Originální popis anglicky:
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 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:
- 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 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:
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
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.
- /etc/services
- The Internet network services list
- /usr/include/netdb.h
- Definition of _PATH_SERVICES
listen(2),
endservent(3),
getservbyname(3),
getservbyport(3),
getservent(3),
setservent(3),
inetd.conf(5),
protocols(5),
inetd(8)
Assigned Numbers RFC, most recently RFC 1700, (AKA STD0002)
Guide to Yellow Pages Service
Guide to BIND/Hesiod Service
