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

See attributes(7) for descriptions of the following attributes: