services - Internet network services list


     services is a plain ASCII file providing a  mapping  between
     friendly  textual  names  for  internet  services, and their
     underlying assigned port numbers and protocol  types.  Every
     networking  program  should  look  into this file to get the
     port number (and protocol) for its service.  The  C  library
     routines  getservent(3), getservbyname(3), getservbyport(3),
     setservent(3), and endservent(3) support querying this  file
     from programs.

     Port numbers are assigned by  the  IANA  (Internet  Assigned
     Numbers  Authority),  and  their current policy is to assign
     both TCP and UDP protocols when  assigning  a  port  number.
     Therefore,  most entries will have two entries, even for TCP
     only services.

     Port numbers below 1024 (so-called 'low numbered' ports) can
     only  be bound to by root (see bind(2), tcp(7), and udp(7).)
     This is so that clients connecting to low numbered ports can
     trust  that  the service running on the port is the standard
     implementation, and not a rogue service run by a user of the
     machine.   Well-known port numbers specified by the IANA are
     normally located in this root only space.

     The presence of an entry for a service in the services  file
     does not necessarily mean that the service is currently run-
     ning on the machine. See inetd.conf(5) for the configuration
     of  Internet  services offered. Note that not all networking
     services are started by inetd(8), and  so  won't  appear  in
     inetd.conf(5).  In  particular,  news (NNTP) and mail (SMTP)
     servers are often initialised from the system boot scripts.

     The  location  of  the   services   file   is   defined   by
     _PATH_SERVICES in /usr/include/netdb.h.  This is usually set
     to /etc/services.

     Each line describes one service, and is of the form:

          service-name   port/protocol   [aliases ...]


               is the friendly name the service is known  by  and
               looked  up under. It is case sensitive. Often, the
               client program is named after the service-name.

     port      is the port number (in decimal) to  use  for  this

     protocol  is the type of protocol to  be  used.  This  field
               should  match  an  entry in the protocols(5) file.
               Typical values include tcp and udp.

     aliases   is an optional space  or  tab  separated  list  of
               other  names  for  this  service (but see the BUGS
               section below). Again, the names are  case  sensi-

     Either spaces or tabs may be used to separate the fields.

     Comments are started by the hash sign (#) and continue until
     the end of the line. Blank lines are skipped.

     The service-name should begin in the  first  column  of  the
     file,  since leading spaces are not stripped.  service-names
     can be any printable characters  excluding  space  and  tab,
     however,  a conservative choice of characters should be used
     to minimise inter-operability problems. Eg:  a-z,  0-9,  and
     hyphen (-) would seem a sensible choice.

     Lines not matching this format should not be present in  the
     file.  (Currently,  they  are  silently  skipped  by getser-
     vent(3), getservbyname(3), and  getservbyport(3).   However,
     this behaviour should not be relied on.)

     As a backwards compatibility feature, the slash (/)  between
     the  port  number  and protocol name can in fact be either a
     slash or a comma (,). Use of the comma in  modern  installa-
     tions is depreciated.

     This file might  be  distributed  over  a  network  using  a
     network-wide   naming   service  like  Yellow  Pages/NIS  or

     A sample services file might look like this:

          netstat         15/tcp
          qotd            17/tcp          quote
          msp             18/tcp          # message send protocol
          msp             18/udp          # message send protocol
          chargen         19/tcp          ttytst source
          chargen         19/udp          ttytst source
          ftp             21/tcp
          # 22 - unassigned
          telnet          23/tcp


     There is a maximum of 35 aliases, due to the way the getser-
     vent(3) code is written.

     Lines longer than BUFSIZ (currently 1024) characters will be
     ignored   by   getservent(3),   getservbyname(3),  and  get-
     servbyport(3).  However, this will also cause the next  line
     to be mis-parsed.


          The Internet network services list

          Definition of _PATH_SERVICES


     getservent(3), getservbyname(3),  getservbyport(3),  setser-
     vent(3),     endservent(3),     protocols(5),     listen(2),
     inetd.conf(5), inetd(8).

     Assigned Numbers RFC, most recently RFC 1700, (AKA STD0002)

     Guide to Yellow Pages Service

     Guide to BIND/Hesiod Service