Tcpd

Computing: Linux: tcpd

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.

There are  two possible modes of operation: execution of tcpd before a service started by inetd, or linking a daemon with the  libwrap  shared library  as  documented  in  the hosts_access(3) manual page. Operation when started by inetd 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->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 else's 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 else's 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 "other" place.

In order  to  monitor  access to the finger service, move the original finger daemon to the "other" place and install tcpd in the place of the original finger daemon. No changes are required to configuration files.


 * 1) mkdir /other/place
 * 2) mv /usr/sbin/in.fingerd /other/place
 * 3) cp tcpd /usr/sbin/in.fingerd

The example assumes that the network daemons live in /usr/sbin. 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):

finger stream  tcp  nowait  nobody  /usr/sbin/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/sbin. 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.

EXAMPLE 3
In the case of daemons that do not live in a common directory ("secret" or otherwise), edit the 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/local/lib/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

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

Edit Log

 * 2005-10-11 Transcribed from Ubuntu linux tcpd manpage