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Manual Reference Pages  -  TNFTPD (8)

NAME

tnftpd - Internet File Transfer Protocol server

CONTENTS

Synopsis
Description
     User authentication
     Display file escape sequences
     Setting up a restricted ftp subtree
Files
See Also
Standards
History
Bugs
Security Considerations

SYNOPSIS

tnftpd [-46DdHlnQqrsUuWwX] [-a anondir] [-C user [@ host]] [-c confdir] [-e emailaddr] [-h hostname] [-L xferlogfile] [-P dataport] [-V version]

DESCRIPTION

tnftpd is the Internet File Transfer Protocol server process. The server uses the TCP protocol and listens at the port specified in the "ftp" service specification; see services(5).

Available options:
-4 When -D is specified, bind to IPv4 addresses only.
-6 When -D is specified, bind to IPv6 addresses only.
-a anondir
  Define anondir as the directory to chroot(2) into for anonymous logins. Default is the home directory for the ftp user. This can also be specified with the ftpd.conf(5) chroot directive.
-C user [@ host]
  Check whether user ( as if connecting from host, if provided ) would be granted access under the restrictions given in ftpusers(5), and exit without attempting a connection. tnftpd exits with an exit code of 0 if access would be granted, or 1 otherwise. This can be useful for testing configurations.
-c confdir
  Change the root directory of the configuration files from "/usr/local/etc" to confdir. This changes the directory for the following files: /usr/local/etc/ftpchroot, /usr/local/etc/ftpusers, /usr/local/etc/ftpwelcome, /usr/local/etc/motd, and the file specified by the ftpd.conf(5) limit directive.
-D Run as daemon. tnftpd will listen on the default FTP port for incoming connections and fork a child for each connection. This is lower overhead than starting tnftpd from inetd(8) and thus might be useful on busy servers to reduce load.
-d Debugging information is written to the syslog using a facility of LOG_FTP.
-e emailaddr
  Use emailaddr for the "%E" escape sequence (see Display file escape sequences)
-H Equivalent to " -h ‘hostname‘ ".
-h hostname
  Explicitly set the hostname to advertise as to hostname. The default is the hostname associated with the IP address that tnftpd is listening on. This ability (with or without -h ), in conjunction with -c confdir, is useful when configuring 'virtual' FTP servers, each listening on separate addresses as separate names. Refer to inetd.conf(5) for more information on starting services to listen on specific IP addresses.
-L xferlogfile
  Log wu-ftpd style 'xferlog' entries to xferlogfile.
-l Each successful and failed FTP session is logged using syslog with a facility of LOG_FTP. If this option is specified more than once, the retrieve (get), store (put), append, delete, make directory, remove directory and rename operations and their file name arguments are also logged.
-n Don’t attempt translation of IP addresses to hostnames.
-P dataport
  Use dataport as the data port, overriding the default of using the port one less that the port tnftpd is listening on.
-Q Disable the use of pid files for keeping track of the number of logged-in users per class. This may reduce the load on heavily loaded FTP servers.
-q Enable the use of pid files for keeping track of the number of logged-in users per class. This is the default.
-r Permanently drop root privileges once the user is logged in. The use of this option may result in the server using a port other than the (listening-port - 1) for PORT style commands, which is contrary to the RFC 959 specification, but in practice very few clients rely upon this behaviour. See SECURITY CONSIDERATIONS below for more details.
-s Require a secure authentication mechanism like Kerberos or S/Key to be used.
-U Don’t log each concurrent FTP session to /var/run/utmp. This is the default.
-u Log each concurrent FTP session to /var/run/utmp, making them visible to commands such as who(1).
-V version
  Use version as the version to advertise in the login banner and in the output of STAT and SYST instead of the default version information. If version is empty or '-' then don’t display any version information.
-W Don’t log each FTP session to /var/log/wtmp.
-w Log each FTP session to /var/log/wtmp, making them visible to commands such as last(1). This is the default.
-X Log wu-ftpd style 'xferlog' entries to the syslog, prefixed with "xferlog: ", using a facility of LOG_FTP. These syslog entries can be converted to a wu-ftpd style xferlog file suitable for input into a third-party log analysis tool with a command similar to:

    sed -ne ’s/^.*xferlog: //p’ /var/log/xferlog Gt] wuxferlog

The file /etc/nologin can be used to disable FTP access. If the file exists, tnftpd displays it and exits. If the file /usr/local/etc/ftpwelcome exists, tnftpd prints it before issuing the "ready" message. If the file /usr/local/etc/motd exists (under the chroot directory if applicable), tnftpd prints it after a successful login. This may be changed with the ftpd.conf(5) directive motd.

The tnftpd server currently supports the following FTP requests. The case of the requests is ignored.
Request      Description
ABOR      abort previous command
ACCT      specify account (ignored)
ALLO      allocate storage (vacuously)
APPE      append to a file
CDUP      change to parent of current working directory
CWD      change working directory
DELE      delete a file
EPSV      prepare for server-to-server transfer
EPRT      specify data connection port
FEAT      list extra features that are not defined in RFC 959
HELP      give help information
LIST      give list files in a directory("ls-lA")
LPSV      prepare for server-to-server transfer
LPRT      specify data connection port
MLSD      list contents of directory in a machine-processable form
MLST      show a pathname in a machine-processable form
MKD      make a directory
MDTM      show last modification time of file
MODE      specify data transfer mode
NLST      give name list of files in directory
NOOP      do nothing
OPTS      define persistent options for a given command
PASS      specify password
PASV      prepare for server-to-server transfer
PORT      specify data connection port
PWD      print the current working directory
QUIT      terminate session
REST      restart incomplete transfer
RETR      retrieve a file
RMD      remove a directory
RNFR      specify rename-from file name
RNTO      specify rename-to file name
SITE      non-standard commands (see next section)
SIZE      return size of file
STAT      return status of server
STOR      store a file
STOU      store a file with a unique name
STRU      specify data transfer structure
SYST      show operating system type of server system
TYPE      specify data transfer type
USER      specify user name
XCUP      change to parent of current working directory (deprecated)
XCWD      change working directory (deprecated)
XMKD      make a directory (deprecated)
XPWD      print the current working directory (deprecated)
XRMD      remove a directory (deprecated)
 

The following non-standard or Unix specific commands are supported by the SITE request.

Request      Description
CHMOD      change mode of a file, e.g. ‘‘SITE CHMOD 755 filename’’
HELP      give help information.
IDLE      set idle-timer, e.g. ‘‘SITE IDLE 60’’
RATEGET      set maximum get rate throttle in bytes/second, e.g. ‘‘SITE RATEGET 5k’’
RATEPUT      set maximum put rate throttle in bytes/second, e.g. ‘‘SITE RATEPUT 5k’’
UMASK      change umask, e.g. ‘‘SITE UMASK 002’’
 

The following FTP requests (as specified in RFC 959 and RFC 2228) are recognized, but are not implemented: ACCT, ADAT, AUTH, CCC, CONF, ENC, MIC, PBSZ, PROT, REIN, and SMNT.

The tnftpd server will abort an active file transfer only when the ABOR command is preceded by a Telnet "Interrupt Process" (IP) signal and a Telnet "Synch" signal in the command Telnet stream, as described in Internet RFC 959. If a STAT command is received during a data transfer, preceded by a Telnet IP and Synch, transfer status will be returned.

tnftpd interprets file names according to the "globbing" conventions used by csh(1). This allows users to use the metacharacters "*?[]{}~".

    User authentication

tnftpd authenticates users according to five rules.

  1. The login name must be in the password data base, passwd(5), and not have a null password. In this case a password must be provided by the client before any file operations may be performed. If the user has an S/Key key, the response from a successful USER command will include an S/Key challenge. The client may choose to respond with a PASS command giving either a standard password or an S/Key one-time password. The server will automatically determine which type of password it has been given and attempt to authenticate accordingly. See skey(1) for more information on S/Key authentication. S/Key is a Trademark of Bellcore.
  2. The login name must be allowed based on the information in ftpusers(5).
  3. The user must have a standard shell returned by getusershell(3). If the user’s shell field in the password database is empty, the shell is assumed to be /bin/sh. As per shells(5), the user’s shell must be listed with full path in /etc/shells.
  4. If directed by the file ftpchroot(5) the session’s root directory will be changed by chroot(2) to the directory specified in the ftpd.conf(5) chroot directive (if set), or to the home directory of the user. This facility may also be triggered by enabling the boolean ftp-chroot in login.conf(5). However, the user must still supply a password. This feature is intended as a compromise between a fully anonymous account and a fully privileged account. The account should also be set up as for an anonymous account.
  5. If the user name is "anonymous" or "ftp", an anonymous FTP account must be present in the password file (user "ftp"). In this case the user is allowed to log in by specifying any password (by convention an email address for the user should be used as the password).

    The server performs a chroot(2) to the directory specified in the ftpd.conf(5) chroot directive (if set), the -a anondir directory (if set), or to the home directory of the "ftp" user.

    The server then performs a chdir(2) to the directory specified in the ftpd.conf(5) homedir directive (if set), otherwise to /.

    If other restrictions are required (such as disabling of certain commands and the setting of a specific umask), then appropriate entries in ftpd.conf(5) are required.

    If the first character of the password supplied by an anonymous user is "-", then the verbose messages displayed at login and upon a CWD command are suppressed.

    Display file escape sequences

When tnftpd displays various files back to the client (such as /usr/local/etc/ftpwelcome and /usr/local/etc/motd), various escape strings are replaced with information pertinent to the current connection.

The supported escape strings are:
Escape
  Description
%c Class name.
%C Current working directory.
%E Email address given with -e .
%L Local hostname.
%M Maximum number of users for this class. Displays "unlimited" if there’s no limit.
%N Current number of users for this class.
%R Remote hostname.
%s If the result of the most recent "%M" or "%N" was not "1", print an "s".
%S If the result of the most recent "%M" or "%N" was not "1", print an "S".
%T Current time.
%U User name.
%% A "%" character.

    Setting up a restricted ftp subtree

In order that system security is not breached, it is recommended that the subtrees for the "ftp" and "chroot" accounts be constructed with care, following these rules (replace "ftp" in the following directory names with the appropriate account name for 'chroot' users):
~ftp Make the home directory owned by "root" and unwritable by anyone.
~ftp/bin Make this directory owned by "root" and unwritable by anyone (mode 555). Generally any conversion commands should be installed here (mode 111).
~ftp/etc Make this directory owned by "root" and unwritable by anyone (mode 555). The files pwd.db (see passwd(5)) and group (see group(5)) must be present for the LIST command to be able to display owner and group names instead of numbers. The password field in passwd(5) is not used, and should not contain real passwords. The file motd, if present, will be printed after a successful login. These files should be mode 444.
~ftp/pub This directory and the subdirectories beneath it should be owned by the users and groups responsible for placing files in them, and be writable only by them (mode 755 or 775). They should not be owned or writable by ftp or its group.
~ftp/incoming
  This directory is where anonymous users place files they upload. The owners should be the user "ftp" and an appropriate group. Members of this group will be the only users with access to these files after they have been uploaded; these should be people who know how to deal with them appropriately. If you wish anonymous FTP users to be able to see the names of the files in this directory the permissions should be 770, otherwise they should be 370.

The following ftpd.conf(5) directives should be used:

    modify guest off

    umask guest 0707

    upload guest on

This will result in anonymous users being able to upload files to this directory, but they will not be able to download them, delete them, or overwrite them, due to the umask and disabling of the commands mentioned above.

~ftp/tmp This directory is used to create temporary files which contain the error messages generated by a conversion or LIST command. The owner should be the user "ftp". The permissions should be 300.

If you don’t enable conversion commands, or don’t want anonymous users uploading files here (see ~ftp/incoming above), then don’t create this directory. However, error messages from conversion or LIST commands won’t be returned to the user. (This is the traditional behaviour.) Note that the ftpd.conf(5) directive upload can be used to prevent users uploading here.

To set up "ftp-only" accounts that provide only FTP, but no valid shell login, you can copy/link /sbin/nologin to /sbin/ftplogin, and enter /sbin/ftplogin to /etc/shells to allow logging-in via FTP into the accounts, which must have /sbin/ftplogin as login shell.

FILES

/usr/local/etc/ftpchroot List of normal users whose root directory should be changed via chroot(2).
/usr/local/etc/ftpd.conf Configure file conversions and other settings.
/usr/local/etc/ftpusers List of unwelcome/restricted users.
/usr/local/etc/ftpwelcome
  Welcome notice before login.
/usr/local/etc/motd Welcome notice after login.
/etc/nologin If it exists, displayed and access is refused.
/var/run/ftpd.pids-CLASS State file of logged-in processes for the tnftpd class 'CLASS'.
/var/run/utmp List of logged-in users on the system.
/var/log/wtmp Login history database.

SEE ALSO

ftp(1), skey(1), who(1), getusershell(3), ftpchroot(5), ftpd.conf(5), ftpusers(5), login.conf(5), syslogd(8)

STANDARDS

tnftpd recognizes all commands in RFC 959, follows the guidelines in RFC 1123, recognizes all commands in RFC 2228 (although they are not supported yet), and supports the extensions from RFC 2389, RFC 2428, and RFC 3659.

HISTORY

The tnftpd command appeared in BSD 4.2 .

Various features such as the ftpd.conf(5) functionality, RFC 2389, and RFC 3659 support was implemented in
.Nx 1.3 and later releases by Luke Mewburn.

BUGS

The server must run as the super-user to create sockets with privileged port numbers (i.e, those less than IPPORT_RESERVED, which is 1024). If tnftpd is listening on a privileged port it maintains an effective user id of the logged in user, reverting to the super-user only when binding addresses to privileged sockets. The -r option can be used to override this behaviour and force privileges to be permanently revoked; see SECURITY CONSIDERATIONS below for more details.

tnftpd may have trouble handling connections from scoped IPv6 addresses, or IPv4 mapped addresses ( IPv4 connection on AF_INET6 socket ). For the latter case, running two daemons, one for IPv4 and one for IPv6, will avoid the problem.

SECURITY CONSIDERATIONS

RFC 959 provides no restrictions on the PORT command, and this can lead to security problems, as tnftpd can be fooled into connecting to any service on any host. With the "checkportcmd" feature of the ftpd.conf(5), PORT commands with different host addresses, or TCP ports lower than IPPORT_RESERVED will be rejected. This also prevents 'third-party proxy ftp' from working. Use of this option is strongly recommended, and enabled by default.

By default tnftpd uses a port that is one less than the port it is listening on to communicate back to the client for the EPRT, LPRT, and PORT commands, unless overridden with -P dataport. As the default port for tnftpd (21) is a privileged port below IPPORT_RESERVED, tnftpd retains the ability to switch back to root privileges to bind these ports. In order to increase security by reducing the potential for a bug in tnftpd providing a remote root compromise, tnftpd will permanently drop root privileges if one of the following is true:

  1. tnftpd is running on a port greater than IPPORT_RESERVED and the user has logged in as a 'guest' or 'chroot' user.
  2. tnftpd was invoked with -r .

Don’t create ~ftp/tmp if you don’t want anonymous users to upload files there. That directory is only necessary if you want to display the error messages of conversion commands to the user. Note that if uploads are disabled with the ftpd.conf(5) directive upload, then this directory cannot be abused by the user in this way, so it should be safe to create.

To avoid possible denial-of-service attacks, SIZE requests against files larger than 10240 bytes will be denied if the current transfer TYPE is 'A' (ASCII).

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