that half the time the bit appears stuck and half
the time you see the correct value.
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$filterdir\fR\fI/nbdkit\-evil\-filter.so\fR 4
.el .IP \fR\f(CI$filterdir\fR\fI/nbdkit\-evil\-filter.so\fR 4
.IX Item "$filterdir/nbdkit-evil-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\-evil\-filter\*(C'\fR first appeared in nbdkit 1.36.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3),
\&\fBnbdkit\-delay\-filter\fR\|(1),
\&\fBnbdkit\-noextents\-filter\fR\|(1),
\&\fBnbdkit\-error\-filter\fR\|(1),
\&\fBnbdkit_parse_probability\fR\|(3).
.SH AUTHORS
.IX Header "AUTHORS"
Richard W.M. Jones
.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.
