Ubuntu Manpage: ftpd — Internet File Transfer Protocol server

Provided by: ftpd-ssl_0.17.36+0.3-2.2_amd64

NAME

     ftpd — Internet File Transfer Protocol server

SYNOPSIS

     ftpd [-AdDhlMnPSU] [-T maxtimeout] [-t timeout] [-u mask] [-z debug] [-z debug=file]
          [-z certsok] [-z certrequired] [-z secure] [-z ssl] [-z verify=flags]
          [-z cacert=cafile] [-z cert=certfile] [-z key=keyfile] [-z cipher=list] [-z legacy]

DESCRIPTION

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:

-4 Use IPv4 addressing only. The default is to offer service for both families, IPv6
and IPv4.

-6 Only provide IPv6 addressing capability.

-A Permit only anonymous ftp connections or accounts listed in /etc/ftpchroot. Other
connection attempts are refused. This option is no longer 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.

-n Use numeric IP addresses in logs instead of doing hostname lookup.

-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.

-z SSL-parameter
This option is only valid if ftpd has been built with SSL (Secure Socket Layer)
support.

secure Don't fall back to unencrypted mode, that is without SSL, if the client
is not explicitly asking for SSL mode. In this server mode ftpd only
accepts connections from SSL enhanced FTP clients with an option similar
to -z secure in active use.

ssl Negotiate SSL at first, then fall back to legacy FTP protocol.

nossl, !ssl
switch off SSL negotiation

debug Enable SSL related debugging. Useless in non-daemon mode.

debug=file Direct the debugging output to file.

certsok Look username up in /etc/ssl.users. The format of this file is lines of
this form: user1,user2:/C=US/...
where user1 and user2 are usernames. If the client certificate is
valid, authenticate with any password. Use a command openssl x509 -noout
-subject to extract the needed fields, all of which are needed.

certrequired
Client certificate is mandatory and the user must be matched to the
corresponding subject identifier listed in /etc/ssl.users.

verify=int Set the SSL verify flags (use combinations of SSL_VERIFY_* from
openssl/ssl.h ).

cacert=ca_file
Use the CA certificates stored in ca_file to verify the identity of the
peer client. The subject names found herein are given to the client for
whatever use they may present. A clever client software is able to
choose its identity hinted by this list.

cert=cert_file
Use the certificate(s) in cert_file instead of the default location
/etc/ftpd-ssl/ftpd.pem. This is a PEM formatted file. The first
certificate identifies the server and the rest of the chain is used for
verification purposes while talking to the peer client.

key=key_file
Use the key stored in key_file, should the certificate file not contain
the required private key.

cipher=ciph_list
Set the preferred ciphers to ciph_list. See openssl/ssl.h for more
information).

legacy This is a compatibility option, which activates a work around during
verification, which the legacy code depended on. It should not be used
now that chains and CA lists are available, but is introduced to ease
the transition to the better implementation.

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
EPRT specify data connection port, either IPv4 or IPv6
EPSV ask for a server port for fetching data
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. In order to read passwd(5) and group(5), the
library libnss_files.so.2 is also needed. 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.

FILES

     /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.
     /etc/ftpd-ssl/ftpd.pem
                      Default certificate and key for SSL authentication.
     /etc/ssl.users   List of trusted users and their subject identifiers.

ENVIRONMENT

     Ftpd accesses a single environment variable:

     SSL_CIPHER  containing a list of acceptable cipher combinations.

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.

HISTORY

     The ftpd command appeared in 4.2BSD.