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  proto-
       cols 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 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), setservent(3), inetd.conf(5), protocols(5), inetd(8)

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

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux                                                               2010-05-22                                                         SERVICES(5)