TCPD(1M)TCPD(1M)NAMEtcpd - 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(1M) for more information.
LOGGING
Connections that are monitored by tcpd are reported through the sys‐
log(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 informa‐
tion from several hosts is merged.
In order to find out where your logs are going, examine the syslog con‐
figuration 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(4) 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 connec‐
tion 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 con‐
nection 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 con‐
sulting each other's identd services.
EXAMPLESATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Committed │
└────────────────────┴─────────────────┘
Sep 15, 2011 TCPD(1M)
