libexec/ftpd/ftpd.8

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.\"     @(#)ftpd.8	8.2 (Berkeley) 4/19/94
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.Dd June 28, 2022
.Dt FTPD 8
.Os
.Sh NAME
.Nm ftpd
.Nd Internet File Transfer Protocol server
.Sh SYNOPSIS
.Nm
.Op Fl 468ABDdEhMmOoRrSUvW
.Op Fl l Op Fl l
.Op Fl a Ar address
.Op Fl P Ar port
.Op Fl p Ar file
.Op Fl T Ar maxtimeout
.Op Fl t Ar timeout
.Op Fl u Ar umask
.Sh DESCRIPTION
The
.Nm
utility is the
Internet File Transfer Protocol
server process.
The server uses the
.Tn TCP
protocol
and listens at the port specified with the
.Fl P
option or in the
.Dq ftp
service specification; see
.Xr services 5 .
.Pp
Available options:
.Bl -tag -width indent
.It Fl 4
When
.Fl D
is specified, accept connections via
.Dv AF_INET
socket.
.It Fl 6
When
.Fl D
is specified, accept connections via
.Dv AF_INET6
socket.
.It Fl 8
Enable transparent UTF-8 mode.
RFC\ 2640 compliant clients will be told that the character encoding
used by the server is UTF-8, which is the only effect of the option.
.Pp
This option does not enable any encoding conversion for server file names;
it implies instead that the names of files on the server are encoded
in UTF-8.
As for files uploaded via FTP, it is the duty of the RFC\ 2640 compliant
client to convert their names from the client's local encoding to UTF-8.
FTP command names and own
.Nm
messages are always encoded in ASCII, which is a subset of UTF-8.
Hence no need for server-side conversion at all.
.It Fl A
Allow only anonymous ftp access.
.It Fl a
When
.Fl D
is specified, accept connections only on the specified
.Ar address .
.It Fl B
With this option set,
.Nm
sends authentication success and failure messages to the
.Xr blacklistd 8
daemon.  If this option is not specified, no communcation with the
.Xr blacklistd 8
daemon is attempted.
.It Fl D
With this option set,
.Nm
will detach and become a daemon, accepting connections on the FTP port and
forking children processes to handle them.
This is lower overhead than starting
.Nm
from
.Xr inetd 8
and is thus useful on busy servers to reduce load.
.It Fl d
Debugging information is written to the syslog using
.Dv LOG_FTP .
.It Fl E
Disable the EPSV command.
This is useful for servers behind older firewalls.
.It Fl h
Disable printing host-specific information, such as the
server software version or hostname, in server messages.
.It Fl l
Each successful and failed
.Xr ftp 1
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.
By default,
.Xr syslogd 8
logs these to
.Pa /var/log/xferlog .
.It Fl M
Prevent anonymous users from creating directories.
.It Fl m
Permit anonymous users to overwrite or modify
existing files if allowed by file system permissions.
By default, anonymous users cannot modify existing files;
in particular, files to upload will be created under a unique name.
.It Fl O
Put server in write-only mode for anonymous users only.
RETR is disabled for anonymous users, preventing anonymous downloads.
This has no effect if
.Fl o
is also specified.
.It Fl o
Put server in write-only mode.
RETR is disabled, preventing downloads.
.It Fl P
When
.Fl D
is specified, accept connections at
.Ar port ,
specified as a numeric value or service name, instead of at the default
.Dq ftp
port.
.It Fl p
When
.Fl D
is specified, write the daemon's process ID to
.Ar file
instead of the default pid file,
.Pa /var/run/ftpd.pid .
.It Fl R
With this option set,
.Nm
will revert to historical behavior with regard to security checks on
user operations and restrictions on PORT requests.
Currently,
.Nm
will only honor PORT commands directed to unprivileged ports on the
remote user's host (which violates the FTP protocol specification but
closes some security holes).
.It Fl r
Put server in read-only mode.
All commands which may modify the local file system are disabled.
.It Fl S
With this option set,
.Nm
logs all anonymous file downloads to the file
.Pa /var/log/ftpd
when this file exists.
.It Fl T
A client may also request a different timeout period;
the maximum period allowed may be set to
.Ar timeout
seconds with the
.Fl T
option.
The default limit is 2 hours.
.It Fl t
The inactivity timeout period is set to
.Ar timeout
seconds (the default is 15 minutes).
.It Fl U
This option instructs ftpd to use data ports in the range of
.Dv IP_PORTRANGE_DEFAULT
instead of in the range of
.Dv IP_PORTRANGE_HIGH .
Such a change may be useful for some specific firewall configurations;
see
.Xr ip 4
for more information.
.Pp
Note that option is a virtual no-op in
.Fx 5.0
and above; both port
ranges are identical by default.
.It Fl u
The default file creation mode mask is set to
.Ar umask ,
which is expected to be an octal numeric value.
Refer to
.Xr umask 2
for details.
This option may be overridden by
.Xr login.conf 5 .
.It Fl v
A synonym for
.Fl d .
.It Fl W
Do not log FTP sessions to the user accounting database.
.El
.Pp
The file
.Pa /var/run/nologin
can be used to disable 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/ftpmotd
exists,
.Nm
prints it after a successful login.
Note the motd file used is the one
relative to the login environment.
This means the one in
.Pa ~ftp/etc
in the anonymous user's case.
.Pp
The ftp server currently supports the following ftp requests.
The case of the requests is ignored.
Requests marked [RW] are
disabled if
.Fl r
is specified.
.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 [RW]"
.It CDUP Ta "change to parent of current working directory"
.It CWD Ta "change working directory"
.It DELE Ta "delete a file [RW]"
.It EPRT Ta "specify data connection port, multiprotocol"
.It EPSV Ta "prepare for server-to-server transfer, multiprotocol"
.It FEAT Ta "give information on extended features of server"
.It HELP Ta "give help information"
.It LIST Ta "give list files in a directory" Pq Dq Li "ls -lgA"
.It LPRT Ta "specify data connection port, multiprotocol"
.It LPSV Ta "prepare for server-to-server transfer, multiprotocol"
.It MDTM Ta "show last modification time of file"
.It MKD Ta "make a directory [RW]"
.It MODE Ta "specify data transfer" Em mode
.It NLST Ta "give name list of files in directory"
.It NOOP Ta "do nothing"
.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 [RW]"
.It RNFR Ta "specify rename-from file name [RW]"
.It RNTO Ta "specify rename-to file name [RW]"
.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 [RW]"
.It STOU Ta "store a file with a unique name [RW]"
.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) [RW]"
.It XPWD Ta "print the current working directory (deprecated)"
.It XRMD Ta "remove a directory (deprecated) [RW]"
.El
.Pp
The following non-standard or
.Ux
specific commands are supported
by the
SITE request.
.Bl -column Request -offset indent
.It Sy Request Ta Sy Description
.It UMASK Ta change umask, e.g. ``SITE UMASK 002''
.It IDLE Ta set idle-timer, e.g. ``SITE IDLE 60''
.It CHMOD Ta "change mode of a file [RW], e.g. ``SITE CHMOD 755 filename''"
.It MD5 Ta "report the files MD5 checksum, e.g. ``SITE MD5 filename''"
.It HELP Ta give help information
.El
.Pp
Note: SITE requests are disabled in case of anonymous logins.
.Pp
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.
To avoid possible denial-of-service attacks, SIZE requests against
files larger than 10240 bytes will be denied if the current transfer
type is ASCII.
.Pp
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.
.Pp
The
.Nm
utility interprets file names according to the
.Dq globbing
conventions used by
.Xr csh 1 .
This allows users to utilize the metacharacters
.Dq Li \&*?[]{}~ .
.Pp
The
.Nm
utility authenticates users according to six rules.
.Bl -enum -offset indent
.It
The login name must be in the password data base
and not have a null password.
In this case a password must be provided by the client before any
file operations may be performed.
.It
The login name must not appear in the file
.Pa /etc/ftpusers .
.It
The login name must not be a member of a group specified in the file
.Pa /etc/ftpusers .
Entries in this file interpreted as group names are prefixed by an "at"
.Ql \&@
sign.
.It
The user must have a standard shell returned by
.Xr getusershell 3 .
.It
If the user name appears in the file
.Pa /etc/ftpchroot ,
or the user is a member of a group with a group entry in this file,
i.e., one prefixed with
.Ql \&@ ,
the session's root will be changed to the directory specified
in this file or to the user's login directory by
.Xr chroot 2
as for an
.Dq anonymous
or
.Dq ftp
account (see next item).
See
.Xr ftpchroot 5
for a detailed description of the format of this file.
This facility may also be triggered by enabling the boolean "ftp-chroot"
capability in
.Xr 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.
.It
If the user name is
.Dq anonymous
or
.Dq ftp ,
an
anonymous 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).
When the
.Fl S
option is set, all transfers are logged as well.
.El
.Pp
In the last case,
.Nm
takes special measures to restrict the client's access privileges.
The server performs a
.Xr chroot 2
to the home directory of the
.Dq ftp
user.
As a special case if the
.Dq ftp
user's home directory pathname contains the
.Pa /./
separator,
.Nm
uses its left-hand side as the name of the directory to do
.Xr chroot 2
to, and its right-hand side to change the current directory to afterwards.
A typical example for this case would be
.Pa /var/spool/ftp/./pub .
In order that system security is not breached, it is recommended
that the
.Dq ftp
subtree be constructed with care, following these rules:
.Bl -tag -width "~ftp/pub" -offset indent
.It Pa ~ftp
Make the home directory owned by
.Dq root
and unwritable by anyone.
.It Pa ~ftp/etc
Make this directory owned by
.Dq root
and unwritable by anyone (mode 555).
The files pwd.db (see
.Xr passwd 5 )
and
.Xr group 5
must be present for the
.Xr ls 1
command to be able to produce owner names rather than numbers.
The password field in
.Xr passwd 5
is not used, and should not contain real passwords.
The file
.Pa ftpmotd ,
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
.Dq ftp
or its group, otherwise guest users
can fill the drive with unwanted files.
.El
.Pp
If the system has multiple IP addresses,
.Nm
supports the idea of virtual hosts, which provides the ability to
define multiple anonymous ftp areas, each one allocated to a different
internet address.
The file
.Pa /etc/ftphosts
contains information pertaining to each of the virtual hosts.
Each host is defined on its own line which contains a number of
fields separated by whitespace:
.Bl -tag -offset indent -width hostname
.It hostname
Contains the hostname or IP address of the virtual host.
.It user
Contains a user record in the system password file.
As with normal anonymous ftp, this user's access uid, gid and group
memberships determine file access to the anonymous ftp area.
The anonymous ftp area (to which any user is chrooted on login)
is determined by the home directory defined for the account.
User id and group for any ftp account may be the same as for the
standard ftp user.
.It statfile
File to which all file transfers are logged, which
defaults to
.Pa /var/log/ftpd .
.It welcome
This file is the welcome message displayed before the server ready
prompt.
It defaults to
.Pa /etc/ftpwelcome .
.It motd
This file is displayed after the user logs in.
It defaults to
.Pa /etc/ftpmotd .
.El
.Pp
Lines beginning with a '#' are ignored and can be used to include
comments.
.Pp
Defining a virtual host for the primary IP address or hostname
changes the default for ftp logins to that address.
The 'user', 'statfile', 'welcome' and 'motd' fields may be left
blank, or a single hyphen '-' used to indicate that the default
value is to be used.
.Pp
As with any anonymous login configuration, due care must be given
to setup and maintenance to guard against security related problems.
.Pp
The
.Nm
utility has internal support for handling remote requests to list
files, and will not execute
.Pa /bin/ls
in either a chrooted or non-chrooted environment.
The
.Pa ~/bin/ls
executable need not be placed into the chrooted tree, nor need the
.Pa ~/bin
directory exist.
.Sh FILES
.Bl -tag -width ".Pa /var/run/ftpd.pid" -compact
.It Pa /etc/ftpusers
List of unwelcome/restricted users.
.It Pa /etc/ftpchroot
List of normal users who should be chroot'd.
.It Pa /etc/ftphosts
Virtual hosting configuration file.
.It Pa /etc/ftpwelcome
Welcome notice.
.It Pa /etc/ftpmotd
Welcome notice after login.
.It Pa /var/run/ftpd.pid
Default pid file for daemon mode.
.It Pa /var/run/nologin
Displayed and access refused.
.It Pa /var/log/ftpd
Log file for anonymous transfers.
.It Pa /var/log/xferlog
Default place for session logs.
.It Pa /var/spool/ftp
Recommended directory for the FTP root directory
(the home directory of the ftp user).
.El
.Sh SEE ALSO
.Xr ftp 1 ,
.Xr umask 2 ,
.Xr getusershell 3 ,
.Xr ftpchroot 5 ,
.Xr login.conf 5 ,
.Xr inetd 8 ,
.Xr syslogd 8
.Sh HISTORY
The
.Nm
utility appeared in
.Bx 4.2 .
IPv6 support was added in WIDE Hydrangea IPv6 stack kit.
.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.
The
possible security holes have been extensively
scrutinized, but are possibly incomplete.