nbdkit-protect-filter - write-protect parts of a plugin
nbdkit --filter=protect plugin protect=START-END [protect=...]nbdkit-protect-filter is a filter for nbdkit(1) that lets you write-protect parts of the underlying plugin. This prevents clients from modifying ranges of the plugin, while still allowing the NBD connection to be writable in general.
One use for this is to prevent clients from adding a phony header to the data to make it look like a non-raw disk format (like adding a qcow2 header), since that can be used in some exploits.
To make the whole plugin readonly, use the nbdkit -r option on the command line, or nbdkit-readonly-filter(1). To give selective access to the server, use nbdkit-ip-filter(1). To serve only a subset of the plugin use nbdkit-offset-filter(1) or nbdkit-partition-filter(1).
Protect the first part of the boot sector from writes, but allow the MBR partition table to be updated:
nbdkit --filter=protect file disk.img protect=0-0x1bdAllow clients to write to a single partition on the disk, but prevent modifications to the partition table, boot sector, other partitions, or any other metadata outside the partition:
$ fdisk -l disk.img
Device Boot Start End Sectors Size Id Type
disk.img1 * 2048 12580863 12578816 6G 83 Linux
$ start=$((2048*512))
$ end=$((12580863*512+511))
$ nbdkit --filter=protect file disk.img protect=~$start-$endNotes:
Data in other partitions and parts of the disk is still readable.
The '~' character complements the range protected. You might need to escape it from the shell using quotes or backslash.
This is different from nbdkit-partition-filter(1) — that filter exposes the partition as a whole device.
Protect the bytes starting at offset START through to offset END (inclusive). Reads are permitted. Writes to the protected range are only permitted if they do not change the content. If an attempt is made to change the content, then the error NBD_EPERM (Operation not permitted) is returned to the client.
START and END can be expressed in decimal, octal (0NN) or hexadecimal (0xNN). If omitted then they default to the start and end of the underlying plugin respectively.
This parameter can be given multiple times to protect several ranges.
This protects the complement of the range START to END.
The filter.
Use nbdkit --dump-config to find the location of $filterdir.
nbdkit-protect-filter first appeared in nbdkit 1.30.
nbdkit(1), nbdkit-file-plugin(1), nbdkit-ip-filter(1), nbdkit-offset-filter(1), nbdkit-partition-filter(1), nbdkit-readonly-filter(1), nbdkit-filter(3).
Richard W.M. Jones
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