NetBSD/libexec/ftpd/ftpd.8

.\"	$NetBSD: ftpd.8,v 1.50 2000/01/13 00:04:31 lukem Exp $
.\"
.\" Copyright (c) 1997-2000 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Luke Mewburn.
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.\" modification, are permitted provided that the following conditions
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.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
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.\"    may be used to endorse or promote products derived from this software
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\"     @(#)ftpd.8	8.2 (Berkeley) 4/19/94
.\"
.Dd January 13, 2000
.Dt FTPD 8
.Os
.Sh NAME
.Nm ftpd
.Nd
Internet File Transfer Protocol server
.Sh SYNOPSIS
.Nm
.Op Fl dlsU
.Op Fl a Ar anondir
.Op Fl c Ar confdir
.Op Fl C Ar user
.Op Fl h Ar hostname
.Sh DESCRIPTION
.Nm
is the Internet File Transfer Protocol server process.
The server uses the
.Tn TCP
protocol and listens at the port specified in the
.Dq ftp
service specification; see
.Xr services 5 .
.Pp
Available options:
.Bl -tag -width Ds
.It Fl a Ar anondir
Define
.Ar anondir
as the directory to
.Xr chroot 2
into for anonymous logins.
Default is the home directory for the ftp user.
.It Fl c Ar confdir
Change the root directory of the configuration files from
.Dq Pa /etc
to
.Ar confdir .
.It Fl C Ar user
Check whether
.Ar user
would be granted access under
the restrictions given in
.Xr ftpusers 5
and exit without attempting a connection.
.Nm
exits with an exit code of 0 if access would be granted, or 1 otherwise.
This can be useful for testing configurations.
.It Fl d
Debugging information is written to the syslog using
.Dv LOG_FTP .
.It Fl h Ar hostname
Explicitly set the hostname to advertise as.
Defaults to the hostname associated with the IP address that
.NM
is listening on.
This ability (with or without
.Fl h ) ,
in conjunction with 
.Fl c Ar confdir ,
is useful when configuring
.Sq virtual
.Tn FTP
servers, each listening on separate addresses as separate names.
Refer to
.Xr inetd.conf 5
for more information on starting services to listen on specific IP addresses.
.It Fl l
Each successful and failed
.Tn FTP
session is logged using syslog with a facility of
.Dv 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.
.It Fl s
Require a secure authentication mechanism like Kerberos or S/Key to be used.
.It Fl U
Each concurrent
.Tn FTP
session is logged to the file
.Pa /var/run/utmp ,
making them visible to commands such as
.Xr who 1 .
.El
.Pp
The file
.Pa /etc/nologin
can be used to disable
.Tn FTP
access.
If the file exists,
.Nm
displays it and exits.
If the file
.Pa /etc/ftpwelcome
exists,
.Nm
prints it before issuing the
.Dq ready
message.
If the file
.Pa /etc/motd
exists,
.Nm
prints it after a successful login.
(This may be changed with the
.Xr ftpd.conf 5
directive
.Sy upload . )
.Pp
The
.Nm
server currently supports the following
.Tn FTP
requests.
The case of the requests is ignored.
.Bl -column "Request" -offset indent
.It Sy Request Ta Sy Description
.It ABOR Ta "abort previous command"
.It ACCT Ta "specify account (ignored)"
.It ALLO Ta "allocate storage (vacuously)"
.It APPE Ta "append to a file"
.It CDUP Ta "change to parent of current working directory"
.It CWD Ta "change working directory"
.It DELE Ta "delete a file"
.It EPSV Ta "prepare for server-to-server transfer"
.It EPRT Ta "specify data connection port"
.It FEAT Ta "list extra features that are not defined in" Cm "RFC 959"
.It HELP Ta "give help information"
.It LIST Ta "give list files in a directory" Pq Dq Li "ls -lgA"
.It LPSV Ta "prepare for server-to-server transfer"
.It LPRT Ta "specify data connection port"
.It MKD Ta "make a directory"
.It MDTM Ta "show last modification time of file"
.It MODE Ta "specify data transfer" Em mode
.It NLST Ta "give name list of files in directory"
.It NOOP Ta "do nothing"
.It OPTS Ta "define persistent options for a given command"
.It PASS Ta "specify password"
.It PASV Ta "prepare for server-to-server transfer"
.It PORT Ta "specify data connection port"
.It PWD Ta "print the current working directory"
.It QUIT Ta "terminate session"
.It REST Ta "restart incomplete transfer"
.It RETR Ta "retrieve a file"
.It RMD Ta "remove a directory"
.It RNFR Ta "specify rename-from file name"
.It RNTO Ta "specify rename-to file name"
.It SITE Ta "non-standard commands (see next section)"
.It SIZE Ta "return size of file"
.It STAT Ta "return status of server"
.It STOR Ta "store a file"
.It STOU Ta "store a file with a unique name"
.It STRU Ta "specify data transfer" Em structure
.It SYST Ta "show operating system type of server system"
.It TYPE Ta "specify data transfer" Em type
.It USER Ta "specify user name"
.It XCUP Ta "change to parent of current working directory (deprecated)"
.It XCWD Ta "change working directory (deprecated)"
.It XMKD Ta "make a directory (deprecated)"
.It XPWD Ta "print the current working directory (deprecated)"
.It XRMD Ta "remove a directory (deprecated)"
.El
.Pp
The following non-standard or
.Ux
specific commands are supported by the SITE request.
.Pp
.Bl -column Request -offset indent
.It Sy Request Ta Sy Description
.It CHMOD Ta "change mode of a file, e.g. ``SITE CHMOD 755 filename''"
.It HELP Ta "give help information."
.It IDLE Ta "set idle-timer, e.g. ``SITE IDLE 60''"
.It RATEGET Ta "set maximum get rate throttle, e.g. ``SITE RATEGET 5k''"
.It RATEPUT Ta "set maximum put rate throttle, e.g. ``SITE RATEPUT 5k''"
.It UMASK Ta "change umask, e.g. ``SITE UMASK 002''"
.El
.Pp
The following
.Tn FTP
requests (as specified in
.Cm RFC 959 )
are recognized, but are not implemented:
ACCT, SMNT, and REIN.
MDTM and SIZE are not specified in
.Cm RFC 959 ,
but will appear in the
next updated
.Tn FTP
RFC.
.Pp
The
.Nm
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
.Cm RFC 959 .
If a
STAT
command is received during a data transfer, preceded by a Telnet IP
and Synch, transfer status will be returned.
.Pp
.Nm
interprets file names according to the
.Dq globbing
conventions used by
.Xr csh 1 .
This allows users to utilize the metacharacters
.Dq Li \&*?[]{}~ .
.Sh User authentication
.Pp
.Nm
authenticates users according to five rules.
.Pp
.Bl -enum -offset indent
.It
The login name must be in the password data base,
.Pa /etc/pwd.db ,
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
.Xr skey 1
for more information on S/Key authentication.
S/Key is a Trademark of Bellcore.
.It
The login name must be allowed based on the information in
.Xr ftpusers 5 .
.It
The user must have a standard shell returned by
.Xr getusershell 3 .
If the user's shell field in the password database is empty, the
shell is assumed to be
.Pa /bin/sh .
.It
If directed by the file
.Xr ftpchroot 5
the session's root will be changed to the user's login directory by
.Xr chroot 2
as for an
.Dq anonymous
or
.Dq 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.
.It
If the user name is
.Dq anonymous
or
.Dq ftp ,
an
anonymous
.Tn FTP
account must be present in the password
file (user
.Dq 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
.Xr chroot 2
to the home directory of the
.Dq ftp
user.
If other restrictions are required (such as disabling of certain
commands and the setting of a specific umask), then appropriate
entries in
.Xr ftpd.conf 5
are required.
.Pp
If the first character of the password supplied by an anonymous user
is
.Dq - ,
then the verbose messages displayed at login and upon a
.Sy CWD
command are suppressed.
.El
.Sh Display file escape sequences
.Pp
When
.Nm
displays various files back to the client (such as
.Pa /etc/ftpwelcome
and
.Pa /etc/motd ) ,
various escape strings are replaced with information pertinent
to the current connection.
.Pp
The supported escape strings are:
.Bl -tag -width "Escape" -offset indent -compact
.It Sy "Escape"
.Sy Description
.It "\&%c"
Class name.
.It "\&%C"
Current working directory.
.It "\&%L"
Local hostname.
.It "\&%M"
Maximum number of users for this class.
Displays
.Dq unlimited
if there's no limit.
.It "\&%N"
Current number of users for this class.
.It "\&%R"
Remote hostname.
.It "\&%T"
Current time.
.It "\&%U"
Username.
.It "\&%\&%"
A
.Dq \&%
character.
.El
.Sh Setting up a restricted ftp subtree
.Pp
In order that system security is not breached, it is recommended
that the
subtrees for the
.Dq ftp
and
.Dq chroot
accounts be constructed with care, following these rules
(replace
.Dq ftp
in the following directory names
with the appropriate account name for
.Sq chroot
users):
.Bl -tag -width "~ftp/incoming" -offset indent
.It Pa ~ftp
Make the home directory owned by
.Dq root
and unwritable by anyone.
.It Pa ~ftp/bin
Make this directory owned by
.Dq root
and unwritable by anyone (mode 555).
Generally any conversion commands should be installed
here (mode 111).
.It Pa ~ftp/etc
Make this directory owned by
.Dq root
and unwritable by anyone (mode 555).
The files
.Pa pwd.db
(see
.Xr passwd 5 )
and
.Pa group
(see
.Xr group 5 )
must be present for the
.Xr LIST
command to be able to display owner and group names instead of numbers.
The password field in
.Xr passwd 5
is not used, and should not contain real passwords.
The file
.Pa motd ,
if present, will be printed after a successful login.
These files should be mode 444.
.It Pa ~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
.Em not
be owned or writable by ftp or its group.
.It Pa ~ftp/incoming
This directory is where anonymous users place files they upload.
The owners should be the user
.Dq 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
.Tn 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.
.Pp
Anonymous users will be 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.
.It Pa ~ftp/tmp
This directory is used to create temporary files which contain
the error messages generated by a conversion or
.Sq LIST
command.
The owner should be the user
.Dq ftp .
The permissions should be 300.
.Pp
If you don't enable conversion commands, or don't want anonymous users
uploading files here (see
.Pa ~ftp/incoming
above), then don't create this directory.
However, error messages from conversion or
.Sq LIST
commands won't be returned to the user.
(This is the traditional behaviour.)
Note that the
.Xr ftpd.conf 5
directive
.Sy upload
can be used to prevent users uploading here.
.El
.Pp
To set up "ftp-only" accounts that provide only
.Tn FTP ,
but no valid shell
login, you can copy/link
.Pa /sbin/nologin
to
.Pa /sbin/ftplogin ,
and enter 
.Pa /sbin/ftplogin 
to
.Pa /etc/shells
to allow logging-in via
.Tn FTP
into the accounts, which must have
.Pa /sbin/ftplogin 
as login shell.
.Sh FILES
.Bl -tag -width /etc/ftpwelcome -compact
.It Pa /etc/ftpchroot
List of normal users who should be
.Xr chroot 2 ed.
.It Pa /etc/ftpd.conf
Configure file conversions and other settings.
.It Pa /etc/ftpusers
List of unwelcome/restricted users.
.It Pa /etc/ftpwelcome
Welcome notice before login.
.It Pa /etc/motd
Welcome notice after login.
.It Pa /etc/nologin
If it exists, displayed and access is refused.
.El
.Sh SEE ALSO
.Xr ftp 1 ,
.Xr skey 1 ,
.Xr who 1 ,
.Xr getusershell 3 ,
.Xr ftpd.conf 5 ,
.Xr ftpchroot 5 ,
.Xr ftpusers 5 ,
.Xr syslogd 8
.Sh STANDARDS
.Nm
recognizes all commands in 
.Cm RFC 959 , 
follows the guidelines in 
.Cm RFC 1123 ,
recognizes all commands in 
.Cm RFC 2228
(although they are not supported yet),
and supports the extensions from
.Cm RFC 2389.
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.2 .
.Pp
The
.Xr ftpd.conf 5
functionality was implemented in
.Nx 1.3
and later releases by Luke Mewburn, based on work by Simon Burge.
.Sh BUGS
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.
.Sh SECURITY CONSIDERATIONS
.Cm RFC 959
provides no restrictions on the PORT command, and this can lead
to security problems, as
.Nm
can be fooled into connecting to any service on any host.
With the
.Dq checkportcmd
feature of the
.Xr ftpd.conf 5 ,
PORT commands with different host addresses, or TCP ports lower than
.Dv IPPORT_RESERVED
will be rejected.
Use of this option is
.Em strongly
recommended.
.Pp
When running on IPv6, connection from IPv4 mapped address
.Pq IPv4 connection to IPv6 socket
is not very stable.
Please run two daemons, one for IPv4 and one for IPv6.
.Pp
Don't create
.Pa ~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
.Xr ftpd.conf 5
directive
.Sy upload ,
then this directory cannot be abused by the user in this way.