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
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), getservby-
port(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 only be bound to by root (see bind(2), tcp(7), and
udp(7).) This is so that clients connecting to low num-
bered 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 cur-
rently 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 initialised from the
system boot scripts.
The location of the services file is defined by _PATH_SER-
VICES in /usr/include/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
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SERVICES(5) Linux Programmer's Manual SERVICES(5)
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 (but see the BUGS
section below). Again, the names are case sensi-
tive.
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
installations is depreciated.
This file might be distributed over a network using a net-
work-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
BUGS
There is a maximum of 35 aliases, due to the way the get-
servent(3) code is written.
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SERVICES(5) Linux Programmer's Manual SERVICES(5)
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.
FILES
/etc/services
The Internet network services list
/usr/include/netdb.h
Definition of _PATH_SERVICES
SEE ALSO
getservent(3), getservbyname(3), getservbyport(3), setser-
vent(3), endservent(3), protocols(5), listen(2),
inetd.conf(5), inetd(8).
Assigned Numbers RFC, most recently RFC 1700, (AKA
STD0002)
Guide to Yellow Pages Service
Guide to BIND/Hesiod Service
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