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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), getservby-
name(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 only be bound to by
root (see bind(2), tcp(7), and udp(7)). This is so clients connecting to low num-
bered ports can trust that the service running on the port is the standard imple-
mentation, 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 ser-
vices are started by inetd(8), and so won’t appear in inetd.conf(5). In particu-
lar, 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
/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 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 ser-
vice (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 minimize
inter-operability problems. E.g., 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 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 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
BUGS
There is a maximum of 35 aliases, due to the way the getservent(3) code is written.
Lines longer than BUFSIZ (currently 1024) characters will be ignored by getser-
vent(3), getservbyname(3), and getservbyport(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), setservent(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
Linux 1996-01-11 SERVICES(5)
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