TCPD(8)                                                   TCPD(8)

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 pro-
       tocol 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 infor-
       mation 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  com-
       mands   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 algo-
       rithm.

       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  mis-
       match.   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 "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/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  (usu-
       ally /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  /some/where/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 direc-
       tory ("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  /some/where/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 con-
       figuration 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  hap-
       pens  is  that  the  client  broadcasts the request to all
       portmap daemons on its network; each portmap  daemon  for-
       wards  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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