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FTPD(8) System Manager's Manual FTPD(8)

ftpd — Internet File Transfer Protocol server

ftpd [-46ADdlMnPSUW] [-m minuid] [-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).

The options are as follows:

When -D is specified, forces ftpd to use IPv4 addresses only.
When -D is specified, forces ftpd to use IPv6 addresses only.
Permit only anonymous FTP connections (unless the -n option is specified), accounts listed in /etc/ftpchroot or users in a login class with the “ftp-chroot” variable set (see below). Other connection attempts are refused.
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.
Debugging information is written to the syslog using LOG_FTP.
Each successful and failed FTP 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.
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.
minuid
Disallow login to user accounts with a UID below minuid. The default is 1000, to prevent access to administrative and daemon accounts. Anonymous access is allowed even if the UID of the FTP user is smaller than minuid.
Do not permit anonymous FTP logins. Normally they are permitted.
Permit illegal port numbers or addresses for PORT command initiated connects. By default ftpd 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.
With this option set, ftpd logs all anonymous downloads to the file /var/log/ftpd when this file exists.
maxtimeout
A client may also request a different timeout period; the maximum period allowed may be set to maxtimeout seconds with the -T option. The default limit is 2 hours.
timeout
The inactivity timeout period is set to timeout seconds (the default is 15 minutes).
Each concurrent FTP session is logged to the file /var/run/utmp, making them visible to commands such as who(1). -U and -W are mutually exclusive.
mask
Force the umask to mask, instead of the default specified in /etc/login.conf (usually 022). Also disallows chmod.
Do not save login records to /var/log/wtmp. -W and -U are mutually exclusive.

The file /etc/nologin can be used to disable FTP access. If the file exists, ftpd displays it and exits. Note: this method will disable non-root logins; see login(1) for further details. If the file /etc/ftpwelcome exists, ftpd prints it before issuing the “ready” message. If the welcome file exists (/etc/motd by default), 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.

ABOR abort previous command
ACCT specify account (not implemented)
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
EPSV prepare for server-to-server transfer
HELP give help information
LIST give list of files in a directory (ls -lgA)
LPRT specify data connection port
LPSV prepare for server-to-server transfer
MDTM show last modification time of file
MKD make a directory
MODE specify data transfer
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
REIN reinitialize (not implemented)
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
SMNT structure mount (not implemented)
STAT return status of server
STOR store a file
STOU store a file with a unique name
STRU specify data transfer
SYST show operating system type of server system
TYPE specify data transfer
USER specify user name; not valid after login
XCUP change to parent of current working directory (deprec.)
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:

CHMOD change mode of a file, e.g., SITE CHMOD 755 filename
HELP give help information
IDLE set idle-timer, e.g., SITE IDLE 60
UMASK change umask, e.g., SITE UMASK 002

The remaining FTP requests specified in Internet RFC 959 are recognized, but not implemented. MDTM and SIZE are specified in RFC 3659.

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 by using the service and type of ftp, as defined in the /etc/login.conf file (see login.conf(5)). An authentication style may be specified by appending with a colon (‘:’) following the authentication style, i.e. “joe:skey”. The allowed authentication styles for ftpd may be explicitly specified by the “auth-ftp” entry in /etc/login.conf.

ftpd authenticates users according to the following rules.

  1. The login name must be in the password database and not have a null password. In this case a password must be provided by the client before any file operations may be performed.
  2. The login name must not appear in the file /etc/ftpusers.
  3. The user account must have a UID not less than minuid.
  4. The user must have a standard shell as described by shells(5).
  5. If the user name appears in the file /etc/ftpchroot, which is a text file containing one user name per line, 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.
  6. 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).

Once a user is authenticated, the user must be approved by any approval script defined (see login.conf(5)). If a valid approval script (by either :approve=...: or :approve-ftp=...: for the user's class) is defined then it is run and must exit with a 0 (success) status. When ftpd is running under the -D flag (and debugging is not turned on) then the approval script will be called with at least the following variables specified via the -v option (see login.conf(5)) to the approve script:

FTPD_HOST The server's (virtual) hostname

For example (the line is broken to fit the page):

/usr/libexec/auth/approve_ftpd -v FTPD_HOST=ftp.mycompany.com \
	username class service

When the user logs in to the anonymous FTP account, 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/etc
Make this directory owned by “root” and unwritable by anyone (mode 511). The files pwd.db (see pwd_mkdb(8)) and group(5) must be present for the ls(1) command to be able to produce owner names rather than numbers. The password field in pwd.db 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/pub
Make this directory mode 555 and owned by “root”. This is traditionally where publicly accessible files are stored for download.

If logging to the /var/log/ftpd file is enabled, information will be written in the following format:

time
The time and date of the download, in ctime(3) format.
elapsed time
The elapsed time, in seconds.
remote host
The remote host (or IP number).
bytes
The number of bytes transferred.
path
The full path (relative to the FTP chroot space) of the file transferred.
type
The type of transfer; either ‘a’ for ASCII or ‘b’ for binary.
unused
Unused field containing a ‘*’, for compatibility.
unused
Unused field containing an ‘o’, for compatibility.
user type
The type of user; either ‘a’ for anonymous or ‘r’ for a real user (should always be anonymous).
name
Either a system login name or the value given for “email address” if an anonymous user.
service name
The network service name (always ftp).
unused
Unused field containing a ‘0’, for compatibility.
real name
The system login name if the connection is not anonymous, or a ‘*’ if it is.

Although fields exist for logging information on real users, this file is only used for anonymous downloads. Unused fields exist only for compatibility with other ftpd implementations.

The ftpd daemon uses the following FTP-specific parameters:

auth-ftp
The list of authentication types available to this class. See login.conf(5).
ftp-chroot
A boolean value. If set, users in this class will be automatically chrooted to the user's login directory.
ftp-dir
A path to a directory. This value overrides the login directory for users in this class. A leading tilde (‘~’) in ftp-dir will be expanded to the user's home directory based on the contents of the password database.
welcome
The path of the file containing the welcome message. If this variable is not set, /etc/motd is used.

For passive mode data connections, ftpd will listen to a random high TCP port. The interval of ports used are configurable using sysctl(8) variables net.inet.ip.porthifirst and net.inet.ip.porthilast.

/etc/ftpchroot
list of normal users who should be chrooted
/etc/ftpusers
list of unwelcome/restricted users
/etc/ftpwelcome
welcome notice
/etc/login.conf
authentication styles
/etc/motd
printed after a successful login
/etc/nologin
displayed and access refused
/var/log/ftpd
log file for anonymous downloads
/var/log/wtmp
login account records
/var/run/utmp
list of users on the system

ftp(1), login(1), skey(1), who(1), chroot(2), ctime(3), group(5), login.conf(5), motd(5), services(5), shells(5), ftp-proxy(8), inetd(8), pwd_mkdb(8), sysctl(8), syslogd(8)

J. Postel and J. Reynolds, FILE TRANSFER PROTOCOL (FTP), RFC 959, October 1985.

P. Hethmon, Extensions to FTP, RFC 3659, March 2007.

The ftpd command appeared in 4.2BSD.

March 31, 2022 OpenBSD-current