services - Internet network services list
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
(3), and endservent
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
Port numbers below 1024 (so-called "low numbered" ports) can be bound
to only by root (see bind
(7), and udp
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
(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
. 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
- is the port number (in decimal) to use for this
- 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.
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). 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:
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
# 22 - unassigned
- The Internet network services list
- Definition of _PATH_SERVICES
Assigned Numbers RFC, most recently RFC 1700, (AKA STD0002).
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