ftpd —
Internet
File Transfer Protocol server
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] |
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
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.
- 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.
- 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(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.
- 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.
- /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.
Ftpd accesses a single environment variable:
SSL_CIPHER
- containing a list of acceptable cipher combinations.
ftp(1),
skey(1),
who(1),
getusershell(3),
ftpusers(5),
syslogd(8)
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
ftpd command appeared in
4.2BSD.