ProFTPD module mod_sftp

## ProFTPD module `mod_sftp`

SFTP versus FTPS
There is a great deal of confusion and misunderstanding surrounding two very different protocols: SFTP and FTPS.

FTPS stands for "FTP over SSL/TLS". It is the existing FTP protocol, made to run over an SSL/TLS connection; in this manner, it is very similar to HTTPS (HTTP over SSL/TLS). In fact, that is where the name "FTPS" comes from. The `mod_tls` module for ProFTPD implements FTPS. Since FTPS is still FTP, any FTPS sessions require multiple TCP connections: one TCP connection for the FTP control channel, and separate TCP connections for each FTP data channel. The need for these multiple connections is undesirable for many network administrators, especially those that wish to restrict all protocols to a single TCP connection which can be passed through firewalls/NAT/router equipment. The network equipment, now, often inspects the application-level data in FTP packets in order to dynamically open the necessary firewall rules for the FTP data channels. However, FTPS encrypts those packets, and thus the network firewall must resort to using a statically configured range of ports, or not allow FTPS. There is a third solution: the `CCC` FTP command, which clears the FTP control channel of encryption, once the user has authenticated. Unfortunately, support for the `CCC` command is not widespread among FTPS clients and servers; the `mod_tls` module does support the `CCC` command.

By contrast, SFTP refers to "Secure File Transfer Protocol", and is not related to FTP in any way. SFTP is based on the SSH2 protocol, which uses binary encoding of messages over a secure channel. Unlike FTP, SSH2 only uses a single TCP connection, and multiplexes multiple transfers or "channels" over that single connection. For this reason, many sites prefer SFTP to FTPS for secure transfer of data.

SSH2 RFCs and SFTP Drafts
For those wishing to learn more about the SSH2 and SFTP protocols, see:

- SSH Assigned Numbers (RFC 4250)
- SSH Architecture (RFC 4251)
- SSH Authentication (RFC 4252)
- SSH Transport Layer (RFC 4253)
- SSH Connection (RFC 4254)
- SSH Keyboard-Interactive Authentication (RFC 4256)
- SSH Channel "Break" Extension (RFC 4335)
- SSH Transport Encryption Modes (RFC 4344)
- SSH Improved Arcfour Modes (RFC 4345)
- SSH Diffie-Hellman Group Exchange Protocol (RFC 4419)
- SSH RSA Key Exchange Protocol (RFC 4432)
- SSH Public Key File Format (RFC 4716)
- SFTP Draft
- SFTP Extensions Draft
- SSH Elliptic Curve Algorithms (RF C5656)

The `mod_sftp` module for ProFTPD
The `mod_sftp` module implements the SSH2 protocol and its SFTP subsystem, for secure file transfer over an SSH2 connection. The `mod_sftp` module supports:

- Public key authentication
- Password authentication (e.g. user authentication via `mod_sql`, `mod_ldap`, `mod_auth_file`, `mod_auth_unix`, `mod_auth_pam`)
- SCP support
- Quotas (via the `mod_quotatab` module)
- FIPS support (see Usage section)
- Throttled transfers (via `TransferRate`, and/or the `mod_shaper` module)
- Blacklisted public keys
- Configurable traffic analysis protection
- Passphrase-protected host keys
- SFTP extensions: check-file, copy-file, vendor-id, version-select, posix-rename@openssh.com, statvfs@openssh.com, fstatvfs@openssh.com, hardlink@openssh.com

This module supports the SFTP and SCP file transfer protocols; it does not support shell access.

The `mod_sftp` module does not currently support:

- `<Anonymous>`
- `ListOptions`
- `MaxRetrieveFileSize`

The `mod_sftp` module is contained in the `mod_sftp/` directory, is intended for ProFTPD 1.3.3rc1 and later, and is not compiled by default. Installation instructions are discussed here.

The most current version of `mod_sftp` is distributed with the ProFTPD source code.

This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit (http://www.openssl.org/). This product includes cryptographic software written by Eric Young (eay@cryptsoft.com).

## Author

Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.

## Directives

- SFTPAcceptEnv
- SFTPAuthMethods
- SFTPAuthorizedHostKeys
- SFTPAuthorizedUserKeys
- SFTPCiphers
- SFTPClientAlive
- SFTPClientMatch
- SFTPCompression
- SFTPCryptoDevice
- SFTPDHParamFile
- SFTPDigests
- SFTPDisplayBanner
- SFTPEngine
- SFTPExtensions
- SFTPHostKey
- SFTPKeyBlacklist
- SFTPKeyExchanges
- SFTPKeyLimits
- SFTPLog
- SFTPMaxChannels
- SFTPOptions
- SFTPPassPhraseProvider
- SFTPRekey
- SFTPTrafficPolicy

### SFTPAcceptEnv

Syntax: SFTPAcceptEnv env1 ...
Default: LANG
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.3rc3 and later

The `SFTPAcceptEnv` directive specifies which environment variables sent by the client, if any, will be used in the session environment. Environment variables are specified by name, and may contain the '*' and '?' wildcard characters. Multiple environment variables may be configured, either separated by whitespace or spread across multiple `SFTPAcceptEnv` directives.

Be warned that some environment variables may have special meaning to underlying libraries; care should be taken in the use of this directive.

### SFTPAuthMethods

Syntax: SFTPAuthMethods meth1 ...
Default: publickey password
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPAuthMethods` directive configures the list of available authentication methods presented to connecting clients. The list of currently supported methods is:

- hostbased
- keyboard-interactive
- publickey
- password

In general, there is no need to use this directive unless only one specific authentication method must be used. Should it be needed, the list of authentication methods to use is provided as a space-delimited list, e.g.:

      # Offer publickey and keyboard-interactive authentication only
      SFTPAuthMethods publickey keyboard-interactive

The hostbased authentication method is only offered to clients if the `SFTPAuthorizedHostKeys` directive is configured.

The publickey authentication method is only offered to clients if the `SFTPAuthorizedUserKeys` directive is configured.

The keyboard-interactive authentication method is only offered to clients if the `mod_sftp_pam` module is present.

As of `proftpd-1.3.6rc2` and later, you can use the `SFTPAuthMethods` directive to configure chains of methods. An authentication chain is a list of authentication methods, all of which must succeed in order for login to succeed. For example, using:

      # Require both publickey and password authentication
      SFTPAuthMethods publickey+password

Note that order of the methods in a chain is important. In the above example, the publickey authentication must succeed first, before password authentication will be offered. If you want require both publickey and password authentication, but that they can be used in any order, you can configure multiple chains:

      # Require both publickey and password authentication, in any order
      SFTPAuthMethods publickey+password password+publickey

As long as any one chain is completed, login will succeed.

You can even require that multiple different keys be used (e.g. a RSA and a DSA public key, or multiple different RSA/DSA keys) using:

      # Require different keys for publickey authentication
      SFTPAuthMethods publickey+publickey

### SFTPAuthorizedHostKeys

Syntax: SFTPAuthorizedHostKeys store1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPAuthorizedHostKeys` directive configures the file that `mod_sftp` will use during the handling of hostbased authentication of users. The configured file may contain several public keys in RFC4716 format, in no particular order.

Note that a `SFTPAuthorizedHostKeys` directive is required for `mod_sftp` to handle sessions which want to use hostbased authentication properly.

Note: In order to use OpenSSH public keys in a `SFTPAuthorizedHostKeys`, you must convert them to the RFC4716 format (see the Usage section). This is done using OpenSSH's `ssh-keygen` tool:

      $ ssh-keygen -e -f /path/to/file

The output from this command can then be added to the `SFTPAuthorizedHostKeys` file.

### SFTPAuthorizedUserKeys

Syntax: SFTPAuthorizedUserKeys store1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPAuthorizedUserKeys` directive configures the file that `mod_sftp` will use during the handling of publickey authentication of users. The configured file may contain several public keys in RFC4716 format, in no particular order.

Example:

      SFTPAuthorizedUserKeys file:~/.sftp/authorized_keys

Note: In order to use OpenSSH public keys in a `SFTPAuthorizedUserKeys`, you must convert them to the RFC4716 format (see the Usage section). This is done using OpenSSH's `ssh-keygen` tool:

      $ ssh-keygen -e -f ~/.ssh/id_rsa.pub

The output from this command can then be added to the `SFTPAuthorizedUserKeys` file.

The configured path for a file containing authorized keys can use the `%u` variable, which will be interpolated with the name of the user being authenticated. This feature supports having per-user files of authorized keys that reside in a central location, rather than requiring (or allowing) users to manage their own authorized keys. For example:

      SFTPAuthorizedUserKeys file:/etc/sftp/authorized_keys/%u

### SFTPCiphers

Syntax: SFTPCiphers algo1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPCiphers` directive is used to specify the list of cipher algorithms that `mod_sftp` should use. The current list of supported cipher algorithms is, in the default order of preference:

- aes256-ctr
- aes192-ctr
- aes128-ctr
- aes256-cbc
- aes192-cbc
- aes128-cbc
- blowfish-ctr
- blowfish-cbc
- cast128-cbc
- arcfour256
- arcfour128
- 3des-ctr
- 3des-cbc

By default, all of the above cipher algorithms are presented to the client, in the above order, during the key exchange.

The "none" cipher (i.e. no encryption) will not be presented to the client by default; any sites which wish to use "none" will have to explicitly configure it via `SFTPCiphers`.

Note that CTR mode ciphers (e.g. the `aes128-ctr`, `aes192-ctr`, and `aes256-ctr` ciphers) are recommended. Any CBC mode cipher allows for the possibility of an attack which causes several bits of plaintext to be leaked; the attack is described here. This attack is on the SSH2 protocol design itself; any SSH2 implementation which conforms to the RFCs will have this weakness.

In general, there is no need to use this directive unless only one specific cipher must be used.

For comparison, the cipher algorithms used by OpenSSH by default are:

- aes128-cbc
- 3des-cbc
- blowfish-cbc
- cast128-cbc
- arcfour128
- arcfour256
- aes192-cbc
- aes256-cbc
- aes128-ctr
- aes192-ctr
- aes256-ctr

Thus if you wanted to configure `mod_sftp` to present the same cipher algorithms as OpenSSH, you would use:

      # Make mod_sftp present the same ciphers as OpenSSH
      SFTPCiphers aes128-cbc 3des-cbc blowfish-cbc cast128-cbc arcfour128 \
                  arcfour256 aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr

### SFTPClientAlive

Syntax: SFTPClientAlive count interval
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.4rc1 and later

The `SFTPClientAlive` directive configured `mod_sftp` to send messages to a client, through the encrypted channel, to request a response from the client. If count client alive messages are sent without receiving any response messages from the client, the client will be disconnected. The interval indicates how much time, in seconds, that `mod_sftp` waits for data from the client before sending a client alive message.

For example, using:

      SFTPClientAlive 3 15

will cause an unresponsive client to be disconnected after approximately 45 seconds.

### SFTPClientMatch

Syntax: SFTPClientMatch pattern key1 val1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPClientMatch` directive is used to tune some of the SSH2/SFTP values based on the connecting client, for better interoperability with those clients. The pattern parameter specifies a POSIX regular expression which will be matched against the connecting client's SSH version banner. If the client's banner version matches pattern, then the configured keys/values are used for that session.

The currently supported SSH2/SFTP keys which can be tuned via `SFTPClientMatch` are:

- channelPacketSize
  The value for this key can be a number, with an optional "KB" (kilobytes) suffix.

  The default `mod_sftp` channel packet size is 32KB.

- channelWindowSize The value for this key can be a number, with an optional "GB" (gigabytes), "MB" (megabytes), or "KB" (kilobytes) suffix.

  The default `mod_sftp` channel window size is 4GB. Using smaller window sizes may be necessary for some clients; larger window sizes can help reduce latency for bulk data transfers.

- sftpProtocolVersion The value for this key can be a specific version number, e.g. "3", or it can be a range such as "1-3". The `mod_sftp` module supports SFTP protocol versions 1-6; using any other protocol versions will result in bad configuration.

- sftpUTF8ProtocolVersion The value for this key is a specific version number, e.g. "3". The `mod_sftp` module uses UTF8 encoding for handling paths/strings in SFTP messages for protocol version 4 and above by default; this key can be used to change the protocol versions for which UTF8 encoding is used.

  For example, if you set:

          SFTPClientMatch .* sftpUTF8ProtocolVersion 1

  then `mod_sftp` will try to use UTF8 for all SFTP protocol versions, all clients.

  Note: the `sftpUTF8ProtocolVersion` key requires NLS support (i.e. the `--enable-nls` configure option).

For example, to make `mod_sftp` only allow SFTP protocol version 3 to WinSCP clients, you would use:

      SFTPClientMatch ".*WinSCP.*" sftpProtocolVersion 3

Or for older OpenSSH clients, you may need to use a smaller channel window size, e.g.:

      SFTPClientMatch "^OpenSSH_3\\.*" channelWindowSize 8MB

### SFTPCompression

Syntax: SFTPCompression on|off|delayed
Default: off
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPCompression` directive enables the use of zlib compression of the payload of SSH2 packets. This can make for smaller packets, but require more CPU time to compress/uncompress the data.

The delayed parameter tells `mod_sftp` to support a custom extension used by OpenSSH, where compression is not actually enabled until after the client has successfully authenticated.

### SFTPCryptoDevice

Syntax: SFTPCryptoDevice driver|"all"|"none"
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPCryptoDevice` directive is used to configure `mod_sftp` to use an OpenSSL "engine", a cryptographic module that OpenSSL library can use for accelerated cryptographic support, or HSM keys, etc.

To use all of the engines compiled into OpenSSL, use:

      SFTPCryptoDevice all

OpenSSL will find, from the list of supported engines, the first one usable, if any. If no usable engines are found, OpenSSL will default to its normal software implementation. If a specific engine is desired as the default engine to use, specify the engine name, e.g.:

      SFTPCryptoDevice chil

The OpenSSL command

      openssl engine

may be used to list all of the engine drivers supported by OpenSSL.

### SFTPDHParamFile

Syntax: SFTPDHParamFile path
Default: dhparams.pem
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPDHParamFile` directive is used to configure the path to a file containing pre-computed Diffie-Hellman (DH) group parameters. These parameters will be used by `mod_sftp` when handling the Diffie-Hellman group exchange (see RFC 4419) portion of the key exchange.

The `SFTPDHParamFile` is produced using OpenSSL, which uses PEM encodings of PKCS#3 `DHparam` structures. It is similar in nature to OpenSSH's `moduli(5)` file.

The `mod_sftp` source code comes with a `dhparams.pem` file which should be sufficient. If for any reason you find that you need to generate your own `SFTPDHParamFile`, use one of the following OpenSSL commands to create this file:

      # openssl dhparam -outform PEM -2 nbits >> dhparams.pem
      # openssl dhparam -outform PEM -5 nbits >> dhparams.pem

Using either `-2` or `-5` as the generator is fine. The nbits value used should vary between 1024 and 8192, inclusive. Beware: this process is quite slow, and CPU/memory intensive!

### SFTPDigests

Syntax: SFTPDigests algo1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPDigests` directive is used to specify the list of MAC digest algorithms that `mod_sftp` should use. The current list of supported MAC algorithms is:

- hmac-sha2-256
- hmac-sha2-512
- hmac-sha1
- hmac-sha1-96
- hmac-md5
- hmac-md5-96
- hmac-ripemd160
- umac-64@openssh.com
- umac-128@openssh.com

By default, all of the above MAC algorithms are presented to the client, in the above order, during the key exchange. Note that some algorithms (e.g. the SHA256 and SHA512 algorithms) may not be supported by the version of OpenSSL used; this will be automatically detected.

The "none" MAC (i.e. no MAC) will not be presented to the client by default; any sites which wish to use "none" will have to explicitly configure it via `SFTPDigests`.

In general, there is no need to use this directive unless only one specific MAC algorithm must be used.

For comparison, the MAC algorithms used by OpenSSH (depending on the version) by default are:

- hmac-md5
- hmac-sha1
- hmac-sha2-256
- hmac-sha2-512
- hmac-ripemd160
- hmac-sha1-96
- hmac-md5-96
- umac-64@openssh.com
- umac-128@openssh.com

Thus if you wanted to configure `mod_sftp` to present the same MAC algorithms as OpenSSH, you would use:

      # Make mod_sftp present the MAC ciphers as OpenSSH
      SFTPDigests hmac-md5 hmac-sha1 hmac-sha2-256 hmac-sha2-512 \
        hmac-ripemd160 hmac-sha1-96 hmac-md5-96 umac-64@openssh.com \
        umac-128@openssh.com

### SFTPDisplayBanner

Syntax: SFTPDisplayBanner path
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPDisplayBanner` directive is used to configure the path of a file which will be sent to the client prior to authentication. Such files are often used to sent terms of use or other such policy terms to connecting clients.

See the Display files howto for more information on the variables that can be used in an `SFTPDisplayBanner` file.

### SFTPEngine

Syntax: SFTPEngine on|off
Default: off
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPEngine` directive toggles the use of the SSH2/SFTP protocols engine (e.g. `mod_sftp`). This is usually used inside a `<VirtualHost>` section to enable SFTP sessions for a particular virtual host. By default `mod_sftp` is disabled for both the main server and all configured virtual hosts.

### SFTPExtensions

Syntax: SFTPExtensions ext1 ... extN
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.3rc3 and later

The `SFTPExtensions` directive can be used to enable/disable support for the various SFTP extensions supported by `mod_sftp`. The extension names used for the `SFTPExtensions` directive are:

- checkFile
- copyFile
- fsync
- vendorID
- versionSelect
- posixRename
- spaceAvailable
- statvfs
- hardlink
- xattr

All extensions except `vendorID` and `xattr` are enabled by default.

To enable an extension, preface the extension name with a '+' (plus) character; to disable the extension, use a '-' (minus) character prefix. For example:

      # Allow the copy-file extension, but not version-select
      SFTPExtensions +copyFile -versionSelect

### SFTPHostKey

Syntax: SFTPHostKey file|agent:/path|"NoRSA"|"NoDSA"|"NoECDSA"
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPHostKey` directive configures the path to a host key file. The `mod_sftp` module uses the keys thus configured as part of the SSH2 key exchange process.

Note that the `SFTPHostKey` directive is required. You can use either an RSA key, a DSA key, and/or ECDSA keys. These should be the exact same host key files as used by your SSH2 server, e.g.:

      SFTPHostKey /etc/ssh_host_dsa_key
      SFTPHostKey /etc/ssh_host_rsa_key
      SFTPHostKey /etc/ssh_host_ecdsa_key

or the equivalents in the `/usr/local/etc/` directory. Using different host keys between your SSH2 server and `mod_sftp` will only cause confusion and problems for any SFTP clients. Most SFTP clients will assume that the host uses the same keys for SSH2 and SFTP; after all, it is the same host (even the same port), just different services. Thus having different host keys for an SSH2 server and `mod_sftp` will cause clients to constantly switch the host keys they record for the same host (and cause confusion for users).

The `SFTPHostKey` directive, as of `proftpd-1.3.5rc1` and later, can also be used to load host keys from an SSH agent such as OpenSSH's `ssh-agent`. For example:

      # Load all of the keys from the ssh-agent configured for server use
      SFTPHostKey agent:%{env:SSH_AUTH_SOCK}

Using an SSH agent for storing host keys allows for configurations where the host keys are not stored on files on the server system, e.g. the keys can be loaded into the SSH agent from a PKCS#11 token.

The `SFTPHostKey` directive, as of `proftpd-1.3.6rc1` and later, can be used to disable host keys configured via `<Global>`. For example, if you have a global RSA key that you wish to disable in all vhosts, you might use the "NoRSA" flag, e.g.:

      <Global>
        SFTPHostKey /path/to/ssh_rsa_host_key
        ...
      </Global>

      <VirtualHost a.b.c.d>
        ...
        # Configure our per-vhost DSA host key
        SFTPHostKey /path/to/ssh_dsa_host_key

        # ...and disable the global RSA host key
        SFTPHostKey NoRSA
        ...
      </VirtualHost>

### SFTPKeyBlacklist

Syntax: SFTPKeyBlacklist "none"|path
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPKeyBlacklist` directive configures the path to a specially formatted dictionary of known weak public keys. This key blacklist, and the code for using it, was primarily developed by Dmitry V. Levin under sponsorship by CivicActions. For the original posting, and further information, see:

      http://www.openwall.com/lists/oss-security/2008/05/27/3

These known weak public keys arose because of a vulnerability in the OpenSSL packages distributed by Debian:

      http://www.debian.org/security/2008/dsa-1571

The `mod_sftp` source code comes with a `blacklist.dat` file which should be sufficient. If for any reason you find that you need to generate your own `SFTPDKeyBlacklist`, use the `blacklist-encode` program mentioned in the above URLs.

### SFTPKeyExchanges

Syntax: SFTPKeyExchanges algo1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPKeyExchanges` directive is used to specify the list of key exchange algorithms that `mod_sftp` should use. The current list of supported key exchange algorithms is:

- curve25519-sha256@libssh.org
- ecdh-sha2-nistp521
- ecdh-sha2-nistp384
- ecdh-sha2-nistp256
- diffie-hellman-group18-sha512
- diffie-hellman-group16-sha512
- diffie-hellman-group14-sha256
- diffie-hellman-group-exchange-sha256
- diffie-hellman-group-exchange-sha1
- diffie-hellman-group14-sha1
- diffie-hellman-group1-sha1
- rsa1024-sha1

By default, all of the above key exchange algorithms are presented to the client, in the above order, during the key exchange.

In general, there is no need to use this directive unless only one specific key exchange algorithm must be used.

### SFTPKeyLimits

Syntax: SFTPKeyLimits limit1 ...
Default: MinimumRSASize 768 MiniumumDSASize 384 MinimumECSize 160None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.6rc3 and later

The `SFTPKeyLimits` directive is used to configure limits on the sizes of keys (in terms of bits), both host keys and user keys. The currently supported limits are:

- `MinimumDSASize`
- `MinimumECSize`
- `MinimumRSASize`

Examples:

      # Require at least 1024-bit RSA keys; leave other default limits alone
      SFTPKeyLimits MinimumRSASize 1024

      # Require at least 256-bit EC keys, and remove other default limits
      SFTPKeyLimits MinimumECSize 256 MinimumDSASize 0 MinimumRSASize 0

### SFTPLog

Syntax: SFTPLog file|"none"
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPLog` directive is used to specify a log file for `mod_sftp`'s reporting on a per-server basis. The file parameter given must be the full path to the file to use for logging.

Note that this path must not be to a world-writable directory and, unless `AllowLogSymlinks` is explicitly set to on (generally a bad idea), the path must not be a symbolic link.

### SFTPMaxChannels

Syntax: SFTPMaxChannels count
Default: 10
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPMaxChannels` directive is used to configure the maximum number of simultaneously open channels permitted for a client. The default value is 10.

A malicious client might try to abuse the server's resources by opening an large number of SSH2 channels and then not using them. Most well-behaved SSH2 clients only ever open one channel.

### SFTPOptions

Syntax: SFTPOptions opt1 ...
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPOptions` directive is used to configure various optional behavior of `mod_sftp`.

For example:

      SFTPOptions IgnoreSFTPUploadPerms IgnoreSCPUploadPerms PessimisticKexinit

The currently implemented options are:

- `AllowInsecureLogin`

  By default, `mod_sftp` will not allow password or keyboard-interactive authentication if either the 'none' cipher or MAC/digest is selected. (These must be explicitly allowed via `SFTPCiphers` and `SFTPDigests` as well.) However, some sites may deliberately wish to allow such logins, as for e.g. performance testing. To allow such logins/sessions, then, use this option.

  Note that this option first appeared in `proftpd-1.3.5`.

- `AllowWeakDH`

  The `mod_sftp` will not use Diffie-Hellman groups of less than 2048 bits, due to weaknesses that can downgrade the security of an SSH session. If for any reason your SFTP/SCP clients require smaller Diffie-Hellman groups, then use this option.

  Note that this option first appeared in `proftpd-1.3.6rc1`.

- `IgnoreFIFOs`

  By default, `mod_sftp` allows uploading and downloading of FIFOs, just as if they were regular files. However, in some cases this can lead to "hangs" on the SFTP/SCP client end, especially when the FIFO reader/writer processes are not running at the time. Thus to tell `mod_sftp` to ignore/reject requests to read from/write to FIFOs, for both SFTP and SCP, use this option.

  Note that this option first appeared in `proftpd-1.3.6rc3`.

- `IgnoreSCPUploadPerms`

  When an SCP client uploads a file, the desired permissions on the file are sent to the server as part of the upload. (This is different from FTP, which does not include the file permissions in an upload.) If you need more FTP-like functionality for any reason and wish to have `mod_sftp` silently ignore any permissions sent by the SCP client, use this option.

- `IgnoreSCPUploadTimes`

  Use this option to have `mod_sftp` silently ignore any timestamps sent by SCP clients as part of the upload.

  Note that this option first appeared in `proftpd-1.3.5rc4`.

- `IgnoreSFTPSetExtendedAttributes`

  Use this option to have `mod_sftp` silently ignore any extended attributes sent by SFTP clients via the `SETSTAT` or `FSETSTAT` SFTP requests.

  Note that this option first appeared in `proftpd-1.3.6rc3`.

- `IgnoreSFTPSetOwners`

  Use this option to have `mod_sftp` silently ignore any ownership changes sent by SFTP clients via the `SETSTAT` or `FSETSTAT` SFTP requests; these are equivalent to an FTP `SITE CHGRP` command.

  Note that this option first appeared in `proftpd-1.3.5rc1`.

- `IgnoreSFTPSetPerms`

  Use this option to have `mod_sftp` silently ignore any permissions sent by SFTP clients via the `SETSTAT` or `FSETSTAT` SFTP requests; these are equivalent to an FTP `SITE CHMOD` command.

  Note that this option first appeared in `proftpd-1.3.4rc2`.

- `IgnoreSFTPSetTimes`

  Use this option to have `mod_sftp` silently ignore any timestamps sent by SFTP clients via the `SETSTAT` or `FSETSTAT` SFTP requests; these are equivalent to an FTP `SITE UTIMES` command.

  Note that this option first appeared in `proftpd-1.3.4rc4`.

- `IgnoreSFTPUploadExtendedAttributes`

  When an SFTP client uploads a file or creates a directory, the desired extended attributes ("xattrs") on the path are sent to the server as part of the upload. (This is different from FTP, which does not include the file attributes in an upload.) If you need more FTP-like functionality for any reason and wish to have `mod_sftp` silently ignore any extended attributes sent by the SFTP client, use this option.

  Note that this option first appeared in `proftpd-1.3.6rc3`.

- `IgnoreSFTPUploadPerms`

  When an SFTP client uploads a file or creates a directory, the desired permissions on the path are sent to the server as part of the upload. (This is different from FTP, which does not include the file permissions in an upload.) If you need more FTP-like functionality for any reason and wish to have `mod_sftp` silently ignore any permissions sent by the SFTP client, use this option.

- `InsecureHostKeyPerms`

  When this option is used, `mod_sftp` will ignore insecure permissions (i.e. group- or world-readable) on `SFTPHostKey` files.

  Note that this option first appeared in `proftpd-1.3.6rc1`.

- `MatchKeySubject`

  When this option is used, if public key authentication is used, the `mod_sftp` module will check if the public key has a "Subject" header. If so, that the value of that header must match the name of the authenticating user/host.

  Note that this option first appeared in `proftpd-1.3.4rc1`.

- `OldProtocolCompat`

  Older clients identity their protocol versions as "1.99", rather than as "2.0". By default, `mod_sftp` will refuse to handle connections from such clients. To enable compatibility with these clients (which tend to be derived from ssh.com/Tectia code), use this option.

  Note that this option automatically enables the `PessimisticKexinit` SFTPOption as well.

  Note that this option first appeared in `proftpd-1.3.4rc1`.

- `PessimisticKexinit`

  As described here, the `mod_sftp` module tries to reduce the connection latency by optimistically sending the `KEXINIT` key exchange message. However, some SSH clients cannot handle this behavior. Use this option to disable the optimistic sending of the `KEXINIT` message.

  Note that this option first appeared in `proftpd-1.3.4rc1`.

### SFTPPassPhraseProvider

Syntax: SFTPPassPhraseProvider path
Default: None
Context: server config
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPPassPhraseProvider` directive is used to specify an external program which will be called, when `mod_sftp` starts up, for each encrypted host key file. The program will be invoked with two command-line arguments, passed on `stdin` to the program:

      servername:portnumber file

where `servername:portnumber` indicates the server using that encrypted certificate key, and file indicates the host key file in question. The program then must print the corresponding passphrase for the key to `stdout`.

The intent is that this external program can perform any security checks necessary, to make sure that the system is not compromised by an attacker, and only when these checks pass successfully will the passphrase be provided. These security checks, and the way the passphrase is determined, can be as complex as you like.

Example:

      SFTPPassPhraseProvider /etc/ftpd/sftp/get-passphrase

### SFTPRekey

Syntax: SFTPRekey "none"|"required" [[interval bytes] timeout]
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPRekey` directive configures the session rekey policy used by the `mod_sftp` module. When an SSH2 client connects, there is an initial exchange of hostkey, cipher, digest, and compression algorithms, followed by sharing of information which leads to the calculation of shared session keys. These keys are used in the various algorithms. The exchange of algorithms and calculation of new session keys can happen at any time during the SSH2 session; this is known as "rekeying".

By default, `mod_sftp` will only require rekeying when the packet sequence number (for sent or received SSH2 packets) reaches a rather high limit. The digest algorithm used, for example, might begin to leak cryptographically sensitive information if used for too many packets (see RFC 4344). Outside of these sequence number limits, however, `mod_sftp` will not attempt to initiate rekeying.

To prevent `mod_sftp` from attempting rekeying, use:

      SFTPRekey none

Note that disabling rekeying altogether is a Very Bad Idea. While rekeying can cause delays in the SSH2 sesssion, it is intended to guard against certain attacks. You should therefore only disable rekeying for certain SSH2 clients which cannot support rekeying. Please report those clients (including the client version) to the mod_sftp maintainer, so that that client can be added to the list of known clients which cannot handle rekeying.

To require rekeying, simply use:

      SFTPRekey required

With this parameter, `mod_sftp` will default to requiring a rekey within 1 hour (3600 seconds) or after 2GB of data sent or received, whichever happens first. There will be no timeout once a rekeying has been requested.

However, administrators wishing to use different rekey intervals or rekey byte limits can use the optional interval (in seconds) and bytes (in MB) parameters, e.g.:

      # Require rekeying after 30 minutes, or 4 GB
      SFTPRekey required 1800 4096

Finally, the paranoid administrator may wish to set a timeout, in seconds, in which a client must complete a session rekeying, otherwise the client will be disconnected. This would look like:

      # Require rekeying after 30 minutes, or 4 GB.  If the rekeying is not
      # completed within 5 minutes of a rekey request, disconnect the client.
      SFTPRekey required 1800 4096 300

Note that normally such rekey timeouts are not necessary.

### SFTPTrafficPolicy

Syntax: SFTPTrafficPolicy policy
Default: None
Context: server config, `<VirtualHost>`, `<Global>`
Module: mod_sftp
Compatibility: 1.3.2rc2 and later

The `SFTPTrafficPolicy` directive is used to configure a measure of protection against traffic analysis attacks. Such attacks use the number of packets sent, the size of the packets, and the timing of the packets to try to gain information about the nature of the data being transmitted.

The SSH2 protocol includes a particular message, `SSH_MSG_IGNORE`, which all implementations (both clients and servers) must handle by dropping on the floor. Implementations, like `mod_sftp`, can use these messages to add random data to the session in the hopes of making such traffic analyses more difficult.

The `SFTPTrafficPolicy` directive supports the following range of policies; the chances columns list the probability that `mod_sftp` will send an `SSH_MSG_IGNORE` message.

Policy Name | Chances | Checks
"none" | Never | Never
"low" | 1 in 1000 | Every 5 seconds
"medium" | 1 in 100 | Every 5 seconds
"high" | 1 in 10 | Every second
"paranoid" | Always | Every second

The default `SFTPTrafficPolicy` setting is "none".

However, if a client negotiates the use of a CBC cipher, then a defensive traffic policy will automatically be used. RFC 4251, Section 9.3.1, specifically mentions the insertion of `SSH_MSG_IGNORE` messages as a way of mitigating the "Rogaway" attack on CBC ciphers. In order to disable this automatic mitigation behavior, you must explicitly configure a `SFTPTrafficPolicy` setting in your configuration, e.g.

      # Disable any automatic protection when CBC ciphers are used
      SFTPTrafficPolicy none

Note that this is not recommended!

## Installation

The `mod_sftp` module is distributed with ProFTPD. For including `mod_sftp` as a staticly linked module, use:

      $ ./configure --enable-openssl --with-modules=mod_sftp ...

Alternatively, `mod_sftp` can be built as a DSO module:

      $ ./configure --enable-dso --enable-openssl --with-shared=mod_sftp ...

Then follow the usual steps:

      $ make
      $ make install

Note that if the `libsodium` library is available, `mod_sftp` will auto-detect this, which enables other supported algorithms. You can ensure that `mod_sftp` compiles with the `libsodium` library like so:

      $ ./configure --enable-openssl --with-modules=mod_sftp ... \
        --with-includes=/path/to/libsodium/include \
        --with-libraries=/path/to/libsodium/lib

## Usage

Access Controls for SFTP Requests
Some of the requests supported by SFTP have FTP command equivalents. Other requests in SFTP have no FTP command equivalent. This makes the use of `<Limit>` sections for access controls in `mod_sftp` a little interesting.

First, the familiar territory. The following shows the `<Limit>` sections for FTP commands (just as for normal FTP sessions) which are honored by `mod_sftp`:

- `<Limit DELE>`
  Applies to the `REMOVE` SFTP request

- `<Limit MKD XMKD>`
  Applies to the `MKDIR` SFTP request

- `<Limit MLSD>`
  Applies to the `OPENDIR` SFTP request

- `<Limit LIST NLST>`
  Applies to the `READDIR` SFTP request, resulting in an empty directory listing.

- `<Limit RETR>`
  Applies to the `READ` SFTP request

- `<Limit RNFR RNTO>`
  Applies to the `RENAME` SFTP request

- `<Limit RMD XRMD>`
  Applies to the `RMDIR` SFTP request

- `<Limit STOR>`
  Applies to the `WRITE` SFTP request

- `<Limit SITE_CHGRP>`
  Applies to the `SETSTAT` and `FSETSTAT` SFTP requests when they change file ownership

- `<Limit SITE_CHMOD>`
  Applies to the `SETSTAT` and `FSETSTAT` SFTP requests when they change file permissions

Thus you can see that `mod_sftp` will reuse much of your existing `<Limit>`-based access controls for SFTP sessions.

For those SFTP requests which have no direct FTP equivalents elsewhere in `proftpd`, the `mod_sftp` module supports some slightly different `<Limit>` sections. Note that the following does not apply to FTP sessions, nor does it imply that FTP supports these commands:

- `<Limit FSTAT>`
  Applies to the `FSTAT` SFTP request

- `<Limit LSTAT>`
  Applies to the `LSTAT` SFTP request

- `<Limit LOCK>`
  Applies to the `LOCK` and `UNLOCK` SFTP requests

- `<Limit OPEN>`
  Applies to the `OPEN` SFTP request

- `<Limit READLINK>`
  Applies to the `READLINK` SFTP request

- `<Limit SETSTAT>`
  Applies to the `FSETSTAT` and `SETSTAT` SFTP requests

- `<Limit STAT>`
  Applies to the `STAT` SFTP request

- `<Limit SYMLINK>`
  Applies to the `LINK` and `SYMLINK` SFTP requests

Not all versions of SFTP support all of these requests; the `LOCK` and `UNLOCK` requests, for example, appear only in later versions of the SFTP protocol.

What about the `READ` and `WRITE` command groups?

- `<Limit READ>`
  Applies to the `SYMLINK` and `READ` SFTP requests

- `<Limit WRITE>`
  Applies to the `LOCK`, `SETSTAT`, `SYMLINK`, and `WRITE` SFTP requests

Environment Variables
The `mod_sftp` module will set the following environment variables whenever an SSH2 client connects:

- `SFTP`
- `SFTP_LIBRARY_VERSION`
- `SSH_CONNECTION`

In addition, the following environment variables will be set (with appropriate values), once the client has successfully exchanged keys with `mod_sftp`:

- `SFTP_CLIENT_BANNER`
- `SFTP_CLIENT_CIPHER_ALGO`
- `SFTP_CLIENT_COMPRESSION_ALGO`
- `SFTP_CLIENT_MAC_ALGO`
- `SFTP_SERVER_CIPHER_ALGO`
- `SFTP_SERVER_COMPRESSION_ALGO`
- `SFTP_SERVER_MAC_ALGO`

And after the client has successfully authenticated, the following variables may be set, depending on the authentication mechanism used:

- `SFTP_USER_PUBLICKEY_ALGO`
- `SFTP_USER_PUBLICKEY_FINGERPRINT`
- `SFTP_USER_PUBLICKEY_FINGERPRINT_ALGO`

These environment variables are intended for informational purposes, e.g. `ExtendedLog` or `SQLLog` directives.

SSH2 Public Key Formats
SSH2 uses public keys for authentication and message protection. The very popular OpenSSH server uses a particular file format for storing public keys in files. However, that format is not portable; it uses a format very specific to OpenSSH. Thus the `mod_sftp` module uses a different format for public keys, specifically, the format specified in RFC 4716. The hope is that this format is more interoperable with other SSH2/SFTP implementations.

This means that if you wish to use your OpenSSH public keys with the `mod_sftp` module, you will need to convert them from OpenSSH's format to the RFC4716 format. Fortunately, this is supported by OpenSSH's `ssh-keygen` utility, e.g.:

      $ ssh-keygen -e -f ~/.ssh/id_rsa.pub

The output from this command can be added to the `SFTPAuthorizedUserKeys` used by `mod_sftp`.

Note that the RFC4716 format is not to be used with the `SFTPHostKey` directive. Even though OpenSSH's host keys have ".pub" public equivalents, host keys are different from the per-user keys that are used for authentication.

Known Client Issues
The following lists some of the various client interoperability issues that have been encountered, and how to address them.

SSH Channel Window Sizes
Some SFTP client implementations do not support the large SSH channel window size that the `mod_sftp` module uses by default. These clients include:

- WS_FTP Pro
- OpenSSH 3.0 and older
- Axway

This FAQ describes the configuration to use for such clients.

SSH Protocol Version
If you are using an old ssh.com client, or perhaps a client which derives from the ssh.com (or Tectia) code, you may see the following error in the `SFTPLog` when trying to connect to `mod_sftp`:

      Bad protocol version 'SSH-1.99-3.2.9.1 SSH Secure Shell (non-commercial)' from x.x.x.x

To support such clients in `mod_sftp`, use `SFTPOptions` to enable compatibility with older protocol versions, e.g.:

      # Support old ssh.com/Tectia clients
      SFTPOptions OldProtocolCompat

SFTP Protocol Versions
Some SFTP clients do not properly handle some of the newer SFTP protocol versions. These clients include:

- WinSCP
- CuteFTP

For these clients, use a `SFTPClientMatch` rule to restrict the SFTP protocol versions offered by the `mod_sftp` module, e.g.:

      # Only support SFTP protocol version 1 through 3 for WinSCP and CuteFTP
      SFTPClientMatch WinSCP|ClientSftp sftpProtocolVersion 1-3

`KEXINIT` Optimization
Some SFTP clients do not like this KEXINIT optimization that the `mod_sftp` module uses. These clients include:

- `Net::SSH::Perl`, `Net::SFTP` Perl modules
- edtFTPj

For these clients, use this configuration to disable the optimization:

      # Disable the KEXINIT optimization at the cost of latency
      SFTPOptions PessimisticKexinit

Cipher Implementations
Some SFTP clients (most notably the SecureBlackBox libraries) do not implement the `blowfish-ctr` cipher correctly. If your site needs to support such clients, then you will need to use the `SFTPCiphers` directive to configure a list of ciphers which does not include `blowfish-ctr`. Clients with this bug will show up in the `SFTPLog` with this log message:

      client sent buggy/malicious packet payload length, ignoring

However, this log message can also be caused by other factors. It is the combination of this error message, and the negotation of the `blowfish-ctr` cipher, that indicates the use of these buggy clients.

FIPS Compliance
FIPS stands for "Federal Information Processing Standard", and refers to a series of information processing standards issued and used by the US government. When speaking of FIPS and cryptographic software, the pertinent standard in particular is FIPS-140.

How then would you build OpenSSL and `mod_sftp`, and use them such that `mod_sftp` would be FIPS-compliant? Using OpenSSL in FIPS mode requires quite a few steps. First, you would configure `proftpd` to use the `mod_sftp` module as normal, assuming your OpenSSL installation has been compiled with FIPS support:

      ./configure --enable-openssl --with-modules=mod_sftp ...

Compiling `proftpd` requires the following, for FIPS support to work properly:

      make CC=/path/to/openssl/bin/fipsld FIPSLD_CC=gcc

The `FIPSLD_CC` variable should point to your normal C compiler, e.g. `gcc`. The use of this `fipsld` program is mandatory. The FIPS standard requires that the linking process happen a very specific way, involving verification of calculated and expected checksums of compiled code, etc. The OpenSSL packages with FIPS support supply this `fipsld` program which will link the compiled code according to the FIPS specifications. If you do not use `fipsld`, then attempts to use OpenSSL in FIPS mode will fail. For example, you would see the following if starting a `proftpd` daemon which has not been linked using `fipsld` while requesting use of FIPS:

      - mod_sftp/0.0: unable to use FIPS mode: (unknown)
      - Fatal: unable to load module 'mod_sftp.c': Operation not permitted

Now, assuming you have compiled and installed your `proftpd` properly, e.g.:

      make CC=/path/to/openssl/bin/fipsld FIPSLD_CC=gcc
      make CC=/path/to/openssl/bin/fipsld FIPSLD_CC=gcc install

you will now be ready to start `proftpd`.

In order for FIPS mode to be effective, OpenSSL must be told to run in FIPS mode from the very beginning. The `mod_sftp` module initializes the OpenSSL library when the `mod_sftp` module is loaded, before the `proftpd.conf` file is parsed. Thus the requesting of FIPS mode cannot be done via a setting in `proftpd.conf`. (Annoying, I know.)

Instead, you must use the -D command-line parameter when starting `proftpd` (see the docs for the `<IfDefine>` and `Define` directives) to define a specific variable, which the `mod_sftp` module will look for. Specifically, you will need to start `proftpd` like thus:

      /path/to/proftpd -DSFTP_USE_FIPS ...

This will define the `SFTP_USE_FIPS` variable; this tells `mod_sftp` to initialize OpenSSL using FIPS mode. When this works, you will see the following when `proftpd` starts up:

      - mod_sftp/0.0: FIPS mode enabled

When `mod_sftp` detects that OpenSSL has been configured for FIPS mode, it will refuse to advertise support for the following cipher algorithms:

- `arcfour128`
- `arcfour256`
- `blowfish-cbc`
- `blowfish-ctr`
- `cast128-cbc`

and for the following digest algorithms:

- `hmac-md5`
- `hmac-md5-96`
- `hmac-ripemd160`

as these algorithms are not compliant with the FIPS specification.

For additional reading on OpenSSL and FIPS, see:

      http://www.openssl.org/docs/fips/SecurityPolicy-1.1.1.pdf
      http://www.openssl.org/docs/fips/UserGuide-1.1.1.pdf

Example Configuration
When using `mod_sftp` to handle SFTP connections, you will want to create a separate `<VirtualHost>` section for the `mod_sftp` configuration. SSH2 (and thus SFTP) requires a different port than FTP; in order to listen on different ports, `proftpd` requires different `<VirtualHost>` sections. You can have multiple `<VirtualHost>` sections for the same address (IP address or DNS name), just different `Port` directives.

      <IfModule mod_sftp.c>
        <VirtualHost a.b.c.d>
          SFTPEngine on
          SFTPLog /etc/proftpd/sftp/sftp.log

          # Configure the server to listen on the normal SSH2 port, port 22
          Port 22

          # Configure the RSA, DSA, and ECDSA host keys, using the same host key
          # files that OpenSSH uses.
          SFTPHostKey /etc/ssh_host_rsa_key
          SFTPHostKey /etc/ssh_host_dsa_key
          SFTPHostKey /etc/ssh_host_ecdsa_key

          # Configure the file used for comparing authorized public keys of users.
          SFTPAuthorizedUserKeys file:~/.sftp/authorized_keys

          # Enable compression
          SFTPCompression delayed

          # Allow the same number of authentication attempts as OpenSSH.
          #
          # It is recommended that you explicitly configure MaxLoginAttempts
          # for your SSH2/SFTP instance to be higher than the normal
          # MaxLoginAttempts value for FTP, as there are more ways to authenticate
          # using SSH2.
          MaxLoginAttempts 6

        </VirtualHost>
      </IfModule>

Logging
The `mod_sftp` module supports different forms of logging. The main module logging is done via the `SFTPLog` directive. For debugging purposes, the module also uses trace logging, via the module-specific log channels:

- ssh2
- sftp
- scp

Thus for trace logging, to aid in debugging, you would use the following in your `proftpd.conf`:

      TraceLog /path/to/sftp-trace.log
      Trace scp:20 sftp:20 ssh2:20

This trace logging can generate large files; it is intended for debugging use only, and should be removed from any production configuration.

Suggested Future Features
The following lists the features I hope to add to `mod_sftp`, according to need, demand, inclination, and time:

- Look up authorized public keys from LDAP directories
- Honor the `MaxTransferPerHost`, `MaxTransfersPerUser`, `MaxRetrieveFileSize` directives

Frequently Asked Questions

Question: When I telnet to port 22 of other SSH2 servers, I see the SSH2 version banner displayed, e.g.:

      # telnet example.com 22
      Trying 1.2.3.4...
      Connected to example.com.
      Escape character is '^]'.
      SSH-1.99-OpenSSH_5.1

However, when I telnet to the port on which `mod_sftp` is listening, I see the version banner plus some data which can't be rendered in the terminal. What is that extra data?
Answer: The short answer is that `mod_sftp` is pre-emptively sending the first key exchange data; it is this data which is not displayed well.

This pre-emptive sending of the first key exchange message is an optimization. The SSH2 protocol (and many SSH2 servers) start off doing:

     (1) client ------------ connect -------------> server
     (2) client <-------- version banner ---------- server
     (3) client --- client's first key exchange --> server
     (4) client <-- server's first key exchange --- server

So you can see that there's some back and forth. The "first key exchange" messages sent by the client are always the same for that client, likewise the server always sends the same "first key exchange" message (based on its configuration). So the above has several TCP packets and ACKs that need to be sent back and forth.

The `mod_sftp` module avoids one unnecessary TCP packet+ACK by sending its version banner and its first key exchange message in the same TCP packet:

     (1) client ------------ connect -------------> server
     (2) client <-- version banner/1st key exch---- server
     (3) client --- client's first key exchange --> server

This helps cut down on the latency during startup, since `mod_sftp` saves on one TCP round-trip time.

This optimization was added after reading this informative paper on SSH and SSL/TLS by Peter Gutmann:

      Performance Characteristics of Application-level Security Protocols

Question: My SSH2/SFTP client dies when trying to connect to `mod_sftp`. The login succeeds, but then the client is disconnected. The client logs show something like:

      ServerMaxPacket(32768) ServerWindow(-1) (from WS_FTP)

or

      debug1: channel 0: open confirm rwindow -1 rmax 32768 (from OpenSSH_3.0.2p1)

Answer: The issue, in short, involves that "-1" value you see, and the particular client implementation in question. (The example error above is from an old OpenSSH client.)

The SSH2 protocol specifications say that implementations must encode the SSH2 channel "window size" as a 32-bit unsigned number. This means that the maximum window size is effectively 4294967295 (i.e. 2 ^ 32 - 1). This maximum window size is what `mod_sftp` uses by default. However, some client implementations use a signed data type for holding this value. This means that the value that `mod_sftp` sends to the client becomes a negative signed number to the client (the large value "wraps around" to the negative value), and the client chokes.

As another example, the WS_FTP Pro client has this same problem; in the client, you will see something like the following error:

      Started subsystem "sftp" on channel 0760a2ce
      error 84350000 initializing sftp protocol
      Sending channel close message for channel 0760a2ce
      SSH Transport closed.

There are two approaches for handling such cases. You can use the `SFTPClientMatch` directive to set a smaller channel window size for that particular client, e.g.:

      SFTPClientMatch ".*WS_FTP.*" channelWindowSize 256MB
      SFTPClientMatch ".*Axway" channelWindowSize 256MB

or you can use `SFTPClientMatch` to set a general channel window size for all clients:

      SFTPClientMatch .* channelWindowSize 256MB

The larger the channel window size, the better. (Note that the SSH2 channel window size has absolutely no relation to the TCP window size.) The smaller this SSH2 channel window size, the more frequently the peer needs to send a "window size adjustment" SSH2 request (which entails its own TCP packet and ACK) in order to keep the window size open. The larger the channel window size, the fewer "window size adjustment" requests the peer has to send, and thus the fewer TCP packets/ACKs are needed. Since each TCP packet adds latency, the smaller channel window sizes can increase the latency for data transfers.

For this reason, my personal recommendation is to only set smaller channel window sizes for those SSH2/SFTP clients which need it; no need to incur the additional latency if the client supports the larger window sizes.

Question: Can I configure `mod_sftp` to be an OpenSSH subsystem, e.g. by using the following in my `sshd_config` file:

      Subsystem  sftp  /usr/local/sbin/proftpd

Answer: No. The `mod_sftp` module cannot be used as an OpenSSH subsystem because proftpd needs more information about the session than is provided to OpenSSH subsystems. ProFTPD needs to know all of the information about the authenticated user, the SSH2 channel IDs being used for the SFTP/SCP sessions, etc.

Question: How can I configure `proftpd` so that it can handle both FTP and SFTP at the same time?
Answer: The key to doing this is to create a `<VirtualHost>` section just for the `mod_sftp` configuration, listening on the address and port that you wish, e.g.:

      # The FTP configuration
      DefaultAddress a.b.c.d
      Port 21

      <IfModule mod_sftp.c>
        <VirtualHost a.b.c.d>
          # The SFTP configuration
          Port 22

          SFTPEngine on
          ...
        </VirtualHost>
      </IfModule>

Note that if you use the above configuration, and you wish to handle any address requested by incoming connections, use `0.0.0.0` as the a.b.c.d IP address. `0.0.0.0` is known as the "wildcard address", and indicates that ProFTPD wants to listen on any/all addresses, on those ports.

Question: How can I configure `proftpd` so that it only accepts SFTP connections (via `mod_sftp`), and no FTP connections?
Answer: Use the following in your `proftpd.conf`:

      <Limit LOGIN>
        DenyAll
      </Limit>

      <VirtualHost a.b.c.d>
        <Limit LOGIN>
          AllowAll
        </Limit>

        SFTPEngine on
        ...
      </VirtualHost>

The idea is to use the `<Limit LOGIN>` to deny all logins to any/all virtual host sections except for the `mod_sftp` section.

Question: I followed the instructions to convert my authorized keys from OpenSSH to RFC4716 format, yet I see the following error in my `SFTPLog`:

      mod_sftp/0.9.7[31050]: line too long (74) on line 1 of 'authorized_keys'

What can I do to fix this?
Answer: OpenSSH's `ssh-keygen` tool had a bug where the RFC4716 keys it would generate might not comply to the RFC; see the OpenSSH bug filed for this issue.

Chances are that the offending line in the RFC4716 key is a "Comment: " header that is too long. Simply delete that "Comment: " line; the `mod_sftp` ignores this header.

Question: I generated an SSH key using `ssh-keygen`, then added it to my `authorized_keys` file using:

      $ cat id_rsa.pub >> authorized_keys

But logging into `mod_sftp` still prompts me for my password. Why?
Answer: The most likely culprit is that the format of your `authorized_keys` file is not what `mod_sftp` expects. Per the `SFTPAuthorizedUserKeys` documentation, `mod_sftp` expects that the keys be in RFC 4716 format. If your `authorized_keys` file contains data that looks like this:

      ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDN1tCK5uHUGl....

rather than looking like this:

      ---- BEGIN SSH2 PUBLIC KEY ----
      AAAAB3NzaC1yc2EAAAADAQABAAABAQDN1tCK5uHUGlpxye+KEmB0EIr9zqvMmgyNdWvJBW
      ...
      PZ1+hOi5QECWQYD0BpiWvj
      ---- END SSH2 PUBLIC KEY ----

Then the format of some of the keys in your `authorized_keys` file is wrong. Make sure that you convert your OpenSSH-generated keys to RFC 4716 format using `ssh-keygen`, before adding it to your `authorized_keys` file, using something like this:

      $ ssh-keygen -e -f ~/.ssh/id_rsa.pub >> authorized_keys

See the Usage documentation for more information.

Question: I'm using the `MaxStoreFileSize` directive in my `mod_sftp` configuration, but when I upload a file using e.g. OpenSSH's `sftp` client, I see the file uploaded past my configured limit. Is this a bug?
Answer: No. The `mod_sftp` module honors a configured `MaxStoreFileSize` limit properly. However, what you see in the SFTP client may not reflect this as you would expect. Some SFTP clients, like that from OpenSSH, will only reflect the failure after the entire file has been uploaded:

      sftp> put /path/to/file.dat
      Uploading /path/to/file.dat to /file.dat
      /path/to/file.dat                           100%  704KB 704.1KB/s   00:00
      Couldn't write to remote file "/file.dat": Failure

In the `SFTPLog` for the above situation, you would most likely see log messages about the SFTP `WRITE` requests being rejected:

      mod_sftp/0.9.9[32297]: error writing 32768 bytes to '/file.dat': File too large (MaxStoreFileSize 204800 exceeded)
      mod_sftp/0.9.9[32297]: error writing 32768 bytes to '/file.dat': File too large (MaxStoreFileSize 204800 exceeded)

There is a discrepancy between what the client shows the user, and what the server is actually doing. The `mod_sftp` module, when handling SFTP uploads, checks the size of the file on disk, after receiving the buffer of data from the network, but before writing that data to disk. If the size of the file on disk exceeds the `MaxStoreFileSize` limit, then that SFTP `WRITE` request is denied.

Many SFTP clients will send multiple SFTP `WRITE` requests concurrently, optimistically hoping that they will succeed; this makes for faster SFTP uploads. This is why you might see multiple "File too large" messages logged by `mod_sftp`. And it also means that the client thinks it has sent more data to the server than has actually been written to disk.

Now, this may lead you to ask "Due to SFTP protocol itself, there is no way to control the file size uploaded by a client, right?" This is not quite correct.

There's no way for an SFTP server to control how much data the SFTP client will send. But there is a way to control how much data is written to disk by the SFTP server; that is what `MaxStoreFileSize` does. But this means that the SFTP client may report how much data it sent over the network, which is not the same as how much data was actually stored by the SFTP server. Believing what your client shows you, though, is always suspect, since what the client shows may not actually reflect the reality of things on the server side of things.

Question: Is it true that using CBC mode ciphers in SSH is insecure?
Answer: It is true that there are a couple of published theoretical attacks against the SSH protocol when CBC ciphers are used, including Rogaway, Wai, and Bellare (see RFC 4251, Section 9.3.1) and "Plaintext Recovery Attacks Against SSH" (CPNI-957037).

However, these attacks can be mitigated by countermeasures in the SSH implementation. The use of `SSH_MSG_IGNORE` messages for the Rogaway attack, and the delayed checking of packet length fields until the MAC check has passed (and then reading in a random amount of additional data from the network) to migitate CPNI-957037, are both implemented within the `mod_sftp` module as security measures.

Question: I am trying to use `ListOptions` to change what is displayed for files in my SFTP clients, but it is not working. Why?
Answer: Unlike FTP, in SFTP it is the client, not the server, which determines what to display to the end user. Thus the `ListOptions` directive is not honored by `mod_sftp`.

Question: If I use a configuration like this:

      <Directory /path/to/upload>
        <Limit WRITE>
          AllowAll
        </Limit>

        <Limit SITE_CHMOD>
          DenyAll
        </Limit>
      </Directory>

then why do my SFTP uploads fail, when the same configuration allows uploads to succeed using FTP?
Answer: When a file is uploaded using FTP, the only parts of the file uploaded are its contents but not the file metadata (e.g. not the file timestamps or permissions). Separate FTP commands are used to sent the file metadata to the server.

However, the SFTP protocol sends the file metadata as part of the normal upload. In the configuration above, the limit on `SITE CHMOD` is being applied to the SFTP upload. If you do not want this, and want your SFTP uploads to be more like your FTP uploads, then you should use:

      SFTPOptions IgnoreSFTPUploadPerms

in your `mod_sftp` configuration.

Question: I use `UserOwner` and `GroupOwner` in my `mod_sftp` configuration, but they are not being applied properly. Is this a bug?
Answer: No.

By default, the `mod_sftp` module drops root privileges as soon as it can, which is just after the user has been authenticated. Without root privileges, `mod_sftp` cannot change the ownership of files uploaded via SFTP/SCP as per `UserOwner`/`GroupOwner` settings.

In ProFTPD 1.3.4rc1, honoring of the `UserOwner` and `GroupOwner` directives was added to the `mod_sftp` module. However, you will also need to add the following to your `mod_sftp` configuration to make them work properly:

      RootRevoke off

This explicitly tells the `mod_sftp` to not drop root privileges after authentication, and instead to keep them for the duration of the session.

In addition, if you are on a Linux machine, you may also need to disable the `mod_cap` module, as that can also restrict what can be done with root privileges:

      <IfModule mod_cap.c>
        CapabilitiesEngine off
      </IfModule>

Question: Why can't I use `ssh` to connect to my proftpd+mod_sftp server? When I try, I see:

      # ssh user@host
      Enter passphrase for key '/home/user/.ssh/id_rsa':
      PTY allocation request failed on channel 0
      shell request failed on channel 0

Answer: The `mod_sftp` module supports file transfers via SFTP and SCP. It does not support shell access, which is what the `ssh` command-line client tries to use. The above error messages from `ssh` show that `mod_sftp` refused the shell access requests.

Question: When I try to start proftpd with `mod_sftp` configured to use my existing host key, it fails with one of the following error messages:

      error reading passphrase for SFTPHostKey 'hostkey': Invalid argument
      error reading passphrase for SFTPHostKey 'hostkey': (unknown)

The permissions on my hostkey are fine, and it is not passphrase-protected. Is this a bug?
Answer: You are probably trying to use a host key generated by Tectia's SSH software (or an older `ssh.com` host key).

The `mod_sftp` module expects the configured `SFTPHostKey` file to be in the same format as used by OpenSSH. To check the format, run this command:

      # grep BEGIN hostkey

If you see either of the following lines:

      -----BEGIN RSA PRIVATE KEY-----
      -----BEGIN DSA PRIVATE KEY-----

then the format should be correct. If instead you see:

      ---- BEGIN SSH2 ENCRYPTED PRIVATE KEY ----

then the format is incorrect. The above line indicates that your host key was generated by Tectia (nee `ssh.com`), and needs to be converted to a different format.

To convert the format of your Tectia host key, use OpenSSH's `ssh-keygen` (not Tectia's `ssh-keygen`) and do the following:

      # ssh-keygen -i -f hostkey > /path/to/new-file

The configure your `SFTPHostKey` directive with the path to the new file.

Question: When I start `proftpd`, I see warnings that look like this:

      - mod_sftp/0.9.7: compiled using OpenSSL version 'OpenSSL 0.9.8n 24 Mar 2010' headers, but linked to OpenSSL version 'OpenSSL 0.9.8q 2 Dec 2010' library

What does this mean?
Answer: That is an informational/warning message.

Some systems are badly maintained by their admins (and/or by the packages installed on the systems), such that the OpenSSL headers can become quite badly out of sync with the OpenSSL libraries. If this discrepancy becomes bad enough, you can see strange behavior from OpenSSL, ranging from random behavior to segfaults. So `mod_sftp` tries to let the admin know about the system's mismatched OpenSSL header/library versions.

Usually a minor OpenSSL version difference like the example above is OK, but it really depends on exactly what changed in OpenSSL, and how.

If you see the above message, it is not a requirement that you recompile `proftpd` against the OpenSSL headers of the same version as the OpenSSL libraries. However, the version discrepancy is a possible source of trouble.

Question: When a client first connects to `mod_sftp` and starts the SSH handshake, the client sees that `mod_sftp` provides an SSH version identifier like "SSH-2.0-mod_sftp/0.9.8". Is there any way I can tell `mod_sftp` to hide/change that version?
Answer: Yes. As of `proftpd-1.3.5rc1`, the `mod_sftp` module will honor the `ServerIdent` directive.

For example, to see just "SSH-2.0-mod_sftp", without any `mod_sftp` version suffix, use the following within your `mod_sftp` `<VirtualHost>`:

      # Tell mod_sftp to not reveal its module version in the SSH version
      # identification
      ServerIdent off

You can also use `ServerIdent` to make `mod_sftp` say it is something else completely, like this:

      # Tell mod_sftp to advertise itself as an OpenSSH server
      ServerIdent on "OpenSSH_5.6p1"

Question: I notice that when I use some SFTP clients (e.g. DreamWeaver) with `mod_sftp`, the network performance is 5-6 times slower than with other SFTP clients. Using the same SFTP client against OpenSSH doesn't show the same slowdown. Network tools such as Wireshark show that `mod_sftp` is sending 2 TCP packets for each SFTP request, whereas OpenSSH only sends 1 TCP packet for each SFTP request. Why is `mod_sftp` so much slower than OpenSSH, but only for some SFTP clients?
Answer: Investigations into this `mod_sftp` behavior have shown it to be caused by the `SFTPTrafficPolicy` behavior, particularly when the SFTP client negotiates the use of a CBC cipher.

The recommended action to take is to disable the use of CBC ciphers, which will cause the `SFTPTrafficPolicy` mitigation to not go into effect, which keeps the number of SSH messages lower, thus improving network latency. Thus the change to make for your `mod_sftp` configuration is:

      # Remove the CBC ciphers from the ciphers we advertise
      SFTPCiphers aes256-ctr aes192-ctr aes128-ctr blowfish-ctr arcfour256 arcfour128 3des-ctr

Question: When I use `mod_sftp` and get a directory listing, I notice that the timestamps on the files are localised (e.g. are in French). When I use OpenSSH, the timestamps are always in English. How I get get `mod_sftp` to behave like OpenSSH, and not localise the timestamps?
Answer: This happens when your `proftpd` has been built with the `--enable-nls` configure option. To check, use ``proftpd -V'`:

      ...
      Features:
        ...
        + NLS support
        ...

This means that `proftpd` honors any locale-related environment variables when it starts up. The reason you see localised timestamps in the directory listings is probably because your environment has something like `LANG=fr_FR.utf8`.

To tell `proftpd` to ignore that environment variable, you need to use this at the top of your `proftpd.conf`:

      Unsetenv LANG

Question: My SFTP client wants to use SFTP protocol version 6, but `mod_sftp` only accepts protocol version 3. These docs say that protocol versions 1-6 are supported. Is this a bug?
Answer: The most likely cause of this behavior is that your `proftpd` installation was not compiled with the `--enable-nls` configure option. The `--enable-nls` configure option is required to enable NLS support, which includes support for UTF8. SFTP protocol versions 4-6 require UTF8 support, which is why the `--enable-nls` configure option is needed.

Question: Why is my `SFTPLog` often spammed with lots of messages that look like:

      requested read offset (20480 bytes) greater than size of '/some/path/to.file' (20307 bytes)

Is there something wrong with my SFTP client, or my configuration?
Answer: The short answer is that these extraneous `READ` requests are normal, and do not indicate any problem or issue.

Many SFTP clients try to improve their download speed by issuing many concurrent `READ` requests, each request with a different offset. Thus the client can read multiple "chunks" simultaneously, and do the reassembly of the "chunks" into the file itself. However, sometimes these SFTP clients do not necessarily know how many such `READ` requests are needed before the end-of-file is reached. So the SFTP client sends more `READ`s that might be necessary; the server will reply with an error if the requested offset is beyond the end of the file. When this "end of file" reply is received by the SFTP client, it will know that it is done downloading. On the server side, though, this behavior results in the above log messages.

Question: When I use my SFTP client, I see something like this when I try to connect to `proftpd` with `mod_sftp`:

      500 SSH-2.0-OPENSSH 6.2 not understood

Why would this happen?
Answer: A "500 ... not understood" is an FTP response, not an SFTP response. This usually happens when your SFTP client connects to an FTP virtual host/port (versus an SFTP virtual host/port). So either a) the SFTP client is connecting to the wrong port on your server, or b) perhaps "SFTPEngine on" is not set in the virtual host for that port.

Question: I try to connect to `proftpd` with `mod_sftp` and the connection fails with an error messages like "PTY allocation request failed on channel 0" or "Server refused to allocate pty". What is going wrong?
Answer: This error message happens when you try to connect to `mod_sftp` using an SSH client which wants a shell session, rather than an SFTP session or an SCP transfer. The `mod_sftp` module does not support shell sessions (unlike OpenSSH). In the `SFTPLog`, you would see such connection attempts logged like so:

      mod_sftp/0.9.9[6304]: unsupported 'pty-req' channel requested, ignoring
      mod_sftp/0.9.9[6304]: unsupported 'shell' channel requested, ignoring

Question: When I create a directory via SFTP, in a directory with the SGID bit set, the created directory does not have the SGID bit set as expected. How do I fix this?
Answer: Preserving the SGID bit for a created directory usually require root privs; the `mod_sftp` module automatically drops all root privs. So to get the directory SGID bit functionality, you will need to use the following in your `mod_sftp` `<VirtualHost>` section:

      RootRevoke off

In addition, if your `proftpd` is running on Linux, you may need to disable the `mod_cap` module, or configure it to preserve the SGID bit (e.g. via the `CAP_FSETID` capability).

Question: In my SFTP logs, I see these warnings:

      WARNING: unable to read SFTPDHParamFile 'path': Permission denied
      unable to open SFTPKeyBlacklist 'path': Permission denied

What can I do to fix this?
Answer: This indicates that the files are not readable (usually world-readable) by the users logging in, or that the directories containing the files are not readable by the users.

Fortunately `mod_sftp` will function without issue if it cannot read either the `SFTPDHParamFile` or the `SFTPKeyBlacklist` files.

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