services(5) — Linux manual page


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

NAME         top

       services - Internet network services list

DESCRIPTION         top

       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 ...]


              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.

              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.

              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         top

              The Internet network services list

              Definition of _PATH_SERVICES

SEE ALSO         top

       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         top

       This page is part of release 5.09 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

Linux                            2020-04-11                      SERVICES(5)

Pages that refer to this page: endservent(3)freeaddrinfo(3)gai_strerror(3)getaddrinfo(3)getnameinfo(3)getservbyname(3)getservbyname_r(3)getservbyport(3)getservbyport_r(3)getservent(3)getservent_r(3)setservent(3)nscd(8)rpc.rquotad(8)rsyslogd(8)