Internet File Transfer Protocol server

is the Internet File Transfer Protocol server process. The server uses the TCP protocol and listens at the port specified in the service specification; see services(5) .

Available options:

-A

Permit only anonymous ftp connections or accounts listed in /etc/ftpchroot. Other connection attempts are refused. This option is nolonger 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() 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 they are set to 49152 and 65535 respectively.

-l

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.

-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.

-P

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.

-S

With this option set, ftpd logs all anonymous transfers to the file /var/log/ftpd when this file exists.

-U

Each concurrent ftp() 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.

debug

Enable SSL related debugging.

ssl

Negotiate SSL at first, then use ftp protocol. In this mode ftpd only accepts connections from SSL enhanced ftp with option -z ssl . (Not yet implemented)

nossl, !ssl

switch of SSL negotiation

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 client certificate is valid, authenticate without password.

certrequired

client certificate is mandatory

secure

Don't switch back to unencrypted mode (no SSL) if SSL is not available.

verify=int

Set the SSL verify flags (SSL_VERIFY_* in ssl/ssl.h ).

cert=cert_file

Use the certificate(s) in cert_file .

key=key_file

Use the key(s) in key_file .

cipher=ciph_list

Set the preferred ciphers to ciph_list . (See ssl/ssl.h ).

The file /etc/nologin can be used to disable ftp access. If the file exists, displays it and exits. If the file /etc/ftpwelcome exists, prints it before issuing the message. If the file /etc/motd exists, prints it after a successful login. If the file .message exists in a directory, prints it when that directory is entered.

The ftp server currently supports the following ftp requests. The case of the requests is ignored.

Request Ta Description

ABOR Ta abort previous command

ACCT Ta specify account (ignored)

ALLO Ta allocate storage (vacuously)

APPE Ta append to a file

CDUP Ta change to parent of current working directory

CWD Ta change working directory

DELE Ta delete a file

HELP Ta give help information

LIST Ta give list files in a directory Pq Dq ls -lgA

MKD Ta make a directory

MDTM Ta show last modification time of file

MODE Ta specify data transfer Em mode

NLST Ta give name list of files in directory

NOOP Ta do nothing

PASS Ta specify password

PASV Ta prepare for server-to-server transfer

PORT Ta specify data connection port

PWD Ta print the current working directory

QUIT Ta terminate session

REST Ta restart incomplete transfer

RETR Ta retrieve a file

RMD Ta remove a directory

RNFR Ta specify rename-from file name

RNTO Ta specify rename-to file name

SITE Ta non-standard commands (see next section)

SIZE Ta return size of file

STAT Ta return status of server

STOR Ta store a file

STOU Ta store a file with a unique name

STRU Ta specify data transfer Em structure

SYST Ta show operating system type of server system

TYPE Ta specify data transfer Em type

USER Ta specify user name

XCUP Ta change to parent of current working directory (deprecated)

XCWD Ta change working directory (deprecated)

XMKD Ta make a directory (deprecated)

XPWD Ta print the current working directory (deprecated)

XRMD Ta remove a directory (deprecated)

The following non-standard or UNIX specific commands are supported by the SITE request.

Sy Request Ta Sy Description

UMASK Ta change umask, e.g. ``SITE UMASK 002''

IDLE Ta set idle-timer, e.g. ``SITE IDLE 60''

CHMOD Ta change mode of a file, e.g. ``SITE CHMOD 755 filename''

HELP Ta 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 conventions used by csh(1) . This allows users to utilize the metacharacters

Ftpd authenticates users according to five rules.

•

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() for more information on S/Key authentication. S/Key is a Trademark of Bellcore.

•

The login name must not appear in the file /etc/ftpusers .

•

The user must have a standard shell returned by getusershell(3) .

•

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() as for an or 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.

•

If the user name is or an anonymous ftp account must be present in the password file (user 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 user. In order that system security is not breached, it is recommended that the subtree be constructed with care, following these rules:

~ftp

Make the home directory owned by and unwritable by anyone (mode 555).

~ftp/bin

Make this directory owned by and unwritable by anyone (mode 511). This directory is required, and should contain at least a statically linked copy of ls() Any programs in this directory should be mode 111 (executable only).

~ftp/etc

Make this directory owned by and unwritable by anyone (mode 511). The files passwd() and group() must be present for the () 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 and unwritable by anyone (mode 511). The libraries () and () (or whatever your () command is linked to) must be present. In order to read passwd() and group(5) , the library () is also needed. Note that if you're using a 2.2.* or later Linux kernel, () 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 This is traditionally where publically accessible files are stored for download.

/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.

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.

The command appeared in