tcpd - access control facility for internet services
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.
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.
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.
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.
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.
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.
The details of using
tcpd depend on pathname information that was
compiled into the program.
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.
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.
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.
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.
The default locations of the host access control tables are:
/etc/hosts.allow
/etc/hosts.deny
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 ([email protected]),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands