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18 ftpd: FTP daemon

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.

     ftpd [option]...
-A
--anonymous-only
Only anonymous login is allowed.
-a auth
--auth=auth
Specify what authentication mechanism to use for incoming connections. Possible values are: ‘kerberos’, ‘kerberos5’, ‘opie’ and ‘default’.

Anonymous logins will continue to work when this option is used.

-D
--daemon
ftpd enters daemon-mode. That allows ftpd to be run without inetd.
-d
--debug
Debugging information is written to the syslog using facility ‘LOG_FTP’.
-l
--logging
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.
-p pidfile
--pidfile=pidfile
Change default location of pidfile.
-q
--no-version
Quiet mode. No information about the version of the ftpd is given to the client.
-T
--max-timeout
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 timeout
--timeout=timeout
The inactivity timeout period is set to timeout seconds (the default is 15 minutes).
-u umask
--umask=umask
Set default umask(base 8).

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.

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
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 CHMOD0 0CHMOD1 1CHMOD2
HELP give help information.

The remaining FTP requests specified in 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. This allows users to utilize the metacharacters ‘*?[]{}~’.

ftpd authenticates users according to three 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.
  2. The login name must not appear in the file /etc/ftpusers.
  3. The user must have a standard shell.
  4. 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 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.
~ftp/bin
Make this directory owned by ‘root’ and unwritable by anyone (mode 555). The program ls must be present to support the list command. This program should be mode 111.
~ftp/etc
Make this directory owned by ‘root’ and unwritable by anyone (mode 555). The files passwd and group 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/pub
Make this directory mode 777 and owned by ‘ftp’. Guests can then place files which are to be accessible via the anonymous account in this directory.

18.1 Configuration files

/etc/ftpusers
List of unwelcome/restricted users.
/etc/ftpwelcome
Welcome notice.
/etc/motd
Welcome notice after login.
/etc/nologin
Displayed and access refused.