Closing Open Holes



September 27, 2000
By Ankit Fadia

With the spread of Hackers and Hacking incidents, the time has come, when not only system administrators of servers of big companies, but also people who connect to the Internet by dialing up into their ISP, have to worry about securing their system. It really does not make much difference whether you have a static IP or a dynamic one, if your system is connected to the Internet, then there is every chance of it being attacked.

This manual is aimed at discussing methods of system security analysis and will shed light on as to how to secure your standalone (also a system connected to a LAN) system.

Open Ports: A Threat to Security?

In the Netstat Tutorial we had discussed how the netstat -a command showed the list of open ports on your system. Well, anyhow, before I move on, I would like to quickly recap the important part. So here goes, straight from the netstat tutorial:

Now, the ??a? option is used to display all open connections on the local machine. It also returns the remote system to which we are connected to, the port numbers of the remote system we are connected to (and the local machine) and also the type and state of connection we have with the remote system.

For Example,

C:\windows>netstat -a

Active Connections


 Proto  Local Address          Foreign Address              State
 TCP    ankit:1031             dwarf.box.sk:ftp                       ESTABLISHED
 TCP    ankit:1036             dwarf.box.sk:ftp-data                 TIME_WAIT
 TCP    ankit:1043             banners.egroups.com:80    FIN_WAIT_2
 TCP    ankit:1045             mail2.mtnl.net.in:pop3       TIME_WAIT
 TCP    ankit:1052             zztop.boxnetwork.net:80   ESTABLISHED
 TCP    ankit:1053             mail2.mtnl.net.in:pop3       TIME_WAIT
 UDP    ankit:1025             *:*
 UDP    ankit:nbdatagram       *:*


Now, let us take a single line from the above output and see what it stands for:

Proto  Local Address          Foreign Address              State
TCP    ankit:1031                dwarf.box.sk:ftp              ESTABLISHED

Now, the above can be arranged as below:

Protocol: TCP (This can be Transmission Control Protocol or TCP, User Datagram Protocol or UDP or sometimes even, IP or Internet Protocol.)

Local System Name: ankit (This is the name of the local system that you set during the Windows setup.)

Local Port opened and being used by this connection: 1031  

Remote System: dwarf.box.sk (This is the non-numerical form of the system to which we are connected.)

Remote Port: ftp (This is the port number of the remote system dwarf.box.sk to which we are connected.)

State of Connection: ESTABLISHED

?Netstat? with the ??a? argument is normally used, to get a list of open ports on your own system i.e. on the local system. This can be particularly useful to check and see whether your system has a Trojan installed or not. Yes, most good Antiviral software are able to detect the presence of Trojans, but, we are hackers, and need to software to tell us, whether we are infected or not. Besides, it is more fun to do something manually than to simply click on the ?Scan? button and let some software do it.

The following is a list of Trojans and the port numbers which they use, if you Netstat yourself and find any of the following open, then you can be pretty sure, that you are infected.



Port 12345(TCP)                   Netbus
Port 31337(UDP)                 Back Orifice

For complete list, refer to the Tutorial on Trojans at: hackingtruths.box.sk/trojans.txt
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Now, the above tutorial resulted in a number of people raising questions like: If the 'netstat -a' command shows open ports on my system, does this mean that anyone can connect to them? Or, How can I close these open ports? How do I know if an open port is a threat to my system's security of not? Well, the answer to all these question would be clear, once you read the below paragraph:

Now, the thing to understand here is that, Port numbers are divided into three ranges:

The Well Known Ports are those from 0 through 1023. This range or ports is bound to the services running on them. By this what I mean is that each port usually has a specific service running on it. You see there is an internationally accepted Port Numbers to Services rule, (refer RFC 1700 Here) which specifies as to on what port number a particular service runs. For Example, By Default or normally FTP runs on Port 21. So if you find that Port 21 is open on a particular system, then it usually means that that particular system uses the FTP Protocol to transfer files. However, please note that some smart system administrators delibrately i.e. to fool lamers run fake services on popular ports. For Example, a system might be running a fake FTP daemon on Port 21. Although you get the same interface like the FTP daemon banner, response numbers etc, however, it actually might be a software logging your prescence and sometimes even tracing you!!!

The Registered Ports are those from 1024 through 49151. This range of port numbers is not bound to any specific service. Actually, Networking utlites like your Browser, Email Client, FTP software opens a random port within this range and starts a communication with the remote server. A port number within this range is the reason why you are able to surf the net or check your email etc.

If you find that when you give the netstat -a command, then a number of ports within this range are open, then you should probably not worry. These ports are simply opened so that you can get your software applications to do what you want them to do. These ports are opened temporarily by various applications to perform tasks. They act as a buffer transfering packets (data) received to the application and vis-a-versa. Once you close the application, then you find that these ports are closed automatically. For Example, when you type www.hotmail.com in your browser, then your browser randomly chooses a Registered Port and uses it as a buffer to communicate with the various remote servers involved.

The Dynamic and/or Private Ports are those from 49152 through 65535. This range is rarely used, and is mostly used by trojans, however some application do tend to use such high range port numbers. For Example,Sun starts their RPC ports at 32768.
So this basically brings us to what to do if you find that Netstat gives you a couple of open ports on your system:

1. Check the Trojan Port List and check if the open port matches with any of the popular ones. If it does then get a trojan Removal and remove the trojan.

2. If it doesn't or if the Trojan Remover says: No trojan found, then see if the open port lies in the registered Ports range. If yes, then you have nothing to worry, so forget about it.

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HACKING TRUTH: A common technique employed by a number of system administrators, is remapping ports. For example, normally the default port for HTTP is 80. However, the system administrator could also remap it to Port 8080. Now, if that is the case, then the homepage hosted at that server would be at:

http://domain.com:8080 instead of
http://domain.com:80

The idea behind Port Remapping is that instead of running a service on a well known port, where it can easily be exploited, it would be better to run it on a not so well known port, as the hacker, would find it more difficult to find that service. He would have to port scan high range of numbers to discover port remapping.

The ports used for remapping are usually pretty easy to remember. They are choosen keeping in mind the default port number at which the service being remapped should be running. For Example, POP by default runs on Port 110. However, if you were to remap it, you would choose any of the following: 1010, 11000, 1111 etc etc