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TCPD(8)                              System Manager's Manual                              TCPD(8)

       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.

       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.

       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 match‐
       ing.  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.

       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 pre‐
       tends 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.

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 con‐
       nections, and may cause noticeable delays in the case of connections from PCs.

       The details of using tcpd depend on pathname information that was compiled into  the  pro‐

       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.

            # mkdir /other/place
            # mv /usr/sbin/in.fingerd /other/place
            # 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.

       This example applies when tcpd expects that the network daemons are left in their original

       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


            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.

       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  log‐

       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 reg‐
       istered 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.

       The default locations of the host access control tables are:


       hosts_access(3), functions provided by the libwrap library.
       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.

       Wietse Venema (wietse AT wzv.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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