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Special Forums IP Networking Help me understand ports and port forwarding please Post 302840477 by DGPickett on Monday 5th of August 2013 04:44:39 PM
Old 08-05-2013
Ports are fields in UDP and TCP packet headers that allow the flow to be divided on a host to 65K different apps. For instance tcp cpnnections could be made from 63K different apps on one host to port 80 web server on the next. Sometimes port numbers imply a protocol, like 80 for http, 25 for smtp, etc. Servers listen on ports and clients get random ports to identify their socket from al others on the host, In IPV4, you have 2^32 hosts and 2^16 ports, so there are 2^96 possible connections. UDP is connectionless, so a "connection" is just a filter on remote host+port and default remote host+port destination on a socket.

IP packets are identified by Host and protocol (such as TCP), and for tcp and udp, by port. Firewalls like iptables key off the host and port. With tcp, you can tell which end is the client (connecting) and which is the server (listening) in the first two packets (syn and syn+ack bits on, respectively). So, you can allow clients inside to connect everywhere outside but not vice-versa. ICMP is an IP sub-protocol that supports IP, TCP, UDP with control and diagnostic messages Some ICMP messages can be toxic if counterfeit.

IPTables also has NAT, the ability to rewrite packets for a new host, port or both going "out", and back to the original host/port for packets coming "in". This is handy if inside hosts are unroutable, like 10.*, or just to hide inside hosts. Some protocols like FTP (which runs on top of, or inside, TCP) put hosts and port numbers in the data stream as well, and some of these NAT knows how to rewrite. All packet rewriting include adjustment of checksums.
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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)
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