Net::SFTP::SftpServer(3) A Perl implementation of the SFTP subsystem with user access controls

SYNOPSIS


use Net::SFTP::SftpServer;
my $sftp = Net::SFTP::SftpServer->new();
$sftp->run();

DESCRIPTION

A Perl port of sftp-server from openssh providing access control on a per user per command basis and improved logging via syslog

The limitations compared with the openssh implementation are as follows:

  • Only files and directories are dealt with - other types are not returned on readdir
  • a virtual chroot is performed - / is treated as the users home directory from the client perspective and all file access to / will be in /<home_path>/<username> home_path is defined on object initialisation not accessed from /etc/passwd The script DOES NOT run under chroot - this prevents it needing SUID to start. The virtual chroot is enforced by the objects and prevent opperations outside the home area
  • all sym linked files or directories are hidden and not accessible on request
  • symlink returns permission denied. Please contact me if you need this functionaility implementing
  • readlink returns file does not exist. Please contact me if you need this functionaility implementing
  • setting of stats (set_stat or set_fstat) is disabled - client will receive permission denied. Please contact me if you need this functionaility implementing
  • permissions for file or dir is defaulted - default set on object initialisation

USAGE

Basic usage: 

  use Net::SFTP::SftpServer;

Import options: 

  :LOG    - Import logging functions for use in callbacks
  :ACTION - Import constants for Allow/Deny of actions

Configuring syslog:

Syslog output mode must be configured in the use statement of the module as follows: 

  use Net::SFTP::SftpServer ( { log => 'local5' }, qw ( :LOG :ACTIONS ) );

Net::SFTP::SftpServer will default to using "daemon" see your system's syslog documentation for more details

Options for object initialisation:

  • debug

    Log debug level information. Deault=0 (note this will create very large log files - use with caution)

  • home

    Filesystem location of user home directories. default=/home

  • file_perms

    Octal file permissions to force on creation of files. Default=0666 or permissions specified by file open command from client

  • dir_perms

    Octal dir permissions to force on creation of directories. Default=0777 or permissions specified by mkdir command from client

  • on_file_sent, on_file_received

    References to callback functions to be called on complete file sent or received. Function will be passed the full path and filename on the filesystem as a single argument

  • use_tmp_upload

    Use temporary upload filenames while a file is being uploaded - this allows a monitoring script to know which files are in transit without having to watch file size. Will be done transparantly to the user, the file will be renamed to the original file name when close. The temportary extension is ``.SftpXFR.$$''. Default=0

  • max_file_size

    Maximum file size (in bytes) which can be uploaded. Default=0 (no limit)

  • valid_filename_char

    Array of valid characters for filenames

  • allow, deny

    Actions allowed or denied - see ``PERMISSIONS'' for details, Default is to allow ALL.

  • fake_ok

    Array of actions (see action contants in ``PERMISSIONS'') which will be given response SSH2_FX_OK instead of SSH2_FX_PERMISSION_DENIED when denied by above deny options. Default=[]

  • log_action_supress

    Array of actions to log quietly (logDetail - syslog debug level), logs messages whenever this action is performed. Default is quiet for SSH2_FXP_READ, SSH2_FXP_WRITE, SSH2_FX_OPENDIR, SSH2_FXP_READDIR, SSH2_FXP_CLOSE, SSH2_FXP_STAT SSH2_FXP_FSTAT and SSH2_FXP_LSTAT override with log_action, see below.

  • log_action

    Array of actions to log loudly (logGeneral - syslog info level), logs messages whenever this action is performed.

  • log_all_status

    Log all status messages at info level. By default SSH2_FX_OK and SSH2_FX_EOF will be logged at debug level.

PERMISSIONS

  ALL                      - All actions
  NET_SFTP_SYMLINKS        - Symlinks in paths to files (recommended deny to enforce chroot)
  NET_SFTP_RENAME_DIR      - Rename directories (recommended deny if also denying SSH2_FXP_MKDIR)
  SSH2_FXP_OPEN
  SSH2_FXP_CLOSE
  SSH2_FXP_READ
  SSH2_FXP_WRITE
  SSH2_FXP_LSTAT
  SSH2_FXP_STAT_VERSION_0
  SSH2_FXP_FSTAT
  SSH2_FXP_SETSTAT         - Automatically denied, not implemented in module
  SSH2_FXP_FSETSTAT        - Automatically denied, not implemented in module
  SSH2_FXP_OPENDIR
  SSH2_FXP_READDIR
  SSH2_FXP_REMOVE
  SSH2_FXP_MKDIR
  SSH2_FXP_RMDIR
  SSH2_FXP_STAT
  SSH2_FXP_RENAME
  SSH2_FXP_READLINK        - Automatically denied, not implemented in module
  SSH2_FXP_SYMLINK         - Automatically denied, not implemented in module

CALLBACKS

Callback functions can be used to perform actions when files are sent or received, for example move a fully downloaded file to a processed directory or move a received file into an input directory. The callback is proided with a Net::SFTP::SftpServer::File object. This object allows access to the file within the virtual chroot environment. It will also return the full filename, or move the file to an explicit location on the full filesystem. Either of these actions will break the chroot and the methods on the object will no longer be available.

The following methods are provided

  • read

    Read the data from the file - as the IO::File->read. Will open the file for reading if it is not already open and read back the data.

  • open

    Will open the file - as IO::File->open but the filename is not supplied.

  • getFilename

    Will return the filename as within the virtual chroot

  • getFullFilenameBREAKCHROOT

    Will return the full filename on the real file system and break the virtual chroot

  • renameBREAKCHROOT

    Takes a single argument of the new filename, will rename the file to that location and break the virtual chroot

LOGGING

If :LOG is used when including Net::SFTP::SftpServer the following logging functions will be available: 

  logError    - syslog with a log level of error
  logWarning  - syslog with a log level of warning
  logGeneral  - syslog with a log level of info
  logDetail   - syslog with a log level of debug, unless object was created with debug=>1 then syslog with a level of info

HARDENED EXAMPLE SCRIPT

The following example script shows how this module can be used to give far greater control over what is allowed on your SFTP server.

This setup is aimed at admins which want to user SFTP uploads but do not wish to grant users a system account. You will also need to set both the SFTP subsystem and the user's shell to the sftp script, eg /usr/local/bin/sftp-server.pl

This configuration:

  • Enforces that users can only access the sftp script, not an ssh shell.
  • Chroots them into their home directory in /var/upload/sftp
  • Sets all file permissions to 0660 and does not permit users to change them.
  • Does not allow symlinks, making directories or renaming directories, but allows all other normal actions.
  • Has a max upload filesize of 200Mb
  • Has a script memory limit of 100Mb for safety
  • Will log actions by user sftptest in debug mode
  • Will only allow alphanumeric plus _ . and - in filenames
  • Will call ActionOnSent and ActionOnReceived respectively when files have been sent or received. 

  #!/usr/local/bin/perl
  use strict;
  use warnings;
  use Net::SFTP::SftpServer ( { log => 'local5' }, qw ( :LOG :ACTIONS ) );
  use BSD::Resource;        # for setrlimit
  use constant DEBUG_USER => {
    SFTPTEST => 1,
  };
  # Security - make sure we have started this as sftp not ssh
  unless ( scalar @ARGV == 2 and
           $ARGV[0] eq '-c'  and
           ($ARGV[1] eq '/usr/local/bin/sftp-server.pl') ){
         logError "SFTP connection attempted for application $ARGV[0] - exiting";
         print "\n\rYou do not have permission to login interactively to this host.\n\r\n\rPlease contact the system administrator if you believe this to be a configuration error.\n\r";
         exit 1;
  }
  my $MEMLIMIT = 100 * 1024 * 1024; # 100 Mb
  # hard limits on process memory usage;
  setrlimit( RLIMIT_RSS,  $MEMLIMIT, $MEMLIMIT );
  setrlimit( RLIMIT_VMEM, $MEMLIMIT, $MEMLIMIT );
  my $debug = (defined DEBUG_USER->{uc(getpwuid($>))} and DEBUG_USER->{uc(getpwuid($>))}) ? 1 : 0;
  my $sftp = Net::SFTP::SftpServer->new(
    debug               => $debug,
    home                => '/var/upload/sftp',
    file_perms          => 0660,
    on_file_sent        => \&ActionOnSent,
    on_file_received    => \&ActionOnReceived,
    use_tmp_upload      => 1,
    max_file_size       => 200 * 1024 * 1024,
    valid_filename_char => [ 'a' .. 'z', 'A' .. 'Z', '0' .. '9', '_', '.', '-' ],
    deny                => ALL,
    allow               => [ (
                                SSH2_FXP_OPEN,
                                SSH2_FXP_CLOSE,
                                SSH2_FXP_READ,
                                SSH2_FXP_WRITE,
                                SSH2_FXP_LSTAT,
                                SSH2_FXP_STAT_VERSION_0,
                                SSH2_FXP_FSTAT,
                                SSH2_FXP_OPENDIR,
                                SSH2_FXP_READDIR,
                                SSH2_FXP_REMOVE,
                                SSH2_FXP_STAT,
                                SSH2_FXP_RENAME,
                             )],
    fake_ok             => [ (
                                SSH2_FXP_SETSTAT,
                                SSH2_FXP_FSETSTAT,
                             )],
  );
  $sftp->run();
  sub ActionOnSent {
    my $fileObject = shift;
     ## Do Stuff
  }
  sub ActionOnReceived {
    my $fileObject = shift;
     ## Do Stuff
  }

DEPENDENCIES

  Stat::lsMode
  Fcntl
  POSIX
  Sys::Syslog
  Errno

AUTHOR

  Simon Day, Pirum Systems Ltd
  cpan <at> simonday.info

COPYRIGHT AND LICENSE

Based on sftp-server.c Copyright (c) 2000-2004 Markus Friedl. All rights reserved.

Ported to Perl and extended by Simon Day Copyright (c) 2009 Pirum Systems Ltd. All rights reserved.

Permission to use, copy, modify, and distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED ``AS IS'' AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

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