SERVICES(5) Linux Programmer's Manual SERVICES(5)
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 from pro-
Port numbers are assigned by the IANA (Internet Assigned Numbers
Authority), and their current policy is to assign both TCP and UDP pro-
tocols 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 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
/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 ...]
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
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 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 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. (Cur-
rently, they are silently skipped by getservent(3), getservbyname(3),
and getservbyport(3). However, this behaviour should not be relied
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 nam-
ing 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
There is a maximum of 35 aliases, due to the way the getservent(3) code
Lines longer than BUFSIZ (currently 1024) characters will be ignored by
getservent(3), getservbyname(3), and getservbyport(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), setservent(3), end-
servent(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)