nbdkit-exportname-filter - adjust export names between client and plugin
nbdkit --filter=exportname plugin [default-export=NAME]
[exportname-list=MODE] [exportname-strict=true] [exportname=NAME]...
[exportdesc=DESC]Some plugins (such as nbdkit-file-plugin(1) and filters (such as nbdkit-ext2-filter(1) are able to serve different content based on the export name requested by the client. The NBD protocol allows a server to advertise the set of export names it is serving. However, the list advertised (or absent) from the plugin may not always match what you want an actual client to see. This filter can be used to alter the advertised list, as well as configuring which export should be treated as the default when the client requests the empty string ("") as an export name.
When the client requests the default export name (""), request the export NAME from the underlying plugin instead of relying on the plugin's choice of default export. Setting NAME to the empty string has the same effect as omitting this parameter.
This parameter determines which exports are advertised to a guest that requests a listing via NBD_OPT_LIST. The default mode is keep to advertise whatever the underlying plugin reports. Mode error causes clients to see an error rather than an export list. Mode empty returns an empty list. Mode defaultonly returns a list that contains only the canonical name of the default export. Mode explicit returns only the exports set by exportname=. Note that the list of advertised exports need not reflect reality: an advertised name may be rejected, or a client may connect to an export name that was not advertised, but learned through other means.
Normally, a client can pass whatever export name it wants, regardless of whether that name is advertised. But setting this parameter to true will cause the connection to fail if a client requests an export name that was not included via an exportname= parameter. At this time, it is not possible to restrict a client to exports advertised by the plugin without repeating that list via exportname; this technical limitation may be lifted in the future.
This parameter adds NAME to the list of advertised exports; it may be set multiple times.
The exportdesc parameter controls what optional descriptions are sent alongside an export name. If set to keep (the default), descriptions are determined by the plugin. If set to none, descriptions from the plugin are ignored (useful if you are worried about a potential information leak). If set to fixed:STRING, the same fixed string description is offered for every export. If set to script:SCRIPT, this filter executes script with $name set to the export to be described, and uses the output of that command as the description.
Suppose that the directory /path/to/dir contains permanent files named file1, file2, and file3. The following commands show various ways to alter the use of export names while serving that directory:
Allow a client requesting "" to get the contents of file2, rather than an error:
nbdkit --filter=exportname file dir=/path/to/dir default-export=file2Do not advertise any exports; a client must know in advance what export names to try:
nbdkit --filter=exportname file dir=/path/to/dir exportname-list=emptyAllow clients to connect to file1 and file3, but not file2:
nbdkit --filter=exportname file dir=/path/to/dir \
exportname-list=explicit exportname-strict=true \
exportname=file1 exportname=file3Offer ls(3) long descriptions alongside each export name:
nbdkit --filter=exportname file dir=/path/to/dir \
exportdesc=script:'ls -l /path/to/dir/"$name"'The filter.
Use nbdkit --dump-config to find the location of $filterdir.
nbdkit-exportname-filter first appeared in nbdkit 1.24.
nbdkit(1), nbdkit-filter(3), nbdkit-ext2-filter(1), nbdkit-extentlist-filter(1), nbdkit-fua-filter(1), nbdkit-nocache-filter(1), nbdkit-noparallel-filter(1), nbdkit-nozero-filter(1), nbdkit-file-plugin(1), nbdkit-info-plugin(1).
Eric Blake
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