2.12 Standard Internet Services
Figure
2.18 lists several standard services that are provided by most
implementations of TCP/IP. Notice that all are provided using both
TCP and UDP and the port number is the same for both protocols.
Often these services are provided by the
inetd daemon on Unix hosts (Section 13.5).
These standard services provide an easy testing facility using the
standard Telnet client. For example, the following tests both the
daytime and echo servers:
|
aix % telnet freebsd daytime
|
|
Trying 12.106.32.254...
|
output by Telnet
client
|
|
Connected to freebsd.unpbook.com.
|
output by Telnet
client
|
|
Escape character is '^]'.
|
output by Telnet
client
|
|
Mon Jul 28 11:56:22 2003
|
output by daytime
server
|
|
Connection closed by foreign host.
|
output by Telnet
client (server closes connection)
|
| |
|
aix % telnet freebsd echo
|
|
Trying 12.106.32.254...
|
output by Telnet
client
|
|
Connected to freebsd.unpbook.com.
|
output by Telnet
client
|
|
Escape character is '^]'.
|
output by Telnet
client
|
|
hello,
world
|
we type this
|
|
hello, world
|
and it is echoed back
by the server
|
|
^]
|
we type control and
right bracket to talk to Telnet client
|
|
telnet> quit
|
and tell client we are
done
|
|
Connection closed.
|
client closes the
connection this time
|
In these two examples, we type the name of the
host and the name of the service (daytime and
echo). These service names are mapped into the port
numbers shown in Figure
2.18 by the /etc/services file, as we will describe in
Section 11.5.
Notice that when we connect to the
daytime server, the server performs the active close,
while with the echo server, the client performs the active
close. Recall from Figure 2.4 that the
end performing the active close is the end that goes through the
TIME_WAIT state.
These "simple services" are often disabled by
default on modern systems due to denial-of-service and other
resource utilization attacks against them.
|
