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Now we will learn about the more technical stuff.
Lets begin with the basics. Ports allow a box to offer the services to another box.
The most common ports are 21,23,25,80,110.
These are the most common ports but there are much more than that. Thats why you need a port scanner to find out which of the ports are open.
Nmap is a good scanner but there are many others.
To install nmap download it in .rpm form.
To download it go to www.insecure.org and they have a lot of information on NMAP.
Then go into bash a linux program similar to dos that you should have learned by now if you didn't go get the book Running Linux.
After the download, type in command from bash.
To run it
It will show all the ports that are open, and the state of the port and the service of the port. It will also to a tcp sequence prediction and a remote operating system guess. The remote operating system guess is extremely important. The nmap program guesses what the servers operating system is. There is another way to do it as i will show below.
lets say port 21 is open.
In bash type in:
The XXXX in quotations stands for the operating system the server is using.
Just by doing that you have found out the operating system.
If you find that the operating system that is shown when you telnet into port 21 is different from the operating system guess during the nmap scan, the server is probabaly faking there operating system. I would go with the operating system guess during the nmap scan. It is probably the more accurate of the two.
Now we should try port 25.
bash$ telnet target.com 25
The XXXX is the daemon that is begin used.
Now you know the SMTP daemon and the version, and the operating system.
If there are other ports that you found go ahead and telnet into them to check it out.
WARNING !! (Telnet is not illegal so you can telnet as much as you want but some ISP may step in and take action if they find a large amount of Telneting. But, as long as they have no proof of you breaking into computers you are fine.)
I realize this tutorial may be confusing so if you have any questions email me a firstname.lastname@example.org or click on this link: Submit Question and I will get back to you as soon as possible.