02-20-2003
That's pretty close, but you are are ignoring the protocols. The IP address gets the packet to the box. The next thing that happens is the IP layer looks at the protocol. The protocol might be TCP, UDP, ICMP, and a few others. So the IP layer hands the packet to the proper protocol layer. This will usually be TCP or UDP. Both TCP and UDP will then look at the port number. If a program has that port open it will get the packet.
The program could be client or server.
Try this:
grep telnet /etc/services
You should see:
telnet 23/tcp
So a telnet daemon might be running and if it is, it would be using port 23 but under the tcp protocol. A packet coming in to port 23 with the udp protocol would be an error of some kind. It is even possible that some other program is using 23/udp but that would be rare.
I don't want to write a complete book here, but one more point... tcp is connection oriented. That means that your average tcp packet has a second port and a second ip address that tell where it came from. So there 4 things to look at:
destination ip address
destination port
source ip address
source ip port
All 4 have to match. That is how you can have 20 telnetd's running. They all have the same local ip address and they use port 23. But it takes all 4 numbers to define a socket.
There is more to this and you really should read a book. I suggest TCP/IP Illustrated Volume 1 by Rich Stevens.
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LEARN ABOUT DEBIAN
aconnect
aconnect(1) General Commands Manual aconnect(1)
NAME
aconnect - ALSA sequencer connection manager
SYNOPSIS
aconnect [-d] [-options] sender receiver
aconnect -i|-o [-options]
aconnect -x
DESCRIPTION
aconnect is a utility to connect and disconnect two existing ports on ALSA sequencer system. The ports with the arbitrary subscription
permission, such as created by aseqview(1), can be connected to any (MIDI) device ports using aconnect. For example, to connect from port
64:0 to 65:0, run as follows:
% aconnect 64:0 65:0
The connection is one-way, and the whole data to the sender port (64:0) is redirected to the receiver port (65:0). When another port (e.g.
65:1) is attached to the same sender port, the data is sent to both receiver ports. For disconnection, use -d option.
% aconnect -d 64:0 65:0
The address can be given using the client's name.
% aconnect External:0 Emu8000:1
Then the port 0 of the client matching with the string "External" is connected to the port 1 of the client matching with the "Emu8000".
Another function of aconnect is to list the present ports on the given condition. The input ports, which may become sender ports, can be
listed with -i option.
% aconnect -i
client 0: 'System' [type=kernel]
0 'Timer '
1 'Announce '
client 64: 'External MIDI-0' [type=kernel]
0 'MIDI 0-0 '
Similarly, to see the output ports, use -o flag.
You can remove all existing exported connections using -x option. This function is useful for terminating the ALSA drivers, because the
modules with sequencer connections cannot be unloaded unless their connections are removed.
OPTIONS
CONNECTION MANAGEMENT
-d, --disconnect
Disconnect the given subscription.
-e, --exclusive
Connect ports with exclusive mode. Both sender and receiver ports can be no longer connected by any other ports.
-r, --real queue
Convert time-stamps of event packets to the current value of the given real-time queue. This is option is, however, not so useful,
since the receiver port must use (not necessarily own) the specified queue.
-t, --tick queue
Like -r option, but time-stamps are converted to the current value of the given tick queue.
LIST PORTS
-i, --input
List existing input (readable) ports. This option is exclusive to -o.
-o, --output
List existing output (writable) ports. This option is exclusive to -i.
-l, --list
List the current connection status. The connected and connecting ports from/to each port are listed together. The suffix flag [ex]
means the connection is exclusive. The suffix flag [real:#] and [tick:#] mean the connection includes real-time and tick conversion
on the listed queue, respectively.
REMOVE ALL CONNECTIONS
-x, --removeall
Remove all exported connections.
SEE ALSO
aseqnet(1), aseqview(1)
AUTHOR
Takashi Iwai <tiwai@suse.de>
August 31, 2000 aconnect(1)