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 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
       (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  /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.

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