Red Hat NETSCAPE MANAGEMENT SYSTEM 6.2 - COMMAND-LINE Guida di Installazione Scaricare il pdf (Pagina 46)

8. Appendix

8.1. Servers, Ports, and Packets

Let's take a quick, non−technical look at some networking concepts, and how they can potentially impact our

own security. We don't need to know much about networking, but a general idea of how things work is

certainly going to help us with firewalls and other related issues.

As you may have noticed Linux is a very network oriented Operating System. Much is done by connecting to

"servers" of one type or another −− X servers, font servers, print servers, etc.

Servers provide "services", which provide various capabilities, both to the local system and potentially other

remote systems. The same server generally provides both functionalities. Some servers work quietly behind

the scenes, and others are more interactive by nature. We may only be aware of a print server when we need

to print something, but it is there running, listening, and waiting for connection requests whether we ever use

it or not (assuming of course we have it enabled). A typical Linux installation will have many, many types of

servers available to it. Default installations often will turn some of these "on".

And even if we are not connected to a real network all the time, we are still "networked" so to speak. Take

our friendly local X server for instance. We may tend to think of this as just providing a GUI interface, which

is only true to a point. It does this by "serving" to client applications, and thus is truly a server. But X

Windows is also capable of serving remote clients over a network −− even large networks like the Internet.

Though we probably don't really want to be doing this ;−)

And yes, if you are not running a firewall or have not taken other precautions, and are connected to the

Internet, it is quite possible that someone −− anyone −− could connect to your X server. X11 "listens" on

TCP "port" 6000 by default. This principle applies to most other servers as well −− they can be easily

connected to, unless something is done to restrict or prevent connections.

In TCP/IP (Transmission Control Protocol/Internet Protocol) networks like we are talking about with Linux

and the Internet, every connected computer has a unique "IP Address". Think of this like a phone number.

You have a phone number, and in order to call someone else, you have to know that phone number, and then

dial it. The phone numbers have to be unique for the system to work. IP address are generally expressed as

"dotted quad" notation, e.g. 152.19.254.81.

On this type of network, servers are said to "listen". This means that they have a "port" opened, and are

awaiting incoming connections to that port. Connections may be local, as is typically the case with our X

server, or remote −− meaning from another computer "somewhere". So servers "listen" on a specific

"port" for incoming connections. Most servers have a default port, such as port 80 for web servers. Or 6000

for X11. See /etc/services for a list of common ports and their associated service.

The "port" is actually just an address in the kernel's networking stack, and is a method that TCP, and other

protocols, use to organize connections and the exchange of data between computers. There are total of 65,536

TCP and UDP ports available, though usually only a relatively few of these are used at any one time. These

are classified as "privileged", those ports below 1024, and "unprivileged", 1024 and above. Most servers use

the privileged ports.

Only one server may listen on, or "bind" to, a port at a time. Though that server may well be able to open

multiple connections via that one port. Computers talk to each other via these "port" connections. One

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Red Hat NETSCAPE MANAGEMENT SYSTEM 6.2 - COMMAND-LINE Guida di Installazione Pagina 46