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. 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.
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 datagram-oriented connections, and may
cause noticeable delays in the case of connections 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/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.
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.
FILES
The default locations of the host access control tables are:
/etc/hosts.allow
/etc/hosts.deny
SEE ALSO
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.
AUTHORS
Wietse Venema (wietse@wzv.win.tue.nl),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands