The Services and Protocols Files
The port numbers on which certain “standard” services are
offered are defined in the Assigned Numbers RFC. To enable server and client
programs to convert service names to these numbers, at least part of
the list is kept on each host; it is stored in a file called
/etc/services
. An entry is made up like this:
service
port
/protocol
[aliases
]
Here, service
specifies the service name,
port
defines the port the service is offered on,
and protocol
defines which transport protocol
is used. Commonly, the latter field is either
udp
or
tcp
. It is possible for a service to be
offered for more than one protocol, as well as offering different services on
the same port as long as the protocols are different. The
aliases
field allows you to specify alternative
names for the same service.
Usually, you don’t have to change the services file that comes along with the network software on your Linux system. Nevertheless, we give a small excerpt from that file in Example 12.2.
Example 12-2. A Sample /etc/services File
# The services file: # # well-known services echo 7/tcp # Echo echo 7/udp # discard 9/tcp sink null # Discard discard 9/udp sink null # daytime 13/tcp # Daytime daytime 13/udp # chargen 19/tcp ttytst source # Character Generator chargen 19/udp ttytst source # ftp-data 20/tcp # File Transfer Protocol (Data) ftp 21/tcp # File Transfer Protocol (Control) telnet 23/tcp # Virtual Terminal Protocol smtp 25/tcp # Simple Mail Transfer Protocol nntp 119/tcp readnews ...
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