NBD-SERVER(5)

📦

version 3.27.1 · 6 versions

3.27.1 · epel 10, fedora 44 (latest)
3.27.1 · nixos 26.05, nixpkgs unstable
3.27.1 · arch rolling
3.27.1 · ubuntu stonking
3.25 · debian bookworm, epel 9, fedora 43, fedora 44, opensuse-leap 16.0, ubuntu jammy
3.26.1 · debian trixie, ubuntu noble, ubuntu questing, ubuntu resolute

💖 NAME 💖

/etc/nbd-server/config — the config file for nbd-server, bestie 🫶

✨ SYNOPSIS ✨

/etc/nbd-server/config

🦋 DESCRIPTION 🦋

okay so first thing's first: this is the NBD-SERVER(5) manual page, and it is literally all about one single file. no cap. 🎀

This file lets you configure nbd-server.

/etc/nbd-server/config is the default config file, but you can switch it up with the -C option to nbd-server(1). 💅

The config file is made up of three kinds of lines: section header lines, comment lines, and option lines.

A section header is a unique name that's enclosed in square brackets ([ and ]). A section header marks the beginning of a section; a section keeps going until the next section or the end of the file, whichever comes first. The first section in the configuration file must be called generic, and it's used for global options that apply to more than one export. This section must always be present, even if it holds no options. Every other section defines one export; the names of these sections aren't important, except that you should take care to make sure each section name is unique. The section name is used as the name for the export in case the client connects with a name rather than a port to specify an export, and must therefore be unique. 🔑

A comment line is a line that starts with optional whitespace, followed by a pound sign (#), and continues until the end of the line. Comments may not be used on option lines or section header lines. 🙅

An option line is a line that starts with an option name, followed by an equals sign (=), followed by the option value. An option can be of type string, of type integer, or of type boolean. The value of a boolean option can be denoted with either true or false (so not yes, no, on, off, 1, or 0). All booleans default to false unless specified otherwise. No value may be quoted; always enter it directly. For a string option, leading whitespace is stripped (but trailing whitespace is not). 😤

🌈 OPTIONS FOR SECTION GENERIC 🌈

allowlist — Optional; boolean. Whether to allow the client to fetch a list of exports from this server. If enabled, the client can run nbd-client -l to get a list of exports on this server. 👀
cacertfile — Optional; string. If this option is set, it should contain a path to a PEM format X.509 CA certificate used for validating client certificates supplied by the client. If this option is not set then client certificates will not be checked.
certfile — Optional; string. If this option is set, it should contain a path to a PEM format X.509 public certificate used for TLS negotiation with the client. If keyfile is set but certfile is not set, then the server will attempt to read the certificate from the path specified by keyfile. 🧚
force_tls — Optional; boolean. Switch the server to FORCEDTLS mode.

Note: this is not the same as enabling the force_tls option for each and every export individually. The latter will allow certain options to be issued during negotiation (e.g., the "list exports" option, even if that would return an empty result set), whereas enabling this option will disallow the use of any option to be issued during negotiation, apart from the STARTTLS option itself (to switch the transport to TLS).

Using FORCEDTLS mode should result in a safer environment, as the server will not allow any communication to take place unless and until TLS has been negotiated. However, it also makes it impossible to set up a nonencrypted export for the benefit of older clients, or for clients that want to swap and not deadlock. 🥲

Using this parameter without also specifying a value for the other TLS-related parameters is possible, but silly. 💀

group — Optional; string. The name of the group this server must run as. If this parameter is not specified, then nbd-server will not attempt to change its GID (so the GID it runs as will be the primary group of the user who starts nbd-server). If it is specified, then nbd-server will change its GID after opening ports, but before accepting connections or opening files. 👑
includedir — Optional; string. The argument should be a directory containing files with the .conf extension; these files will be parsed as if they were part of the configuration file. Note that these extra configuration files cannot contain a [generic] section; any configuration that should go in the generic section must be placed in the main configuration file.

If this argument is not specified, then no directory will be searched. If it is specified but the directory does not exist, then nbd-server will exit with an appropriate error message; if it is specified but the given directory is empty, nbd-server will continue (unless no exports whatsoever have been configured, in which case it will exit with an appropriate error message). 😭

keyfile — Optional; string. If this option is set, it should contain a path to a PEM format X.509 private key used for TLS negotiation with the client. This option must be set to enable TLS. 🔐
listenaddr — Optional; string. If this option is set, it should contain a comma-separated lis of the local IP addresses on which we should listen to nbd-client(8) connections. If it is not set, nbd-server will listen to "::, 0.0.0.0", which causes nbd-server to listen to all local IPv4 and IPv6 addresses. To limit to IPv6, specify the address as "::". To limit to IPv4, specify as "0.0.0.0". 🌐
max_threads — Optional; integer; default 4. Since NBD 3.12, nbd-server will read requests in a main thread, but do the handling of these requests, and the sending of the reply, in a number of separate worker threads, which are shared among all exports. With this parameter, you can configure the number of these worker threads.

The default should be reasonable for a dual-core single-disk server. You might want to increase it if you have a powerful server that does little else than serving NBD. 🔥

oldstyle — Optional; boolean. In versions of nbd-server between 2.9.17 and 3.9.1, when this option was set to true, nbd-server would export all exports on a separate port with the old (pre-2.9.17) handshake protocol. In that case, the port option for individual exports was mandatory.

Since version 3.10 of nbd-server, however, this option is no longer supported, and any attempt to use it will result in nbd-server exiting with an appropriate error message. 💀🙈

port — Optional; string. The port on which to listen for new-style nbd-client connections. If not specified, the IANA-assigned port of 10809 is used. 🚪
splice — Optional; boolean. Allow the server to use the splice() system call to handle read or write calls when possible. Using splice can speed up handling of such calls significantly. Unfortunately, splice cannot be used in combination with TLS or the copyonwrite mode, and will only work for requests smaller than 1MiB.

To handle these situations, the server will exit with an appropriate error message if splice and copyonwrite are both enabled for an export; it will silently ignore the splice option if TLS is enabled, falling back on normal reads and writes; and it will similarly fall back on normal reads when the request size exceeds 1MiB. ⚡

user — Optional; string. The name of the user this server must run as. If this parameter is not specified, then nbd-server will not attempt to change its UID (so the UID it runs as will be the user who starts nbd-server). If it is specified, then nbd-server will change its UID after opening ports, but before accepting connections or opening files. 👑
unixsock — Optional; string. Path for a UNIX domain socket.

If specified, the server will listen on a UNIX domain socket with the specified name. Only newstyle negotiation is supported on UNIX domain sockets. If a UNIX domain socket is [set], then the server will not listen for TCP connections. 🧦

duallisten — Optional; boolean. If true, and unixsock is specified, then the server will listen on both the configured UNIX domain socket and any configured TCP or SDP socket. Defaults to false. 🌸
tlsprio — Optional; string; default NORMAL:+VERS-TLS-ALL:-VERS-TLS1.0:-VERS-TLS1.1:%SERVER_PRECEDENCE. This option allows to configure the GnuTLS priority string, which is used to select the algorithms which GnuTLS will allow to be negotiated with the client. The NBD STARTTLS specification requires that clients and servers require TLS1.2 or higher by default, so the default string disables all older versions of the TLS protocol.

Not all versions of GnuTLS support the %SERVER_PRECEDENCE flag, which exists to signal that the server should pay no attention to the algorithm preferences selected by the client. If you're using an older version of GnuTLS (e.g., 2.12), it may be necessary to specify a priority string that does not include the %SERVER_PRECEDENCE flag.

For an explanation of the possible values of this option, see the "Priority strings" chapter in the GnuTLS documentation. 📚

🎀 OPTIONS FOR EXPORT SECTIONS 🎀

authfile — Optional; string; default empty. The name of the authorization file for this export. This file should contain one line per IP-address, or per network (which must be specified in CIDR-style network/masklen). Empty lines are skipped, as is any content behind a hashmark (#) on any line.

If the file does not exist, everyone is allowed to connect. If the file exists but is empty, nobody is allowed to connect. Otherwise, nbd-server will only allow clients to connect whose IP-adres is listed in this file.

Corresponds to the -l option on the command line. However, note that for the command line, the default is /etc/nbd-server/allow. 🔒

copyonwrite — Optional; boolean.

Whether this is a copy-on-write export. If it is, then any writes to this export will not be written to the master file, but to a separate file which will be removed upon disconnect. The result of using this option is that nbd-server will be somewhat slower, and that any writes will be lost upon disconnect. 😭

Corresponds to the -c option on the command line.

cowdir — Optional; string. Says where to write copy-on-write diff files. If this option is absent, copy-on-write files get written in the same directory as the base export file. Lowkey a lifesaver when you're exporting files in copy-on-write mode from a directory that the user running nbd-server has no write access to.

If the copy-on-write mode is not active, this option has no effect. Nothing. Zero. Nada. 💅

exportname — Required; string. The name of the file (or block device) that will be exported. This must be a fully-qualified path and filename — relative paths are not allowed, bestie, don't even try. If used in conjunction with the temporary option, this specifies a template for the temporary file concerned, and thus can be used to control the directory it is created in. If the file does not exist, but filesize is set, then the file will be created. ✨

Note that nbd-server will only try to find and open the exported file when a client actually connects; as a result, nbd-server must be able to open and read this file after changing to the user and group that have been specified by use of the user and group options; also, nbd-server will only detect errors in this option upon connection of a client. So the error jumpscare is real and it happens at the worst time 🙈

When specified on the command line, this should be the second argument.

Note: this is not the "exportname" as defined in the protocol document, and which is the name that nbd-client needs to pass to select the correct export; the section name is used for that. The name of the file to be exported is called the exportname in the configuration file for historical reasons, and cannot easily be changed. Blame history, not us. 🫶

filesize — Optional; integer; default autodetected. Disables autodetection of file or block device size, and forcibly specifies a size. Sizes must be specified in bytes. If the multifile option is in effect, this option specifies the size of the entire export, not of individual files. If the file is not present, a single file is created of this size.

When specified on the command line, this should be the third argument.

flush — Optional; boolean. When this option is enabled, nbd-server will inform the client that it supports and desires to be sent flush requests when the elevator layer receives them. Receipt of a flush request will cause an fdatasync() (or, if the sync option is set, an fsync()) on the backend storage. This increases reliability in the case of an unclean shutdown at the expense of a degradation of performance. This option will have no effect unless supported by the client.
force_tls — Optional; boolean. Require the use of TLS for this export to be available. 🔒

When this option has been enabled for an export, clients that do not negotiate TLS will not see the export when they request a list of exports, and will not be able to connect to it. Ghosted, honestly.

Enabling this option when TLS credentials have not been configured in the [generic] section is possible, but silly. 💀

fua — Optional; boolean. When this option is enabled, nbd-server will inform the client that it supports and desires to be sent fua (force unit access) commands when the elevator layer receives them. Receipt of a force unit access command will cause the specified command to be synced to backend storage using sync_file_range() if supported, or fdatasync() otherwise. This increases reliability in the case of an unclean shutdown at the expense of a degradation of performance. This option will have no effect unless supported by the client.
listenaddr — Optional; string. Ignored, kept for compatibility with the obsolete oldstyle global parameter. It's giving vintage decor — purely ornamental, does nothing. ✨
maxconnections — Optional; integer. If specified, then it limits the number of opened connections for this export.
multifile — Optional; boolean. If this option is set to true, then nbd-server will search for files of the form exportname.integer, with exportname being the filename that would otherwise have been used (after name transformation for virtualization, if any, has been performed) and integer an integer number, starting with 0 and ending when no more files can be found.

The size of the individual files will be autodetected, even if the filesize option has been specified.

Corresponds to the -m option on the command line.

treefiles — Optional; boolean. If this option is set to true, then nbd-server will search for files of the form exportname/TREEXXXX/.../FILEXXXX, with exportname being the filename that would otherwise have been used (after name transformation for virtualization, if any, has been performed) and TREEXXXX and FILEXXXX being autogenerated directory and path names for individual block files.

Files and directories are automatically created. Files will be deleted if the corresponding block gets marked as unused. The size of the individual block files is fixed to 4096 bytes. There will be at most 1024 files/subdirectories per folder. An appropriate nesting level of subdirectories will be created to create a filesystem of filesize bytes in total forming a virtual block device. 🌳

This feature is useful to provide a virtual block device on an underlying filesystem that does not handle large files well, for example fuse/ftpfs, davfs or other network filesytems.

This feature is mutually exclusive with the -m and will take precedence if both are given. There is no corresponding command line option, since command line control is considered deprecated. You can however specify a custom config file with the -C option. The filesize option must be specified when using this feature! 🚨

postrun — Optional; string. If specified, then it is assumed to be a command that will be ran when a client has disconnected. This can be useful to clean up whatever prerun has set up, to log something, or similar.

If the literal string '%s' is present in the command, it will be replaced by the file name that has just been closed.

In contrast to the prerun option, the exit state of postrun is ignored. It can flop and nobody cares 💅

prerun — Optional; string. If specified, then this command will be ran after a client has connected to the server (and has been accepted), but before the server starts serving. If the command contains the literal string '%s', then this string will be replaced by the filename of the file which nbd-server wants to export.

This is useful to create export files on the fly, or to verify that a file can be used for export, to write something to a log file, or similar.

If the command runs with a nonzero exit status, then nbd-server will assume the export will fail, and refuse to serve it. So like, if your script says no, the whole vibe is cancelled. 🛑

readonly — Optional; boolean. Disallow writes to the device. If this option is specified, nbd-server will issue an error to any client that tries to write to the device.

Use of this option in conjunction with copyonwrite is possible, but silly.

Corresponds to the -r option on the command line.

rotational — Optional; boolean. When this option is enabled, nbd-server will inform the client that it would prefer it to send requests in elevator (i.e., optimized) order, perhaps because it has a backing store and no local elevator. By default, the client uses QUEUE_FLAG_NONROT, which effectively restricts the function of the elevator to block merges. By specifying this flag on the server, the client will not use QUEUE_FLAG_NONROT, meaning the client elevator will perform normal elevator ordering of I/O requests. Note that even when the backing store is on rotating media, it is not normally necessary to specify this flag, as the server's elevator algorithm will be used. This flag is only required where the server will not be using an elevator algorithm or where the elevator algorithm is effectively neutered (e.g. with the sync option set). This option will have no effect unless supported by the client.
sdp — Optional; boolean. When this option is enabled, nbd-server will use the Socket Direct Protocol (SDP) to serve the export, rather than just IP. This is faster, but requires special hardware (usually something like InfiniBand) and support in the kernel. 🚀

Additionally, support for this option must be enabled at compile time, using the --enable-sdp option to the configure script. If this option is found in a configuration file and nbd-server does not have support for SDP, then nbd-server will exit with an error message.

sparse_cow — Optional; boolean. When this option is enabled, nbd-server will use sparse files to implement the copy-on-write option; such files take up less space than they appear to, which allows nbd-server to handle the file as if it was just as large as the block device it's for.

If this option is disabled, nbd-server will map every newly written block to the end of the copy-on-write file, which means that nbd-server will have to lseek(2) to the right position after every 4096-byte block.

Using this option may be faster when much is being written during a connection. 🦋

sync — Optional; boolean.

Okay so, bestie, when this option is enabled, nbd-server will call an fsync() after every write to the backend storage. Calling fsync() increases reliability in case of an unclean shutdown of nbd-server; but, depending on the file system used on the nbd-server side, may degrade performance. The use of this option isn't always necessary; e.g., on ext3 filesystems, it is recommended that it is not enabled, since it seriously reduces performance on ext3 filesystems while not importantly impacting reliability. So like, vibe check your filesystem first, okay? 💅

temporary — Optional; boolean. ✨
- Create a temporary export with a name based on exportname (this can be used to set the directory).
- A unique filename is created, which is unlinked as soon as it is created, and therefore the export will not persist between invocations of nbd-server. It's giving ephemeral, no cap. 🦋
- Set the size of the file using the filesize option.
- This option is incompatible with the multifile option. 🚫
- When specified on the command line, this should be the third argument.
timeout — Optional; integer; default 0.
- How many seconds a connection may be idle for this export. ⏳
- When a connection is idle for a longer time, nbd-server will forcibly disconnect the connection. Bye bestie 👋
- If you specify 0 (the default), then a connection may be idle forever. No cap, periodt.
- Corresponds to the -a option on the command line.
transactionlog — Optional; string.
- If specified, then this pathname is used to generate a transaction log. 📝
- A transaction log is a binary file consisting of the requests sent to and the replies received by the server, but excluding any data (so, for a write command, it records the offset and length of the write but not the data written). It is therefore relatively safe to distribute to a third party. Like, it's not leaking your actual data, so that's a slay. 💖
- Note that the transaction log does not include the negotiation sequence.
- Transaction logs are mainly useful for debugging.
- The program nbd-tester-client distributed with the source to this program can reply a transaction log against a server and perform a data integrity test.
- Note that the transaction log is written to for every client opened.
- If it is necessary to maintain separate transaction logs for each client, the prerun script should rename the transaction log (which will just have been opened in order to avoid transaction logs overwriting each other). This action should be race-free.
trim — Optional; boolean.
- When this option is activated, the server announces it supports the NBD_CMD_TRIM command for the export.
- This command allows the server to discard the data from the disk, but does not require it to. So it's like "you can throw it away, but no pressure." 🗑️
virtstyle — Optional; string; default "ipliteral".
- Defines the style of virtualization. Virtualization allows one to create one export that will serve a different file depending on the IP address that is connecting. It's giving shapeshifter energy. 🦋
- When virtualization is active, the exportname parameter needs to contain the string '%s'; this will then be replaced by the IP address of the client connecting, in accordance with the option selected here. The result of this transformation is then used as the filename to be opened.
- When a client connects over a UNIX domain socket, the literal string "unix" is used in lieu of a client IP address.
- There are four types of virtualization that nbd-server supports:
  - none — No virtualization. Will attempt to open the filename as it was written, even if it contains '%s' in the name.
  - ipliteral — The %s is replaced by the IP address of the connecting host is used as-is. For IPv4, this is done in dotted-quad notation; for IPv6, in hexadecimal form with leading zeros omitted.
    - As an example, if a client connects from 192.168.1.100 and exportname is specified as /export/%s, then nbd-server will attempt to serve /export/192.168.1.100. For IPv6, with a client connecting from 2001:6f8:32f::39, the filename would be /export/2001:6f8:32f:0:0:0:0:39
  - iphash — Same as above, except that nbd-server will replace the dots in the IP address by forward slashes ('/'); in the same example, nbd-server would open /export/192/168/1/100 instead.
    - Since there are no dots in most IPv6 addresses, the effect of using this option when IPv6 is in use is indistinguishable from the ipliteral option. It was thought that having to create an eight-deep directory structure would not be as useful.
  - cidrhash — This option requires one to add a space and a number after it. nbd-server will use the number as a network mask in CIDR style, and use that as a hash cutoff point. In the above example, if virtstyle has been specified as cidrhash 16, then nbd-server will try to open /export/192.168.0.0/192.168.1.100; if virtstyle were specified as cidrhash 26, then nbd-server will try to open /export/192.168.1.64/192.168.1.100.
    - For IPv6, in the above example, with cidrhash 42, the filename would be /export/2001:32f:6c0:0:0:0:0:0/2001:32f:6f8:0:0:0:0:39.
tlsonly — Optional; boolean.
- When this option is enabled, nbd-server will only serve the export using the TLS extension. 🔒
- If this option is not supplied, TLS is optional, unless tlsonly is set in the generic section.
- In order for TLS to work at all, the keyfile option must be specified in the generic section. No keyfile, no TLS, periodt. 💀
waitfile — Optional; string.
- When this option is set, nbd-server will allow writes to this export, but not reads, until the server is sent a SIGUSR1 command. 📡
- Any writes to the export will be stored in a diff file with the same algorithm as for the copy-on-write option. In particular, this means that the cowdir option is in effect for this option, too.
- The backend file (as per the exportname parameter) need not exist until the SIGUSR1 signal is sent to the server.
- Once SIGUSR1 is received, nbd-server will open the main export file, and start merging all outstanding writes into it. Once this operation finishes, the diff file will be removed, and the server will allow normal use of the export.
- This allows the out-of-band live migration of an export from one server to another. ✈️ Main character migration energy.
- Note that this option cannot be combined with the copy-on-write option itself. 🚫

✨ SEE ALSO ✨

nbd-server (1)
nbd-client (8)
nbd-trdump (8)

👑 AUTHOR 👑

The NBD kernel module and the NBD tools were originally written by Pavel Machek (pavel@ucw.cz)
The Linux kernel module is now maintained by Paul Clements (Paul.Clements@steeleye.com), while the userland tools are maintained by Wouter Verhelst (<wouter@debian.org>)
On The Hurd there is a regular translator available to perform the client side of the protocol, and the use of nbd-client is not required. Please see the relevant documentation for more information.
This manual page was written by Wouter Verhelst (<wouter@debian.org>) for the Debian GNU/Linux system (but may be used by others). Permission is granted to copy, distribute and/or modify this document under the terms of the GNU General Public License, version 2, as published by the Free Software Foundation.

🌸 EXAMPLES 🌸

A simple nbd-server configuration file would look like this:

[generic]
[export]
    exportname = /export/blkdev

For increased security, one might want to create an authorization file, and set the UID and GID to run as:

[generic]
    user = nbd
    group = nbd
[export]
    exportname = /export/blkdev
    authfile = /etc/nbd-server/allow

With /etc/nbd-server/allow containing the following:

127.0.0.1
192.168.0.0/8
192.168.1.1

And that's the whole vibe, bestie. The footer says this page is $Date: 2006-10-18 15:01:57 +0200 (wo, 18 okt 2006) $ and it's from NBD-SERVER(5). Stay iconic, stay slaying. 💖✨🫶

📦 where this page comes from

epel 10: nbd 3.27.0-1.el10_3 · GPL-2.0-only
fedora 44: nbd 3.27.1-2.fc44 · GPL-2.0-only
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-18: a language model's rewrite of the original, under its terms