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Linux 2.6 - man page for services (linux section 5)

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

       services - Internet network services list

       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),  getservby-
       port(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 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 usu-
       ally 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 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.  E.g., 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), getservbyname(3), and getservbyport(3).  However,  this
       behavior should not be relied on.)

       This  file  might  be  distributed over a network using a network-wide naming 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

	      The Internet network services list

	      Definition of _PATH_SERVICES

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

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

       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2010-05-22				      SERVICES(5)

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