services man page on CentOS

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

       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  pro‐

       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  only  be
       bound to 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
       /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 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 (but see the BUGS	 section  below).  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 inter-operability 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. (Cur‐
       rently,	they  are silently skipped by getservent(3), getservbyname(3),
       and getservbyport(3).  However, this behaviour  should  not  be	relied

       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 installations is depreciated.

       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

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

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

	      The Internet network services list

	      Definition of _PATH_SERVICES

       listen(2),  endservent(3),  getservbyname(3), getservbyport(3), getser‐
       vent(3), setservent(3), inetd.conf(5), protocols(5), inetd(8)

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

       Guide to Yellow Pages Service

       Guide to BIND/Hesiod Service

Linux				  1996-01-11			   SERVICES(5)

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