services

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
       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 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), 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), 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 5.08 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                              2020-04-11                        SERVICES(5)