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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LEARN ABOUT OSF1
port_names
ports(7) Miscellaneous Information Manual ports(7)
NAME
ports, port_names - Device (tty and lp) names for serial and parallel ports
SYNOPSIS
Default Serial Ports:
/dev/tty00
/dev/tty01 (not present on a single-port system)
Parallel Port:
/dev/lp0
DESCRIPTION
AlphaStation and AlphaServer systems provide one or two 9-pin serial communication ports. These ports are usually labelled 1 (COMM1) and 2
(COMM2), but they may be identified by different icons. Using the appropriate serial cable and terminator, you can connect a serial
printer, external modem, or character-cell terminal to a serial port. Most AlphaStation and AlphaServer systems also provide one parallel
port, for use with a parallel printer.
When you add a device to your system, the installation documentation may instruct you to map the device pathname to the port. These
devices are located in the /dev directory.
For serial-line ports, the two default device pathnames are: This pathname always maps to 1, COMM1, the lowest port number, an icon for a
terminal console, or the only serial port (on a single-port system). This pathname always maps to 2, COMM2, the next numbered port, or (if
one serial port is labeled with an icon for a terminal console) the remaining serial port.
If your system hardware has been extended to include additional serial ports, the pathnames /dev/tty02, /dev/tty03, and so forth, may also
be available to you. However, most systems have only /dev/tty00 and /dev/tty01 as the device pathnames for serial ports.
The one parallel port on an AlphaStation or AlphaServer may be labeled with the word printer or a printer icon. On some systems, the paral-
lel port may not be labeled. The device pathname for the parallel port is /dev/lp0. Currently, Tru64 UNIX does not fully support parallel
printers, so fewer devices are connected to this port as compared to serial ports.
If you are connecting a terminal console to your system, it must be connected to the serial port mapped to /dev/tty00. For other serial
devices, it does not matter which of the serial ports you choose for the connection. For example, suppose you are setting up a system that
has two serial ports, labeled 1 and 2. You intend to use a serial-line terminal rather than a workstation monitor as the system console and
also want to connect a serial-line printer to the system. In this case, you must connect the terminal to the port labeled 1 (with the
device pathname /dev/tty00). Therefore, you must connect the printer to the remaining port labeled 2 (with the device pathname /dev/tty01).
If, for the same type of system, you intend to use a workstation monitor as the system console, it does not matter which serial port you
use for a serial-line printer or modem. In other words, you can connect the printer to either port 1 (with pathname /dev/tty00) or port 2
(with pathname /dev/tty01). When prompted to enter a /dev/tty** pathname by the lprsetup script or the Print configuration tool in the CDE
Application Manager, you would specify /dev/tty00 if you connected the printer to port 1 or /dev/tty01 if you connected the printer to port
2.
See the System Administration manual for more information on setting up consoles (including remote consoles) and printers. See the
modem(7) reference page for more information on setting up modems.
SEE ALSO
Commands: lprsetup(8)
Devices: ace(7), modem(7)
System Administration delim off
ports(7)