Red Hat NETSCAPE MANAGEMENT SYSTEM 6.2 - COMMAND-LINE Guida di Installazione Scaricare il pdf (Pagina 48)

One more point on ports: ports are only accessible if there is something listening on that port. No one can

force a port open if there is no service or daemon listening there, ready to handle incoming connection

requests. A closed port is a totally safe port.

And a final point on the distinction between clients and servers. The example above did not have a telnet or

ftp server in the LISTENER section in the netstat example above. In other words, no such servers were

running locally. You do not need to run a telnet or ftp server daemon in order to connect to somebody

else's telnet or ftp server. These are only for providing these services to others that would be making

connections to you. Which you don't really want to be doing in most cases. This in no way effects the ability

to use telnet and ftp client software.

8.2. Common Ports

A quick run down of some commonly seen and used ports, with the commonly associated service name, and

risk factor. All have some risk. It is just that some have historically had more exploits than others. That is

how they are evaluated below, and not necessarily to be interpreted as whether any given service is safe or

not.

1−19, assorted protocols, many of which are antiquated, and probably none of which are needed on a modern

system. If you don't know what any of these are, then you definitely don't need them. The echo service (port

7) should not be confused with the common ping program. Leave all these off.

20 − FTP−DATA. "Active" FTP connections use two ports: 21 is the control port, and 20 is where the data

comes through. Passive FTP does not use port 20 at all. Low risk, but see below.

21 − FTP server port, aka File Transfer Protocol. A well entrenched protocol for transferring files between

systems. Very high risk, and maybe the number one crack target.

22 − SSH (Secure Shell), or sometimes PCAnywhere. Low to moderate risk (yes there are exploits even

against so called "secure" services).

23 − Telnet server. For LAN use only. Use ssh instead in non−secure environments. Moderate risk.

25 − SMTP, Simple Mail Transfer Protocol, or mail server port, used for sending outgoing mail, and

transferring mail from one place to another. Moderate risk. This has had a bad history of exploits, but has

improved lately.

37 − Time service. This is the built−in inetd time service. Low risk. For LAN use only.

53 − DNS, or Domain Name Server port. Name servers listen on this port, and answer queries for resolving

host names to IP addresses. High Risk.

67 (UDP) − BOOTP, or DHCP, server port. Low risk. If using DHCP on your LAN, this does not need to be

exposed to the Internet.

68 (UDP) − BOOTP, or DHCP, client port. Low risk.

Security Quick−Start HOWTO for Red Hat Linux

8.2. Common Ports 45

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Red Hat NETSCAPE MANAGEMENT SYSTEM 6.2 - COMMAND-LINE Guida di Installazione Pagina 48