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 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 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:
<2>service-name port<3>/<2>protocol <1>[<2>aliases ...<1>]
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
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. (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:
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 getservent(3) , getservbyname(3) , and getservbyport(3) . However, this will also cause the next line to be mis-parsed.