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tcpd \- access control facility for internet services
The \fItcpd\fR program can be set up to monitor incoming requests for
\fItelnet\fR, \fIfinger\fR, \fIftp\fR, \fIexec\fR, \fIrsh\fR,
\fIrlogin\fR, \fItftp\fR, \fItalk\fR, \fIcomsat\fR 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. Functionality may be limited when the protocol underneath TLI is
not an internet protocol.
There are two possible modes of operation: execution of \fItcpd\fP
before a service started by \fIinetd\fP, or linking a daemon with
the \fIlibwrap\fP shared library as documented in the \fIhosts_access\fR(3)
manual page. Operation when started by \fIinetd\fP
is as follows: whenever a request for service arrives, the
\fIinetd\fP daemon is tricked into running the \fItcpd\fP program
instead of the desired server. \fItcpd\fP logs the request and does
some additional checks. When all is well, \fItcpd\fP runs the
appropriate 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.
Connections that are monitored by
.I tcpd
are reported through the \fIsyslog\fR(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.
.I 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
\fIhosts_access\fR(5) manual page.
The authentication scheme of some protocols (\fIrlogin, rsh\fR) 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.
.I 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,
.I tcpd
concludes that it is dealing with a host that pretends to have someone
elses host name.
If the sources are compiled with -DPARANOID,
.I tcpd
will drop the connection in case of a host name/address mismatch.
Otherwise, the hostname can be matched with the \fIPARANOID\fR wildcard,
after which suitable action can be taken.
.I 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.
.SH RFC 931
When RFC 931 etc. lookups are enabled (compile-time option) \fItcpd\fR
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 datagram-oriented
connections, and may cause noticeable delays in the case of connections
from PCs.
The details of using \fItcpd\fR depend on pathname information that was
compiled into the program.
This example applies when \fItcpd\fR expects that the original network
daemons will be moved to an "other" place.
In order to monitor access to the \fIfinger\fR 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.
.in +5
# mkdir /other/place
# mv /usr/sbin/in.fingerd /other/place
# cp tcpd /usr/sbin/in.fingerd
The example assumes that the network daemons live in /usr/sbin. On some
systems, network daemons live in /usr/sbin or in /usr/libexec, or have
no `in.\' prefix to their name.
This example applies when \fItcpd\fR expects that the network daemons
are left in their original place.
In order to monitor access to the \fIfinger\fR service, perform the
following edits on the \fIinetd\fR configuration file (usually
.ti +5
finger stream tcp nowait nobody /usr/sbin/in.fingerd in.fingerd
.ti +5
finger stream tcp nowait nobody /usr/sbin/tcpd in.fingerd
The example assumes that the network daemons live in /usr/sbin. 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 \fItcpd\fR. Send a `kill -HUP\' to the \fIinetd\fR(8)
process to make the changes effective.
In the case of daemons that do not live in a common directory ("secret"
or otherwise), edit the \fIinetd\fR configuration file so that it
specifies an absolute path name for the process name field. For example:
ntalk dgram udp wait root /usr/sbin/tcpd /usr/local/lib/ntalkd
Only the last component (ntalkd) of the pathname will be used for
access control and logging.
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 services are registered with the \fIwait\fR
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 \fIrpc/tcp\fR in the inetd configuration file. The
only non-trivial service that is affected by this limitation is
\fIrexd\fR, which is used by the \fIon(1)\fR command. This is no great
loss. On most systems, \fIrexd\fR is less secure than a wildcard in
RPC broadcast requests (for example: \fIrwall, rup, rusers\fR) always
appear to come from the responding host. What happens is that the
client broadcasts the request to all \fIportmap\fR daemons on its
network; each \fIportmap\fR daemon forwards the request to a local
daemon. As far as the \fIrwall\fR etc. daemons know, the request comes
from the local host.
The default locations of the host access control tables are:
hosts_access(3), functions provided by the libwrap library.
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.
Wietse Venema (,
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
\" @(#) tcpd.8 1.5 96/02/21 16:39:16