ftpd -- Internet File Transfer Protocol server
ftpd [-AdDhlMPSU] [-T maxtimeout] [-t timeout] [-u mask]
Ftpd 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:
-A Permit only anonymous ftp connections or accounts listed in
/etc/ftpchroot. Other connection attempts are refused. This
option is nolonger effective if PAM is enabled. Please refer to
the README file for instructions to doing this with PAM.
-d Debugging information is written to the syslog using LOG_FTP.
-D With this option set, ftpd will detach and become a daemon,
accepting connections on the FTP port and forking child processes
to handle them. This has lower overhead than starting ftpd from
inetd(8) and is thus useful on busy servers to reduce load.
-h The server will use data ports in the high port range for passive
connections. This range is defined by the IPPORT_HIFIRSTAUTO and
IPPORT_HILASTAUTO defines in <netinet/in.h>. In OpenBSD they are
set to 49152 and 65535 respectively.
-l Each successful and failed ftp(1) session is logged using syslog
with a facility of LOG_FTP. If this option is specified twice,
the retrieve (get), store (put), append, delete, make directory,
remove directory and rename operations and their filename arguments
are also logged.
-M Enables multihomed mode. Instead of simply using ~ftp for anonymous
transfers, a directory matching the fully qualified name of
the IP number the client connected to, and located inside ~ftp is
used instead.
-p Disable passive mode ftp connections. This is useful if you are
behind a firewall that refuses connections to arbitrary high numbered
ports. Many ftp clients try passive mode first and do not
always react gracefully to a server that refuses connections to
the port it asked the client to connect to.
-P Permit illegal port numbers or addresses for PORT command initiated
connects. By default ftpd(8) violates the RFC and thus constrains
the PORT command to non-reserved ports and requires it
use the same source address as the connection came from. This
prevents the "FTP bounce attack" against services on both the
local machine and other local machines.
-S With this option set, ftpd logs all anonymous transfers to the
file /var/log/ftpd when this file exists.
-U Each concurrent ftp(1) session is logged to the file
/var/run/utmp, making them visible to commands such as who(1).
This option at present is unsupporte and will always silently
fail.
-T A client may also request a different timeout period; the maximum
period allowed may be set to timeout seconds with the -T option.
The default limit is 2 hours.
-t The inactivity timeout period is set to timeout seconds (the
default is 15 minutes).
-u Change the default umask from 027 to mask.
The file /etc/nologin can be used to disable ftp access. If the file
exists, ftpd displays it and exits. If the file /etc/ftpwelcome exists,
ftpd prints it before issuing the ``ready'' message. If the file
/etc/motd exists, ftpd prints it after a successful login. If the file
.message exists in a directory, ftpd prints it when that directory is
entered.
The ftp 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
HELP give help information
LIST give list files in a directory (``ls -lgA'')
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
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
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.
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 five rules.
1. The login name must be in the password data base, /etc/passwd,
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 not appear in the file /etc/ftpusers.
3. The user must have a standard shell returned by
getusershell(3).
4. If the user name appears in the file /etc/ftpchroot the session's
root will be changed to the user's login directory by
chroot(2) as for an ``anonymous'' or ``ftp'' account (see next
item). 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).
In the last case, ftpd takes special measures to restrict the client's
access privileges. The server performs a chroot(2) 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, following
these rules:
~ftp Make the home directory owned by ``root'' and unwritable
by anyone (mode 555).
~ftp/bin Make this directory owned by ``root'' and unwritable by
anyone (mode 511). This directory is required, and
should contain at least a statically linked copy of
ls(1.) Any programs in this directory should be mode 111
(executable only).
~ftp/etc Make this directory owned by ``root'' and unwritable by
anyone (mode 511). 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 passwords. The
file motd, if present, will be printed after a successful
login. These files should be mode 444.
~ftp/lib Make this directory owned by ``root'' and unwritable by
anyone (mode 511). The libraries ld-linux.so.2 and
libc.so.6 (or whatever your ls command is linked to) must
be present. Note that if you're using a 2.2.* or later
Linux kernel, ld-linux.so.2 must be executable as well as
readable (555). All other files should be mode 444.
~ftp/pub Make this directory mode 555 and owned by ``root''. This
is traditionally where publically accessible files are
stored for download.
/etc/ftpusers List of unwelcome/restricted users.
/etc/ftpchroot List of normal users who should be chroot'd.
/etc/ftpwelcome Welcome notice.
/etc/motd Welcome notice after login.
/etc/nologin Displayed and access refused.
/var/run/utmp List of users on the system.
/var/log/ftpd Log file for anonymous transfers.
ftp(1), skey(1), who(1), getusershell(3), ftpusers(5), syslogd(8)
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.
The ftpd command appeared in 4.2BSD.
Linux NetKit (0.17) September 14, 1999 Linux NetKit (0.17)
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