rclone/docs/content/sftp.md

title	description	versionIntroduced
SFTP	SFTP	v1.36

{{< icon "fa fa-server" >}} SFTP

SFTP is the Secure (or SSH) File Transfer Protocol.

The SFTP backend can be used with a number of different providers:

{{< provider_list >}} {{< provider name="Hetzner Storage Box" home="https://www.hetzner.com/storage/storage-box" config="/sftp/#hetzner-storage-box">}} {{< provider name="rsync.net" home="https://rsync.net/products/rclone.html" config="/sftp/#rsync-net">}} {{< /provider_list >}}

SFTP runs over SSH v2 and is installed as standard with most modern SSH installations.

Paths are specified as remote:path. If the path does not begin with a / it is relative to the home directory of the user. An empty path remote: refers to the user's home directory. For example, rclone lsd remote: would list the home directory of the user configured in the rclone remote config (i.e /home/sftpuser). However, rclone lsd remote:/ would list the root directory for remote machine (i.e. /)

Note that some SFTP servers will need the leading / - Synology is a good example of this. rsync.net and Hetzner, on the other hand, requires users to OMIT the leading /.

Note that by default rclone will try to execute shell commands on the server, see shell access considerations.

Configuration

Here is an example of making an SFTP configuration. First run

rclone config

This will guide you through an interactive setup process.

No remotes found, make a new one?
n) New remote
s) Set configuration password
q) Quit config
n/s/q> n
name> remote
Type of storage to configure.
Choose a number from below, or type in your own value
[snip]
XX / SSH/SFTP
   \ "sftp"
[snip]
Storage> sftp
SSH host to connect to
Choose a number from below, or type in your own value
 1 / Connect to example.com
   \ "example.com"
host> example.com
SSH username
Enter a string value. Press Enter for the default ("$USER").
user> sftpuser
SSH port number
Enter a signed integer. Press Enter for the default (22).
port>
SSH password, leave blank to use ssh-agent.
y) Yes type in my own password
g) Generate random password
n) No leave this optional password blank
y/g/n> n
Path to unencrypted PEM-encoded private key file, leave blank to use ssh-agent.
key_file>
Remote config
--------------------
[remote]
host = example.com
user = sftpuser
port =
pass =
key_file =
--------------------
y) Yes this is OK
e) Edit this remote
d) Delete this remote
y/e/d> y

This remote is called remote and can now be used like this:

See all directories in the home directory

rclone lsd remote:

See all directories in the root directory

rclone lsd remote:/

Make a new directory

rclone mkdir remote:path/to/directory

List the contents of a directory

rclone ls remote:path/to/directory

Sync /home/local/directory to the remote directory, deleting any excess files in the directory.

rclone sync --interactive /home/local/directory remote:directory

Mount the remote path /srv/www-data/ to the local path /mnt/www-data

rclone mount remote:/srv/www-data/ /mnt/www-data

SSH Authentication

The SFTP remote supports three authentication methods:

• Password
• Key file, including certificate signed keys
• ssh-agent

Key files should be PEM-encoded private key files. For instance /home/$USER/.ssh/id_rsa. Only unencrypted OpenSSH or PEM encrypted files are supported.

The key file can be specified in either an external file (key_file) or contained within the rclone config file (key_pem). If using key_pem in the config file, the entry should be on a single line with new line ('\n' or '\r\n') separating lines. i.e.

key_pem = -----BEGIN RSA PRIVATE KEY-----\nMaMbaIXtE\n0gAMbMbaSsd\nMbaass\n-----END RSA PRIVATE KEY-----

This will generate it correctly for key_pem for use in the config:

awk '{printf "%s\\n", $0}' < ~/.ssh/id_rsa

If you don't specify pass, key_file, or key_pem or ask_password then rclone will attempt to contact an ssh-agent. You can also specify key_use_agent to force the usage of an ssh-agent. In this case key_file or key_pem can also be specified to force the usage of a specific key in the ssh-agent.

Using an ssh-agent is the only way to load encrypted OpenSSH keys at the moment.

If you set the ask_password option, rclone will prompt for a password when needed and no password has been configured.

Certificate-signed keys

With traditional key-based authentication, you configure your private key only, and the public key built into it will be used during the authentication process.

If you have a certificate you may use it to sign your public key, creating a separate SSH user certificate that should be used instead of the plain public key extracted from the private key. Then you must provide the path to the user certificate public key file in pubkey_file.

Note: This is not the traditional public key paired with your private key, typically saved as /home/$USER/.ssh/id_rsa.pub. Setting this path in pubkey_file will not work.

Example:

[remote]
type = sftp
host = example.com
user = sftpuser
key_file = ~/id_rsa
pubkey_file = ~/id_rsa-cert.pub

If you concatenate a cert with a private key then you can specify the merged file in both places.

Note: the cert must come first in the file. e.g.

cat id_rsa-cert.pub id_rsa > merged_key

Host key validation

By default rclone will not check the server's host key for validation. This can allow an attacker to replace a server with their own and if you use password authentication then this can lead to that password being exposed.

Host key matching, using standard known_hosts files can be turned on by enabling the known_hosts_file option. This can point to the file maintained by OpenSSH or can point to a unique file.

e.g. using the OpenSSH known_hosts file:

[remote]
type = sftp
host = example.com
user = sftpuser
pass = 
known_hosts_file = ~/.ssh/known_hosts

Alternatively you can create your own known hosts file like this:

ssh-keyscan -t dsa,rsa,ecdsa,ed25519 example.com >> known_hosts

There are some limitations:

• rclone will not manage this file for you. If the key is missing or wrong then the connection will be refused.
• If the server is set up for a certificate host key then the entry in the known_hosts file must be the @cert-authority entry for the CA

If the host key provided by the server does not match the one in the file (or is missing) then the connection will be aborted and an error returned such as

NewFs: couldn't connect SSH: ssh: handshake failed: knownhosts: key mismatch

or

NewFs: couldn't connect SSH: ssh: handshake failed: knownhosts: key is unknown

If you see an error such as

NewFs: couldn't connect SSH: ssh: handshake failed: ssh: no authorities for hostname: example.com:22

then it is likely the server has presented a CA signed host certificate and you will need to add the appropriate @cert-authority entry.

The known_hosts_file setting can be set during rclone config as an advanced option.

ssh-agent on macOS

Note that there seem to be various problems with using an ssh-agent on macOS due to recent changes in the OS. The most effective work-around seems to be to start an ssh-agent in each session, e.g.

eval `ssh-agent -s` && ssh-add -A

And then at the end of the session

eval `ssh-agent -k`

These commands can be used in scripts of course.

Shell access

Some functionality of the SFTP backend relies on remote shell access, and the possibility to execute commands. This includes checksum, and in some cases also about. The shell commands that must be executed may be different on different type of shells, and also quoting/escaping of file path arguments containing special characters may be different. Rclone therefore needs to know what type of shell it is, and if shell access is available at all.

Most servers run on some version of Unix, and then a basic Unix shell can be assumed, without further distinction. Windows 10, Server 2019, and later can also run a SSH server, which is a port of OpenSSH (see official installation guide). On a Windows server the shell handling is different: Although it can also be set up to use a Unix type shell, e.g. Cygwin bash, the default is to use Windows Command Prompt (cmd.exe), and PowerShell is a recommended alternative. All of these have behave differently, which rclone must handle.

Rclone tries to auto-detect what type of shell is used on the server, first time you access the SFTP remote. If a remote shell session is successfully created, it will look for indications that it is CMD or PowerShell, with fall-back to Unix if not something else is detected. If unable to even create a remote shell session, then shell command execution will be disabled entirely. The result is stored in the SFTP remote configuration, in option shell_type, so that the auto-detection only have to be performed once. If you manually set a value for this option before first run, the auto-detection will be skipped, and if you set a different value later this will override any existing. Value none can be set to avoid any attempts at executing shell commands, e.g. if this is not allowed on the server.

When the server is rclone serve sftp, the rclone SFTP remote will detect this as a Unix type shell - even if it is running on Windows. This server does not actually have a shell, but it accepts input commands matching the specific ones that the SFTP backend relies on for Unix shells, e.g. md5sum and df. Also it handles the string escape rules used for Unix shell. Treating it as a Unix type shell from a SFTP remote will therefore always be correct, and support all features.

Shell access considerations

The shell type auto-detection logic, described above, means that by default rclone will try to run a shell command the first time a new sftp remote is accessed. If you configure a sftp remote without a config file, e.g. an on the fly remote, rclone will have nowhere to store the result, and it will re-run the command on every access. To avoid this you should explicitly set the shell_type option to the correct value, or to none if you want to prevent rclone from executing any remote shell commands.

It is also important to note that, since the shell type decides how quoting and escaping of file paths used as command-line arguments are performed, configuring the wrong shell type may leave you exposed to command injection exploits. Make sure to confirm the auto-detected shell type, or explicitly set the shell type you know is correct, or disable shell access until you know.

Checksum

SFTP does not natively support checksums (file hash), but rclone is able to use checksumming if the same login has shell access, and can execute remote commands. If there is a command that can calculate compatible checksums on the remote system, Rclone can then be configured to execute this whenever a checksum is needed, and read back the results. Currently MD5 and SHA-1 are supported.

Normally this requires an external utility being available on the server. By default rclone will try commands md5sum, md5 and rclone md5sum for MD5 checksums, and the first one found usable will be picked. Same with sha1sum, sha1 and rclone sha1sum commands for SHA-1 checksums. These utilities normally need to be in the remote's PATH to be found.

In some cases the shell itself is capable of calculating checksums. PowerShell is an example of such a shell. If rclone detects that the remote shell is PowerShell, which means it most probably is a Windows OpenSSH server, rclone will use a predefined script block to produce the checksums when no external checksum commands are found (see shell access). This assumes PowerShell version 4.0 or newer.

The options md5sum_command and sha1_command can be used to customize the command to be executed for calculation of checksums. You can for example set a specific path to where md5sum and sha1sum executables are located, or use them to specify some other tools that print checksums in compatible format. The value can include command-line arguments, or even shell script blocks as with PowerShell. Rclone has subcommands md5sum and sha1sum that use compatible format, which means if you have an rclone executable on the server it can be used. As mentioned above, they will be automatically picked up if found in PATH, but if not you can set something like /path/to/rclone md5sum as the value of option md5sum_command to make sure a specific executable is used.

Remote checksumming is recommended and enabled by default. First time rclone is using a SFTP remote, if options md5sum_command or sha1_command are not set, it will check if any of the default commands for each of them, as described above, can be used. The result will be saved in the remote configuration, so next time it will use the same. Value none will be set if none of the default commands could be used for a specific algorithm, and this algorithm will not be supported by the remote.

Disabling the checksumming may be required if you are connecting to SFTP servers which are not under your control, and to which the execution of remote shell commands is prohibited. Set the configuration option disable_hashcheck to true to disable checksumming entirely, or set shell_type to none to disable all functionality based on remote shell command execution.

Modified time

Modified times are stored on the server to 1 second precision.

Modified times are used in syncing and are fully supported.

Some SFTP servers disable setting/modifying the file modification time after upload (for example, certain configurations of ProFTPd with mod_sftp). If you are using one of these servers, you can set the option set_modtime = false in your RClone backend configuration to disable this behaviour.

About command

The about command returns the total space, free space, and used space on the remote for the disk of the specified path on the remote or, if not set, the disk of the root on the remote.

SFTP usually supports the about command, but it depends on the server. If the server implements the vendor-specific VFS statistics extension, which is normally the case with OpenSSH instances, it will be used. If not, but the same login has access to a Unix shell, where the df command is available (e.g. in the remote's PATH), then this will be used instead. If the server shell is PowerShell, probably with a Windows OpenSSH server, rclone will use a built-in shell command (see shell access). If none of the above is applicable, about will fail.

{{< rem autogenerated options start" - DO NOT EDIT - instead edit fs.RegInfo in backend/sftp/sftp.go then run make backenddocs" >}}

Standard options

Here are the Standard options specific to sftp (SSH/SFTP).

--sftp-host

SSH host to connect to.

E.g. "example.com".

Properties:

• Config: host
• Env Var: RCLONE_SFTP_HOST
• Type: string
• Required: true

--sftp-user

SSH username.

Properties:

• Config: user
• Env Var: RCLONE_SFTP_USER
• Type: string
• Default: "$USER"

--sftp-port

SSH port number.

Properties:

• Config: port
• Env Var: RCLONE_SFTP_PORT
• Type: int
• Default: 22

--sftp-pass

SSH password, leave blank to use ssh-agent.

NB Input to this must be obscured - see rclone obscure.

Properties:

• Config: pass
• Env Var: RCLONE_SFTP_PASS
• Type: string
• Required: false

--sftp-key-pem

Raw PEM-encoded private key.

If specified, will override key_file parameter.

Properties:

• Config: key_pem
• Env Var: RCLONE_SFTP_KEY_PEM
• Type: string
• Required: false

--sftp-key-file

Path to PEM-encoded private key file.

Leave blank or set key-use-agent to use ssh-agent.

Leading ~ will be expanded in the file name as will environment variables such as ${RCLONE_CONFIG_DIR}.

Properties:

• Config: key_file
• Env Var: RCLONE_SFTP_KEY_FILE
• Type: string
• Required: false

--sftp-key-file-pass

The passphrase to decrypt the PEM-encoded private key file.

Only PEM encrypted key files (old OpenSSH format) are supported. Encrypted keys in the new OpenSSH format can't be used.

NB Input to this must be obscured - see rclone obscure.

Properties:

• Config: key_file_pass
• Env Var: RCLONE_SFTP_KEY_FILE_PASS
• Type: string
• Required: false

--sftp-pubkey-file

Optional path to public key file.

Set this if you have a signed certificate you want to use for authentication.

Leading ~ will be expanded in the file name as will environment variables such as ${RCLONE_CONFIG_DIR}.

Properties:

• Config: pubkey_file
• Env Var: RCLONE_SFTP_PUBKEY_FILE
• Type: string
• Required: false

--sftp-key-use-agent

When set forces the usage of the ssh-agent.

When key-file is also set, the ".pub" file of the specified key-file is read and only the associated key is requested from the ssh-agent. This allows to avoid Too many authentication failures for *username* errors when the ssh-agent contains many keys.

Properties:

• Config: key_use_agent
• Env Var: RCLONE_SFTP_KEY_USE_AGENT
• Type: bool
• Default: false

--sftp-use-insecure-cipher

Enable the use of insecure ciphers and key exchange methods.

This enables the use of the following insecure ciphers and key exchange methods:

• aes128-cbc
• aes192-cbc
• aes256-cbc
• 3des-cbc
• diffie-hellman-group-exchange-sha256
• diffie-hellman-group-exchange-sha1

Those algorithms are insecure and may allow plaintext data to be recovered by an attacker.

This must be false if you use either ciphers or key_exchange advanced options.

Properties:

• Config: use_insecure_cipher
• Env Var: RCLONE_SFTP_USE_INSECURE_CIPHER
• Type: bool
• Default: false
• Examples:
  • "false"
    • Use default Cipher list.
  • "true"
    • Enables the use of the aes128-cbc cipher and diffie-hellman-group-exchange-sha256, diffie-hellman-group-exchange-sha1 key exchange.

--sftp-disable-hashcheck

Disable the execution of SSH commands to determine if remote file hashing is available.

Leave blank or set to false to enable hashing (recommended), set to true to disable hashing.

Properties:

• Config: disable_hashcheck
• Env Var: RCLONE_SFTP_DISABLE_HASHCHECK
• Type: bool
• Default: false

Advanced options

Here are the Advanced options specific to sftp (SSH/SFTP).

--sftp-known-hosts-file

Optional path to known_hosts file.

Set this value to enable server host key validation.

Leading ~ will be expanded in the file name as will environment variables such as ${RCLONE_CONFIG_DIR}.

Properties:

• Config: known_hosts_file
• Env Var: RCLONE_SFTP_KNOWN_HOSTS_FILE
• Type: string
• Required: false
• Examples:
  • "~/.ssh/known_hosts"
    • Use OpenSSH's known_hosts file.

--sftp-ask-password

Allow asking for SFTP password when needed.

If this is set and no password is supplied then rclone will:

• ask for a password
• not contact the ssh agent

Properties:

• Config: ask_password
• Env Var: RCLONE_SFTP_ASK_PASSWORD
• Type: bool
• Default: false

--sftp-path-override

Override path used by SSH shell commands.

This allows checksum calculation when SFTP and SSH paths are different. This issue affects among others Synology NAS boxes.

E.g. if shared folders can be found in directories representing volumes:

rclone sync /home/local/directory remote:/directory --sftp-path-override /volume2/directory

E.g. if home directory can be found in a shared folder called "home":

rclone sync /home/local/directory remote:/home/directory --sftp-path-override /volume1/homes/USER/directory

Properties:

• Config: path_override
• Env Var: RCLONE_SFTP_PATH_OVERRIDE
• Type: string
• Required: false

--sftp-set-modtime

Set the modified time on the remote if set.

Properties:

• Config: set_modtime
• Env Var: RCLONE_SFTP_SET_MODTIME
• Type: bool
• Default: true

--sftp-shell-type

The type of SSH shell on remote server, if any.

Leave blank for autodetect.

Properties:

• Config: shell_type
• Env Var: RCLONE_SFTP_SHELL_TYPE
• Type: string
• Required: false
• Examples:
  • "none"
    • No shell access
  • "unix"
    • Unix shell
  • "powershell"
    • PowerShell
  • "cmd"
    • Windows Command Prompt

--sftp-md5sum-command

The command used to read md5 hashes.

Leave blank for autodetect.

Properties:

• Config: md5sum_command
• Env Var: RCLONE_SFTP_MD5SUM_COMMAND
• Type: string
• Required: false

--sftp-sha1sum-command

The command used to read sha1 hashes.

Leave blank for autodetect.

Properties:

• Config: sha1sum_command
• Env Var: RCLONE_SFTP_SHA1SUM_COMMAND
• Type: string
• Required: false

--sftp-skip-links

Set to skip any symlinks and any other non regular files.

Properties:

• Config: skip_links
• Env Var: RCLONE_SFTP_SKIP_LINKS
• Type: bool
• Default: false

--sftp-subsystem

Specifies the SSH2 subsystem on the remote host.

Properties:

• Config: subsystem
• Env Var: RCLONE_SFTP_SUBSYSTEM
• Type: string
• Default: "sftp"

--sftp-server-command

Specifies the path or command to run a sftp server on the remote host.

The subsystem option is ignored when server_command is defined.

Properties:

• Config: server_command
• Env Var: RCLONE_SFTP_SERVER_COMMAND
• Type: string
• Required: false

--sftp-use-fstat

If set use fstat instead of stat.

Some servers limit the amount of open files and calling Stat after opening the file will throw an error from the server. Setting this flag will call Fstat instead of Stat which is called on an already open file handle.

It has been found that this helps with IBM Sterling SFTP servers which have "extractability" level set to 1 which means only 1 file can be opened at any given time.

Properties:

• Config: use_fstat
• Env Var: RCLONE_SFTP_USE_FSTAT
• Type: bool
• Default: false

--sftp-disable-concurrent-reads

If set don't use concurrent reads.

Normally concurrent reads are safe to use and not using them will degrade performance, so this option is disabled by default.

Some servers limit the amount number of times a file can be downloaded. Using concurrent reads can trigger this limit, so if you have a server which returns

Failed to copy: file does not exist

Then you may need to enable this flag.

If concurrent reads are disabled, the use_fstat option is ignored.

Properties:

• Config: disable_concurrent_reads
• Env Var: RCLONE_SFTP_DISABLE_CONCURRENT_READS
• Type: bool
• Default: false

--sftp-disable-concurrent-writes

If set don't use concurrent writes.

Normally rclone uses concurrent writes to upload files. This improves the performance greatly, especially for distant servers.

This option disables concurrent writes should that be necessary.

Properties:

• Config: disable_concurrent_writes
• Env Var: RCLONE_SFTP_DISABLE_CONCURRENT_WRITES
• Type: bool
• Default: false

--sftp-idle-timeout

Max time before closing idle connections.

If no connections have been returned to the connection pool in the time given, rclone will empty the connection pool.

Set to 0 to keep connections indefinitely.

Properties:

• Config: idle_timeout
• Env Var: RCLONE_SFTP_IDLE_TIMEOUT
• Type: Duration
• Default: 1m0s

--sftp-chunk-size

Upload and download chunk size.

This controls the maximum size of payload in SFTP protocol packets. The RFC limits this to 32768 bytes (32k), which is the default. However, a lot of servers support larger sizes, typically limited to a maximum total package size of 256k, and setting it larger will increase transfer speed dramatically on high latency links. This includes OpenSSH, and, for example, using the value of 255k works well, leaving plenty of room for overhead while still being within a total packet size of 256k.

Make sure to test thoroughly before using a value higher than 32k, and only use it if you always connect to the same server or after sufficiently broad testing. If you get errors such as "failed to send packet payload: EOF", lots of "connection lost", or "corrupted on transfer", when copying a larger file, try lowering the value. The server run by rclone serve sftp sends packets with standard 32k maximum payload so you must not set a different chunk_size when downloading files, but it accepts packets up to the 256k total size, so for uploads the chunk_size can be set as for the OpenSSH example above.

Properties:

• Config: chunk_size
• Env Var: RCLONE_SFTP_CHUNK_SIZE
• Type: SizeSuffix
• Default: 32Ki

--sftp-concurrency

The maximum number of outstanding requests for one file

This controls the maximum number of outstanding requests for one file. Increasing it will increase throughput on high latency links at the cost of using more memory.

Properties:

• Config: concurrency
• Env Var: RCLONE_SFTP_CONCURRENCY
• Type: int
• Default: 64

--sftp-set-env

Environment variables to pass to sftp and commands

Set environment variables in the form:

VAR=value

to be passed to the sftp client and to any commands run (eg md5sum).

Pass multiple variables space separated, eg

VAR1=value VAR2=value

and pass variables with spaces in quotes, eg

"VAR3=value with space" "VAR4=value with space" VAR5=nospacehere

Properties:

• Config: set_env
• Env Var: RCLONE_SFTP_SET_ENV
• Type: SpaceSepList
• Default:

--sftp-ciphers

Space separated list of ciphers to be used for session encryption, ordered by preference.

At least one must match with server configuration. This can be checked for example using ssh -Q cipher.

This must not be set if use_insecure_cipher is true.

Example:

aes128-ctr aes192-ctr aes256-ctr aes128-gcm@openssh.com aes256-gcm@openssh.com

Properties:

• Config: ciphers
• Env Var: RCLONE_SFTP_CIPHERS
• Type: SpaceSepList
• Default:

--sftp-key-exchange

Space separated list of key exchange algorithms, ordered by preference.

At least one must match with server configuration. This can be checked for example using ssh -Q kex.

This must not be set if use_insecure_cipher is true.

Example:

sntrup761x25519-sha512@openssh.com curve25519-sha256 curve25519-sha256@libssh.org ecdh-sha2-nistp256

Properties:

• Config: key_exchange
• Env Var: RCLONE_SFTP_KEY_EXCHANGE
• Type: SpaceSepList
• Default:

--sftp-macs

Space separated list of MACs (message authentication code) algorithms, ordered by preference.

At least one must match with server configuration. This can be checked for example using ssh -Q mac.

Example:

umac-64-etm@openssh.com umac-128-etm@openssh.com hmac-sha2-256-etm@openssh.com

Properties:

• Config: macs
• Env Var: RCLONE_SFTP_MACS
• Type: SpaceSepList
• Default:

--sftp-host-key-algorithms

Space separated list of host key algorithms, ordered by preference.

At least one must match with server configuration. This can be checked for example using ssh -Q HostKeyAlgorithms.

Note: This can affect the outcome of key negotiation with the server even if server host key validation is not enabled.

Example:

ssh-ed25519 ssh-rsa ssh-dss

Properties:

• Config: host_key_algorithms
• Env Var: RCLONE_SFTP_HOST_KEY_ALGORITHMS
• Type: SpaceSepList
• Default:

{{< rem autogenerated options stop >}}

Limitations

On some SFTP servers (e.g. Synology) the paths are different for SSH and SFTP so the hashes can't be calculated properly. For them using disable_hashcheck is a good idea.

The only ssh agent supported under Windows is Putty's pageant.

The Go SSH library disables the use of the aes128-cbc cipher by default, due to security concerns. This can be re-enabled on a per-connection basis by setting the use_insecure_cipher setting in the configuration file to true. Further details on the insecurity of this cipher can be found in this paper.

SFTP isn't supported under plan9 until this issue is fixed.

Note that since SFTP isn't HTTP based the following flags don't work with it: --dump-headers, --dump-bodies, --dump-auth.

Note that --timeout and --contimeout are both supported.

rsync.net

rsync.net is supported through the SFTP backend.

See rsync.net's documentation of rclone examples.

Hetzner Storage Box

Hetzner Storage Boxes are supported through the SFTP backend on port 23.

See Hetzner's documentation for details