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Name

ftpd - DARPA Internet File Transfer Protocol server

Synopsis

ftpd [-A | -a] [-C] [-c] [-d] [-E] [-l] [-v] [-T maxtimeout] [-t time_out] [-p port] [-U ftpusers-file] [-u umask] [-r realm-file] [-s srvtab] [-w{ip|maxhostlen[,{striplocal|nostriplocal}]}]

Description

Ftpd is the DARPA 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:

-A
Connections are only allowed for users who can authenticate via the ftp AUTH mechanism. (Anonymous ftp may also be allowed if it is configured.) Ftpd will ask the user for a password if one is required.

-a
Connections are only allowed for users who can authenticate (via the ftp AUTH mechanism) and who are authorized to connect to the named account without a password. (Anonymous ftp may also be allowed if it is configured.)

-C
Non-anonymous users need local credentials (for example, to authenticate to remote fileservers), and so they should be prompted for a password unless they forwarded credentials as part of authentication.

-c
Allow the CCC (Clear Command Channel) command to be used. This allows less secure connections, and should probably only be used when debugging.

-d
Debugging information is written to the syslog. (Identical to -v)

-E
Don’t allow passwords to be typed across unencrypted connections.

-l
Each ftp(1) session is logged in the syslog. If this flag appears twice, additional information about operations performed (such as files retrieved, directories created, etc.) will be logged via syslog. If it appears three times, some other statistics such as the number of bytes transferred will be logged via syslog as well.

-v
Debugging information is written to the syslog. (Identical to -d)

-T maxtimeout
A client may request a maximum timeout period allowed set to timeout seconds with the -T option. The default limit is 2 hours. This is different from the normal inactivity timeout specified by the -t option (see below).

-t timeout
The inactivity timeout period is set to timeout seconds (the default is 15 minutes).

-p port
Run as a server and accept a connection on port. Normally the ftp server is invoked by inetd(8) .

-U ftpusers-file
Sets the full path and name of the ftpusers file to use. The default value is normally /etc/ftpusers.

-u umask
Sets the umask for the ftpd process. The default value is normally 027.

-r realm-file
Sets the name of the krb.conf file to use. The default value is normally set by /etc/krb5.conf.

-s srvtab
Sets the name of the srvtab file to use for Kerberos V4 authentication. The default value is normally /etc/srvtab.

-w {ip|maxhostlen[,{striplocal|nostriplocal}]}
Controls the form of the remote hostname passed to login(1) . Specifying ip results in the numeric IP address always being passed to login(1) . Specifying a number, maxhostlen, sets the maximum length of the hostname passed to login(1) before it will be passed as a numeric IP address. If maxhostlen is 0, then the system default, as determined by the utmp or utmpx structures, is used. The nostriplocal and striplocal options, which must be preceded by a comma, control whether or not the local host domain is stripped from the remote hostname. By default, the equivalent of striplocal is in effect.

The ftp server currently supports the following ftp requests; case is not distinguished.

Request Description

ABOR
abort previous command
ACCT
specify account (ignored)
ADAT
send an authentication protocol message
ALLO
allocate storage (vacuously)
APPE
append to a file
AUTH
specify an authentication protocol to be performed
CCC
set the command channel protection mode to “Clear” (no protection). Only available if the -c command-line option was given.
CDUP
change to parent of current working directory
CWD
change working directory
DELE
delete a file
ENC
send a privacy and integrity protected command (given in argument)
HELP
give help information
LIST
give list files in a directory (’’ls -lgA’’)
MIC
send an integrity protected command (given in argument)
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
PBSZ
specify a protection buffer size
PORT
specify data connection port
PROT
specify a protection level under which to protect data transfers
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. E.g., SITE HELP

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 the following rules:

1. The user name must be in the password data base, /etc/passwd.

2. An AUTH command must be accepted, the ensuing authentication protocol (conducted via ADAT commands and replies) must successfully complete, and the authenticated user must permitted access. Otherwise, a valid password which is not null must be provided by the client.

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

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

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 this is given as the client host’s name).

In the last case, ftpd takes special measures to restrict the client’s access privileges. The server performs a chroot(2) command 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; the following rules are recommended.

~ftp Make the home directory owned by ‘’ftp’’ and unwritable by anyone.

~ftp/bin
Make this directory owned by the super-user and unwritable by anyone. The program ls(1) must be present to support the list command. This program should have mode 111.

~ftp/etc
Make this directory owned by the super-user and unwritable by anyone. 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 encrypted passwords. These files should be mode 444.

~ftp/pub
Make this directory mode 777 and owned by ‘’ftp’’. Users should then place files which are to be accessible via the anonymous account in this directory.

If an ADAT command succeeds, the control channel must be either integrity or privacy protected. In this case, the MIC and ENC commands are the only commands allowed over the control channel. The argument to the MIC command is a base 64 encoded string which, when decoded, is an ftp command integrity protected with a cryptographic checksum. The argument to the ENC command is a base 64 encoded string which, when decoded, is an ftp command privacy and integrity protected with encryption.

If an ADAT command succeeds, ftp replies will also be either integrity or privacy protected.

If an ADAT command succeeds, the data channel can also be integrity or privacy protected. The PROT command accepts S for integrity and P for privacy protection. Unless an ADAT command succeeds, the only protection level accepted by the PROT command is C (clear).

See Also

ftp(1) , getusershell(3) , syslogd(8)

Lunt, S. J., FTP Security Extensions, Internet Draft, November 1993.

Bugs

The anonymous account is inherently dangerous and should avoided when possible.

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.


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