Tribblix: manual page: tcpd.8

TCPD(8) Maintenance Commands and Procedures TCPD(8)

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.

Operation 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.

LIBWRAP INTERFACE

The same monitoring and access control functionality provided by the
tcpd standalone program is also available through the libwrap shared
library interface. Some programs, including the Solaris inetd daemon,
have been modified to use the libwrap interface and thus do not
require replacing the real server programs with tcpd. The libwrap
interface is also more efficient and can be used for inetd internal
services. See inetd(8) for more information.

LOGGING

Connections that are monitored by tcpd are reported through the
syslog(3C) 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.

Warning: If the local system runs an RFC 931 server it is important
that it be configured NOT to use TCP Wrappers, or that TCP Wrappers
be configured to avoid RFC 931-based access control for this service.
If you use usernames in the access control files, make sure that you
have a hosts.allow entry that allows the RFC 931 service (often
called "identd" or "auth") without any username restrictions. Failure
to heed this warning can result in two hosts getting in an endless
loop of consulting each other's identd services.

EXAMPLES

ATTRIBUTES