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.
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.
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(1M) for more
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(4) 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
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.
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.
See attributes(5) for descriptions of the following attributes: