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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  proto-
       col  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),  get-
       servbyname(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 clients connecting to low  num-
       bered  ports  can trust that the service running on the port is the standard imple-
       mentation, 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 ser-
       vices are started by inetd(8), and so won't appear in inetd.conf(5).   In  particu-
       lar, 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  ser-
                 vice  (but see the BUGS section below).  Again, the names are case sensi-
                 tive.

       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

FILES
       /etc/services
              The Internet network services list

       <netdb.h>
              Definition of _PATH_SERVICES

SEE ALSO
       listen(2), endservent(3), getservbyname(3), getservbyport(3),  getservent(3),  set-
       servent(3), inetd.conf(5), protocols(5), inetd(8)

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-09-23                       SERVICES(5)

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