hi all .sorry for last thread about hack , but someone told me he can access my PC over the internet and thats why i asked that question
anyway when i test open port on my localhost it gave me this :

[root@Hamzah-PC ~]# nmap -sS localhost

Starting Nmap 5.00 ( http://nmap.org ) at 2009-10-13 23:05 EEST
Interesting ports on localhost (127.0.0.1):
Not shown: 995 closed ports
PORT STATE SERVICE
22/tcp open ssh
25/tcp open smtp
111/tcp open rpcbind
631/tcp open ipp
3306/tcp open mysql

am not good enough with networking ,can someone explain me what this port mean ? and how to open and close each one ?
also what you Device me to protect my PC ?

Hi 7mza,

as an example, the open port 22 means that the ssh-daemon is listning on port 22 for incoming requests, which means that anybody can access your computer via ssh.

Look at the /etc/services file, which lists the services and the related ports. Also read "man services".

To close the port stop the sshd daemon. How this is to do depends on the distribution. Mostly something like /etc/init.d/sshd stop. The name of the directory where the scripts for the daemon-programs reside may differ.

I think this will point you in the right direction to understand the other open ports.

Markus

If you need or want to leave sshd up, then it would be the responsibility of a firewall to block access to the system where sshd is running. That firewall typically is a separate piece of network gear (like the firewall built into a home router).

If you have no external hardware firewall, then you probably should think about at least using the software firewall facilities built into some (most?) distributions. There is one built into Fedora by default. (Mine is off at home as my Linksys router gives me a false sense of security; others might argue that it is a misplaced false sense of security...)

Others could advise about config'ing the builtin firewall, but in the Fedora gui, you can go to system->administration->firewall to see how or if it is set up.

You can also use the menu System->Admin->Services to turn off those services.
Note that you really need to run that nmap from the outside of your router to see what ports are being forwarded to your system from the internet.
If this is just a home pc with no other systems, you prob don't need them open.
OTOH, localhost (127.0.0.1) is how your machine talks to itself, try nmap against the (probably) 192.168.x.x address.
Use

ifconfig -a

to get the 192.168.x.x. address.

You'll prob find almost no services running if this is a default install.

On localhost you almost definitely don't need ssh & rpcbind (used by nfs, nis).
Really you need to research each of those and see if you need them.
A simpler way is just to turn them off one at a time and see if anything breaks.