Provided by: tcpd_7.6.q-25_amd64 
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
TCPD(8)