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ftpd

NAME

ftpd - DARPA Internet File Transfer Protocol server

SYNOPSIS

/usr/kerberos/sbin/ftpd [-A | -a] [-C] [-c] [-d] [-E] [-l] [-v] [-T maxtimeout] [-t timeout] [-p port] [-U ftpusers-file] [-u umask] [-r realm-file] [-s srvtab] [-w{ip|maxhostlen[,{striplocal|nostriplocal}]}]

DESCRIPTION

Ftpd is the DARPA 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:

-A

Connections are only allowed for users who can authenticate via the ftp AUTH mechanism. (Anonymous ftp may also be allowed if it is configured.) Ftpd will ask the user for a password if one is required.

-a

Connections are only allowed for users who can authenticate (via the ftp AUTH mechanism) and who are authorized to connect to the named account without a password. (Anonymous ftp may also be allowed if it is configured.)

-C

Non-anonymous users need local credentials (for example, to authenticate to remote fileservers), and so they should be prompted for a password unless they forwarded credentials as part of authentication.

-c

Allow the CCC (Clear Command Channel) command to be used. This allows less secure connections, and should probably only be used when debugging.

-d

Debugging information is written to the syslog. (Identical to -v)

-E

Don't allow passwords to be typed across unencrypted connections.

-l

Each ftp(1) session is logged in the syslog. If this flag appears twice, additional information about operations performed (such as files retrieved, directories created, etc.) will be logged via syslog. If it appears three times, some other statistics such as the number of bytes transferred will be logged via syslog as well.

-v

Debugging information is written to the syslog. (Identical to -d)

-T maxtimeout

A client may request a maximum timeout period allowed set to timeout seconds with the -T option. The default limit is 2 hours. This is different from the normal inactivity timeout specified by the -t option (see below).

-t timeout

The inactivity timeout period is set to timeout seconds (the default is 15 minutes).

-p port

Run as a server and accept a connection on port. Normally the ftp server is invoked by inetd(8).

-U ftpusers-file

Sets the full path and name of the ftpusers file to use. The default value is normally /etc/ftpusers.

-u umask

Sets the umask for the ftpd process. The default value is normally 027.

-w {ip|maxhostlen[,{striplocal|nostriplocal}]}

Controls the form of the remote hostname passed to login(1). Specifying ip results in the numeric IP address always being passed to login(1). Specifying a number, maxhostlen, sets the maximum length of the hostname passed to login(1) before it will be passed as a numeric IP address. If maxhostlen is 0, then the system default, as determined by the utmp or utmpx structures, is used. The nostriplocal and striplocal options, which must be preceded by a comma, control whether or not the local host domain is stripped from the remote hostname. By default, the equivalent of striplocal is in effect.

The ftp server currently supports the following ftp requests; case is not distinguished.

Request

Description

ABOR

abort previous command

ACCT

specify account (ignored)

ADAT

send an authentication protocol message

ALLO

allocate storage (vacuously)

APPE

append to a file

AUTH

specify an authentication protocol to be performed

CCC

set the command channel protection mode to "Clear" (no protection). Only available if the -c command-line option was given.

CDUP

change to parent of current working directory

CWD

change working directory

DELE

delete a file

ENC

send a privacy and integrity protected command (given in argument)

HELP

give help information

LIST

give list files in a directory (``ls -lgA'')

MIC

send an integrity protected command (given in argument)

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

PASS

specify password

PASV

prepare for server-to-server transfer

PBSZ

specify a protection buffer size

PORT

specify data connection port

PROT

specify a protection level under which to protect data transfers

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

UMASK

change umask. E.g., SITE UMASK 002

IDLE

set idle-timer. E.g., SITE IDLE 60

CHMOD

change mode of a file. E.g., SITE CHMOD 755 filename

HELP

give help information. E.g., SITE HELP

The remaining ftp requests specified in Internet RFC 959 are recognized, but not implemented. MDTM and SIZE are not specified in RFC 959, but will appear in the next updated FTP RFC.

The ftp 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.

Ftpd interprets file names according to the ``globbing'' conventions used by csh(1). This allows users to utilize the metacharacters ``*?[]{}~''.

Ftpd authenticates users according to the following rules:

  1.

The user name must be in the password data base, /etc/passwd.

  2.

An AUTH command must be accepted, the ensuing authentication protocol (conducted via ADAT commands and replies) must successfully complete, and the authenticated user must permitted access. Otherwise, a valid password which is not null must be provided by the client.

  3.

The user name must not appear in the file /etc/ftpusers.

  4.

The user must have a standard shell returned by getusershell(3).

  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 this is given as the client host's name).

In the last case, ftpd takes special measures to restrict the client's access privileges. The server performs a chroot(2) command to the home directory of the ``ftp'' user. In order that system security is not breached, it is recommended that the ``ftp'' subtree be constructed with care; the following rules are recommended.

~ftp

Make the home directory owned by ``ftp'' and unwritable by anyone.

~ftp/bin

Make this directory owned by the super-user and unwritable by anyone. The program ls(1) must be present to support the list command. This program should have mode 111.

~ftp/etc

Make this directory owned by the super-user and unwritable by anyone. The files passwd(5) and group(5) must be present for the ls command to be able to produce owner names rather than numbers. The password field in passwd is not used, and should not contain real encrypted passwords. These files should be mode 444.

~ftp/pub

Make this directory mode 777 and owned by ``ftp''. Users should then place files which are to be accessible via the anonymous account in this directory.

If an ADAT command succeeds, the control channel must be either integrity or privacy protected. In this case, the MIC and ENC commands are the only commands allowed over the control channel. The argument to the MIC command is a base 64 encoded string which, when decoded, is an ftp command integrity protected with a cryptographic checksum. The argument to the ENC command is a base 64 encoded string which, when decoded, is an ftp command privacy and integrity protected with encryption.

If an ADAT command succeeds, ftp replies will also be either integrity or privacy protected.

If an ADAT command succeeds, the data channel can also be integrity or privacy protected. The PROT command accepts S for integrity and P for privacy protection. Unless an ADAT command succeeds, the only protection level accepted by the PROT command is C (clear).

SEE ALSO

ftp(1), getusershell(3), syslogd(8)

Lunt, S. J., FTP Security Extensions, Internet Draft, November 1993.

BUGS

The anonymous account is inherently dangerous and should avoided when possible.

The server must run as the super-user to create sockets with privileged port numbers. It maintains an effective user id of the logged in user, reverting to the super-user only when binding addresses to sockets. The possible security holes have been extensively scrutinized, but are possibly incomplete.

HISTORY

The ftpd command appeared in 4.2BSD.