services - Internet network services list
services is a plain ASCII file providing a mapping between
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
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
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 that 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 run-
ning 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 initialised from the system boot scripts.
The location of the services file is defined by
_PATH_SERVICES in /usr/include/netdb.h. This is usually set
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
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 (but see the BUGS
section below). Again, the names are case sensi-
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 minimise inter-operability problems. Eg: 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 getser-
vent(3), getservbyname(3), and getservbyport(3). However,
this behaviour should not be relied on.)
As a backwards compatibility feature, the slash (/) between
the port number and protocol name can in fact be either a
slash or a comma (,). Use of the comma in modern installa-
tions is depreciated.
This file might be distributed over a network using a
network-wide naming service like Yellow Pages/NIS or
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
There is a maximum of 35 aliases, due to the way the getser-
vent(3) code is written.
Lines longer than BUFSIZ (currently 1024) characters will be
ignored by getservent(3), getservbyname(3), and get-
servbyport(3). However, this will also cause the next line
to be mis-parsed.
The Internet network services list
Definition of _PATH_SERVICES
getservent(3), getservbyname(3), getservbyport(3), setser-
vent(3), endservent(3), protocols(5), listen(2),
Assigned Numbers RFC, most recently RFC 1700, (AKA STD0002)
Guide to Yellow Pages Service
Guide to BIND/Hesiod Service