tcpd
access control facility for internet services
add an example, a script, a trick and tips
examples
The details of using tcpd depend on pathname information
that was compiled into the program.
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.
Functionality may be limited when the protocol underneath
TLI is not an internet protocol.
There are two
possible modes of operation: execution of tcpd before
a service started by inetd, or linking a daemon with
the libwrap shared library as documented in the
hosts_access(3) manual page. Operation when started
by inetd 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
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.
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.
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/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.
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):
finger stream tcp nowait nobody /usr/sbin/in.fingerd in.fingerd
becomes:
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 tcpd. Send a ’kill -HUP´ to the
inetd(8) process to make the changes effective.
example 3
In the case of daemons that do not live in a common directory
("secret" or otherwise), 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 /usr/sbin/tcpd /usr/local/lib/ntalkd
Only the last component (ntalkd) of the pathname will be used for
access control and logging.
files
The default locations of the host access control tables are:
/etc/hosts.allow
/etc/hosts.deny
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.
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.
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.
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
datagram-oriented connections, and may cause noticeable delays in
the case of connections from PCs.
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 services 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.
see also
hosts_access,
functions provided by the libwrap library.
hosts_access, format of the tcpd access control tables.
syslog.conf, format of the syslogd control file.
inetd.conf, format of the inetd control file.
authors
Wietse Venema
(wietse[:at:]wzv.win.tue[:dot:]nl),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands