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SERVICES(5) Linux Programmer's Manual SERVICES(5)

NAME

     services - Internet network services list

DESCRIPTION

     services  is  a  plain  ASCII  file  providing a mapping between human-
     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 pro-
     tocols 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  be  bound
     to  only by root (see bind(2), tcp(7), and udp(7)).  This is so 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 running 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 initialized from the system boot scripts.

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

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

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

     where:

     service-name
               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 service.

     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.  Again, the names are case sensitive.

     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  minimize  compatibility  problems.   For
     example, 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  getservent(3),  getservby-
     name(3),  and  getservbyport(3).   However, this behavior should not be
     relied on.)

     This file might be distributed over a network using a network-wide nam-
     ing service like Yellow Pages/NIS or BIND/Hesiod.

     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

FILES

     /etc/services
            The Internet network services list

     <netdb.h>
            Definition of _PATH_SERVICES