Help me understand ports and port forwarding please Post: 302840477

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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

08-05-2013

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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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roar_socket_listen

roar_socket_listen(3)					System Manager's Manual: RoarAudio				     roar_socket_listen(3)

NAME

       roar_socket_listen - Opens a new listen socket

SYNOPSIS

       #include <roaraudio.h>

       int roar_socket_listen(int type, char * host, int port);

DESCRIPTION

       Opens a new listening socket for network clients to connect to.	You may accept new connections via accept(2).

PARAMETERS

       type   The type of the Socket. See the list below.

       host   The  host  or  path  name  to  listen  on.  This depends on the type argument.  For TCP/IP to listen on any interface use a value of
	      "0.0.0.0".

       port   The port number to listen on.  This may be ignored by types not supporting port numbers.

SOCKET TYPES

       ROAR_SOCKET_TYPE_UNKNOWN
	      The type will be auto detected. Don't use this if you know the type. Only use this to support user given addresses and  enable  auto
	      detection.

       ROAR_SOCKET_TYPE_TCP
	      Use  a TCP/IP connection. This is standard mode for networking.  host is the hostname or IP to listen on and port is the port number
	      to use.

       ROAR_SOCKET_TYPE_INET
	      Same as above but deprecated. Use ROAR_SOCKET_TYPE_TCP.

       ROAR_SOCKET_TYPE_UNIX
	      UNIX Domain Socket. This should be default for local connections.  host is the filename of the socket.

       ROAR_SOCKET_TYPE_DECNET
	      DECnet socket.

       ROAR_SOCKET_TYPE_FORK
	      This is to fork a roard to communicate with. Can't be use to listen on, just to connect to.  host and port is ignored at the moment.

       ROAR_SOCKET_TYPE_FILE
	      This is used to open a plain file. Can't be use to listen on, just to connect to.  host is the filename to connect to.

RETURN VALUE

       On success these calls return filehandle.  On error, -1 is returned.

BUGS

       Listening on UDP sockets is not supported at the moment.

EXAMPLES

       FIXME

SEE ALSO

       accept(2), libroar(7), RoarAudio(7).

RoarAudio							     May 2011						     roar_socket_listen(3)

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