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TCPD(8) TCPD(8)
NAME
tcpd - access control facility for internet services
DESCRIPTION
The tcpd program can be set up to monitor incoming requests for telnet, finger,
ftp, exec, rsh, rlogin, tftp, talk, comsat and other services that have a one-to-
one mapping onto executable files.
The program supports both 4.3BSD-style sockets and System V.4-style TLI. Function-
ality may be limited when the protocol underneath TLI is not an internet protocol.
Operation is as follows: whenever a request for service arrives, the inetd daemon
is tricked into running the tcpd program instead of the desired server. tcpd logs
the request and does some additional checks. When all is well, tcpd runs the appro-
priate server program and goes away.
Optional features are: pattern-based access control, client username lookups with
the RFC 931 etc. protocol, protection against hosts that pretend to have someone
elses host name, and protection against hosts that pretend to have someone elses
network address.
LOGGING
Connections that are monitored by tcpd are reported through the syslog(3) facility.
Each record contains a time stamp, the client host name and the name of the
requested service. The information can be useful to detect unwanted activities,
especially when logfile information from several hosts is merged.
In order to find out where your logs are going, examine the syslog configuration
file, usually /etc/syslog.conf.
ACCESS CONTROL
Optionally, tcpd supports a simple form of access control that is based on pattern
matching. The access-control software provides hooks for the execution of shell
commands when a pattern fires. For details, see the hosts_access(5) manual page.
HOST NAME VERIFICATION
The authentication scheme of some protocols (rlogin, rsh) relies on host names.
Some implementations believe the host name that they get from any random name
server; other implementations are more careful but use a flawed algorithm.
tcpd verifies the client host name that is returned by the address->name DNS server
by looking at the host name and address that are returned by the name->address DNS
server. If any discrepancy is detected, tcpd concludes that it is dealing with a
host that pretends to have someone elses host name.
If the sources are compiled with -DPARANOID, tcpd will drop the connection in case
of a host name/address mismatch. Otherwise, the hostname can be matched with the
PARANOID wildcard, after which suitable action can be taken.
HOST ADDRESS SPOOFING
Optionally, tcpd disables source-routing socket options on every connection that it
deals with. This will take care of most attacks from hosts that pretend to have an
address that belongs to someone elses network. UDP services do not benefit from
this protection. This feature must be turned on at compile time.
RFC 931
When RFC 931 etc. lookups are enabled (compile-time option) tcpd will attempt to
establish the name of the client user. This will succeed only if the client host
runs an RFC 931-compliant daemon. Client user name lookups will not work for data-
gram-oriented connections, and may cause noticeable delays in the case of connec-
tions from PCs.
EXAMPLES
The details of using tcpd depend on pathname information that was compiled into the
program.
EXAMPLE 1
This example applies when tcpd expects that the original network daemons will be
moved to an "other" place.
In order to monitor access to the finger service, move the original finger daemon
to the "other" place and install tcpd in the place of the original finger daemon.
No changes are required to configuration files.
# mkdir /other/place
# mv /usr/etc/in.fingerd /other/place
# cp tcpd /usr/etc/in.fingerd
The example assumes that the network daemons live in /usr/etc. On some systems,
network daemons live in /usr/sbin or in /usr/libexec, or have no ‘in.´ prefix to
their name.
EXAMPLE 2
This example applies when tcpd expects that the network daemons are left in their
original place.
In order to monitor access to the finger service, perform the following edits on
the inetd configuration file (usually /etc/inetd.conf or /etc/inet/inetd.conf):
finger stream tcp nowait nobody /usr/etc/in.fingerd in.fingerd
becomes:
finger stream tcp nowait nobody /some/where/tcpd in.fingerd
The example assumes that the network daemons live in /usr/etc. On some systems,
network daemons live in /usr/sbin or in /usr/libexec, the daemons have no ‘in.´
prefix to their name, or there is no userid field in the inetd configuration file.
Similar changes will be needed for the other services that are to be covered by
tcpd. Send a ‘kill -HUP´ to the inetd(8) process to make the changes effective.
AIX users may also have to execute the ‘inetimp´ command.
EXAMPLE 3
In the case of daemons that do not live in a common directory ("secret" or other-
wise), edit the inetd configuration file so that it specifies an absolute path name
for the process name field. For example:
ntalk dgram udp wait root /some/where/tcpd /usr/local/lib/ntalkd
Only the last component (ntalkd) of the pathname will be used for access control
and logging.
BUGS
Some UDP (and RPC) daemons linger around for a while after they have finished their
work, in case another request comes in. In the inetd configuration file these ser-
vices are registered with the wait option. Only the request that started such a
daemon will be logged.
The program does not work with RPC services over TCP. These services are registered
as rpc/tcp in the inetd configuration file. The only non-trivial service that is
affected by this limitation is rexd, which is used by the on(1) command. This is no
great loss. On most systems, rexd is less secure than a wildcard in
/etc/hosts.equiv.
RPC broadcast requests (for example: rwall, rup, rusers) always appear to come from
the responding host. What happens is that the client broadcasts the request to all
portmap daemons on its network; each portmap daemon forwards the request to a local
daemon. As far as the rwall etc. daemons know, the request comes from the local
host.
FILES
The default locations of the host access control tables are:
/etc/hosts.allow
/etc/hosts.deny
SEE ALSO
hosts_access(5), format of the tcpd access control tables.
syslog.conf(5), format of the syslogd control file.
inetd.conf(5), format of the inetd control file.
AUTHORS
Wietse Venema (wietse AT wzv.nl),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
TCPD(8)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2009-01-10 12:51 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)