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TCPD
!!!TCPD
NAME
DESCRIPTION
LOGGING
ACCESS CONTROL
HOST NAME VERIFICATION
HOST ADDRESS SPOOFING
RFC 931
EXAMPLES
EXAMPLE 1
EXAMPLE 2
EXAMPLE 3
BUGS
FILES
SEE ALSO
AUTHORS
----
!!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.
!!LOGGING


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.
!!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-
''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.
!!EXAMPLES


The details of using ''tcpd'' depend on pathname
information that was compiled into the program.
!!EXAMPLE 1


This example applies when ''tcpd'' expects that the
original network daemons will be moved to an
''


In order to monitor access to the ''finger'' service,
move the original finger daemon to the
''


# mkdir /other/place
# mv /usr/etc/in.fingerd /other/place
# cp tcpd /usr/etc/in.fingerd
The example assumes that the network daemons live in /usr/etc. On some systems, network daemons live in /usr/sbin or in /usr/libexec, or have no `in.' prefix to their name.
!!EXAMPLE 2


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'' or
''/etc/inet/inetd.conf''):


finger  stream  tcp  nowait  nobody  /usr/etc/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/etc. 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. AIX
users may also have to execute the `inetimp'
command.
!!EXAMPLE 3


In the case of daemons that do not live in a common
directory (
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/sbin/in.ntalkd
Only the last component (ntalkd) of the pathname will be used for access control and logging.
!!BUGS


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.
!!FILES


The default locations of the host access control tables
are:


/etc/hosts.allow
/etc/hosts.deny
!!SEE ALSO


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.
!!AUTHORS


Wietse Venema (wietse@wzv.win.tue.nl),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
----
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Last edited on Tuesday, June 4, 2002 12:31:20 am by "perry"
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