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:

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:

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

The mod_sftp module does not currently support:

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

Syntax: SFTPAcceptEnv env1 ...
Default: LANG
Context: "server config", <VirtualHost>, <Gl obal>
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:

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.


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:

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:

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:

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 RFC4419) 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:

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:

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


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.


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:

All extensions except vendorID 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
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.


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:

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.


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. Note: all of the configured SFTPOptions parameters must appear on the same line in the configuration; only the first SFTPOptions directive that appears in the configuration is used.

For example:

  SFTPOptions IgnoreSFTPUploadPerms IgnoreSCPUploadPerms PessimisticKexinit

The currently implemented options are:


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 RFC4344). 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

To install mod_sftp, go to the third-party module area in the proftpd source code and unpack the mod_sftp source tarball:
  cd proftpd-dir/contrib/
  tar zxvf /path/to/mod_sftp-version.tar.gz
after unpacking the latest proftpd-1.3.2 source code. For including mod_sftp as a staticly linked module:
  ./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


Usage

Compiler Warnings
When compiling mod_sftp, you may notice the following compiler warning:

  contrib/mod_sftp/mod_sftp.a(fxp.o): In function `fxp_handle_create':
  fxp.c:(.text+0x4dc8): warning: the use of `mktemp' is dangerous, better use `mkstemp'
This warning can be ignored.

The mktemp(3) function is generally considered unsafe because, in the past, many applications would use this function to create the names of temporary files. The actual problem was not the mktemp(3) function itself, but rather that the applications did not check to see if the name generated by mktemp(3) already existed in filesytem first. This lead to race conditions where local users could create symlinks of the generated names, and the application would overwrite the symlinked file.

In the case of mod_sftp, however, the mktemp(3) function is not used to create files. Instead, it is used to create arbitrary unique strings that are used as "handles", as references, in the protocol. These strings have no relation to any actual path on the filesystem, and thus do not suffer from the above race condition.

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:

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:

Note that not all versions of SFTP support all of these requests; the LOCK and UNLOCK requests, for example, appear only in later versions of SFTP.

What about the READ and WRITE command groups?

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

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

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:

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:

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:

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:

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 "scp", "sftp", and "ssh2" log channels. 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:

    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>
    

    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: 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 RFC4251, 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.

    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: 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).



    Author: $Author: castaglia $
    Last Updated: $Date: 2014/03/03 05:40:13 $


    © Copyright 2008-2014 TJ Saunders
    All Rights Reserved