Netcat Tutorial

Submitted by Adam Palmer on Sat, 06/11/2005 - 2:20pm.

Netcat Tutorial
A Article By Adam Palmer © Learn Security Online, Inc. 2004-2005


Introduction 3
Netcat Syntax 4
Netcat Installation 6
Simple File Transfer 7
Tar 9
Simple socket reply 10
inetd 11
talking to syslogd -r 12
Internetworking Basics 13
nc-e 14
Scanning 15
Spoofing 16
Simple response service 17
Advanced Uses 18
Windows Command Shell 19
Unauthorized Proxying 20
Cryptcat 20
Final Thoughts 21
Command cheat sheet 22


What is Netcat?

"Netcat is a simple Unix utility which reads and writes data across network connections, using TCP or UDP protocol. It is designed to be a reliable "back-end" tool that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities. Netcat, or "nc" as the actual program is named, should have been supplied long ago as
another one of those cryptic but standard Unix tools." Taken from the README of the netcat source tree, this description sums up the uses of netcat perfectly.

Netcats homepage is:

Throughout this tutorial, I will be giving examples on Linux systems. The official Netcat homepage makes no reference to Windows systems, however I have successfully built Netcat from source under Cygwin, and you can find a Win32 copy built by @Stake from: and all examples used
below are fully supported under Windows. © Learn Security Online, Inc. 2004-2005

Netcat Syntax

adam@adamp:~$nc -h

connect to

somewhere: nc [-options] hostname

port[s] [ports] ..
listen for
inbound: nc -l -p port [-options]
[hostname] [port]

-e prog programto exec after connect [dangerous!!]
-b allow

-g gateway source-routing hop point[s], up to 8
-G num source-routing

pointer: 4, 8, 12, ...
-h this
-i secs delayinterval for lines sent, ports scanned
-l listen
mode, for inbound connects
-n numeric-onlyIP addresses, no DNS
-o file hex
dump of traffic
-p port localport number
-r randomize
local and remote ports
-q secs quitafter EOF on stdin and delay of secs
-s addr local
source address
-t answer
TELNET negotiation
-u UDP

-v verbose
[use twice to be more verbose]

-w secs timeout
for connects and final net reads

-z zero-I/
mode [used for scanning]

numbers can be individual or ranges: lo-hi [inclusive] © Learn Security Online, Inc. 2004-2005

Netcat Installation

I will cover here three installation methods.

a) On a debian or similar machine:

apt-get install netcat will do the trick:

adamp:~# apt-get install netcatReading Package Lists... DoneBuilding Dependency Tree... Done

The following NEW packages will be installed:

0 packages upgraded, 1 newly installed, 0 toremove and 0 not upgraded.
Need to get 63.3kB of archives. After unpacking190kB will be used.
Get:1 stable/mainnetcat 1.10-21 [63.3kB]
Fetched 63.3kB in 2s (27.9kB/s)
Selecting previously deselected package netcat.
(Readingdatabase ... 39433 files and directories currently installed.)
Unpacking netcat (from.../netcat_1.10-21_i386.deb) ...
Setting up netcat (1.10-21) ...

b) And for those that prefer RPMs:

rpm Uvh netcat-version.rpm

c) And for those that prefer the source:

We will start by wgeting the source:


We will now untar, cd to the directory we have untarred the source codeto, and run the configure script.

adam@adamp:~$ tar -xzf netcat-0.7.1.tar.gzadam@adamp:~$ cd netcat-0.7.1adam@adamp:~/netcat-0.7.1$ ./configure

The configure script should run through with no trouble, as netcat has very few dependencies.
We then run make:

adam@adamp:~/netcat-0.7.1$ make

This will run through and will compile your source, which again should complete simply and successfully. You
can then run make install if you have the necessary privileges, or you could simply run src/netcat which will have
been built after a successful make. At this point, you should now have a successful build of netcat
somewhere on your system. © Learn Security Online, Inc. 2004-2005

Simple File Transfer

So as an example, I will start two copies of netcat on the same machine locally:

adam@adamp:~$ netcat -l -p 1111

Here, using the -l switch, we are able to specify that netcat should go into listen mode i.e. to listen on
the specified port. Using p 1111 we are able to specify that we are using port 1111. To summarize,
netcat will sit and listen for TCP connections on port 1111 and print any data it receives out to the
screen. In another window we start netcat as:

adam@adamp:~$ netcat 1111

This will connect to host (Locally) on port 1111.
We are now able to have a full two way data transmission, in Window 1:

adam@adamp:~$ netcat -l -p 1111This message was typed in WINDOW1This message was typed in WINDOW2Now I'm going to end communication with ^C (Ctrl-C)

And in Window 2:

adam@adamp:~$ netcat 1111This message was typed in WINDOW1This message was typed in WINDOW2Now I'm going to end communication with ^C (Ctrl-C)

This is the most basic use of netcat described. Here, we are using a BASH shell, and thus we may pipe |
data to and from netcat, as well as using the redirection (>, >>, <, <<) to allow netcat to integrate into
the shell environment. We will now examine using netcat with one of the redirection operators. Lets
say we wanted to simply transmit a plaintext file. In one window, we will start netcat as:

adam@adamp:~$ netcat -l -p 1111 > outputfile

This will run netcat with the same parameters specified above, except it will redirect
all text received into outputfile.

adam@adamp:~$ echo > infile << EOF
> This is a test file.
> I am going to attempt to transmit this.
> Using Netcat.

Here, we have created some text in a file, and this is the file we are going to attempt to transmit:

adam@adamp:~$ cat infile | netcat 1111 q 10adam@adamp:~$ © Learn Security Online, Inc. 2004-2005

Hopefully this has now been transmitted to the otherside:
adam@adamp:~$ cat outputfile
This is a test file.

I am going to attempt to transmit this.
Using Netcat.

And here we can confirm that it has. The -q 10 in the command line will quit after EOF (Otherwise
netcat will hang waiting for more input for cat and we will have to terminate it manually). The
parameter 10 causes it to quit after 10 seconds anyway.


Now, there is no reason why we can’t integrate tar and netcat together, and use this to transmit a
directory across a netcat socket:

On one side: tar zcfp - /path/to/directory | nc -w 3 1234

The tar statement before the pipe tars and compresses (using gzip) every file within that directory,
before printing its output to stdout (The screen). It is then caught by the pipe, and piped to nc which in
this example, connects to on port 1234 and sends it the data which would normally hit the
screen. The w 3 switch causes nc to allow for a 3 second timeout (In the event of a temporary
disconnection or similar).

On the other side: nc -l -p 1234 | tar xvfpz

This will listen on port 1234 for a connection, and will pass any data received to tar. Using the option v
we can print out filenames to screen:

Simple Socket Reply

With what we have learned so far, we are easily able to get netcat to listen in on a socket, and pump out
any data we wish when it receives a connection.

As an example:

while true; do echo "Leave me alone" | netcat -l -p 1234 w10; done

Consider this line. Firstly lets examine echo "Leave me alone" | netcat -l -p 1234 -w10

What we are doing here, is listening in on port 1234 with a wait time of 10 seconds. If/when we receive
a connection, pipe the results of echo "Leave me alone" to netcat. The w 10 is necessary, as otherwise
any connection made in will remain open forever. We can also optionally add a v in to the netcat
command line which will give us verbose information, i.e. who is connecting.

Every time a connection times out (either with the w 10 command line switch, or because a connection
has been made and then closed), netcat will exit. As this is not what we want, we put the command line
within a standard BASH: while CONDITION; do STATEMENT; done clause, which when the
condition is set to true will run forever.


If you build netcat with GAPING_SECURITY_HOLE defined, you can use it as an "inetd" substitute
to test experimental network servers that would otherwise run under "inetd".

A script or program will have its input and output hooked to the network the same way, perhaps sans
some fancier signal handling.

Given that most network services do not bind to a particular local address, whether they are under
"inetd" or not, it is possible for netcat avoid the "address already in use" error by binding to a specific

This lets you [as root, for low ports] place netcat "in the way" of a standard service, since inbound
connections are generally sent to such specifically-bound listeners first and fall back to the ones bound
to "any".

This allows for a one-off experimental simulation of some service, without having to screw around
with inetd.conf. Running with -v turned on and collecting a connection log from standard error is

Netcat as well can make an outbound connection and then run a program or script on the originating
end, with input and output connected to the same network port.

This "inverse inetd" capability could enhance the backup-server concept described above or help
facilitate things such as a "network dialback" concept.

The possibilities are many and varied here; if such things are intended as security mechanisms, it may
be best to modify netcat specifically for the purpose instead of wrapping such functions in scripts.

Speaking of inetd, netcat will function perfectly well *under* inetd as a TCP connection redirector for
inbound services, like a "plug-gw" without the authentication step.

This is very useful for doing stuff like redirecting traffic through your firewall out to other places like
web servers and mail hubs, while posing no risk to the firewall machine itself.

Put netcat behind inetd and tcp_wrappers, perhaps thusly:

www stream tcp nowait nobody /etc/tcpd /bin/nc -w 3 realwww 80

and you have a simple and effective "application relay" with access control and logging. Note use of
the wait time as a "safety" in case realwww isn't reachable or the calling user aborts the connection --
otherwise the relay may hang there forever.

Inetd/tcp_wrappers and netcat information, courtesy of:

Talking to syslogd -r

Syslog Daemons running with the r switch log not only their own hosts data but accept remote UDP
broadcasts. They listen in on UDP port 514.

"echo '<0>message' | nc -w 1 -u loggerhost 514"

If loggerhost is running syslogd r and can accept your messages.

Note the -u switch here, to put netcat into UDP mode. Specifying the <0> before your message ensures
that your message receives top priority within syslog (kern.emerg)

Internetworking Basics

For the purposes of this section, machine refers to an x86 compatible PC with a connection to the
Internet through some means, terminated by a standardized TCP/IP stack.

Each machine on the Internet today comes shipped with a standard, compatible TCP/IP stack. This
stack guarantees the use of 65535 ports, and IPv4 protocol compatibility.

Below we can see the OSI model. This explains in terms of 7 layers, how data is constructed at one
host and received at the next.

In short; Data is constructed on the left by an application, encodes it with a transport (TCP) which takes
it over the network (IP), resolves MACs of local devices (Data Link) and then passes a constructed
packet to the network card which transmits (Physical) it over the wire (at which point the opposite
happens at the other end).

You may have intelligent devices such as switches along the way. These for example may be wise up
to layer 5 for example and not only route according to MAC address (Layer 2) but inspect and firewall
packets based on findings up to Layer 5 (Simple firewalling) or even Layer 7 (Packet inspection).

"The OSI, or Open System Interconnection, model defines a networking framework for implementing
protocols in seven layers. Control is passed from one layer to the next, starting at the application layer
in one station, proceeding to the bottom layer, over the channel to the next station and back up the
hierarchy." (Courtesy of:

nc -e

We have already discussed the basics of redirection with netcat. Netcat has a e switch which we can
use to execute a program on connection. There are a couple of viable and legitimate uses for this, i.e.
running as nc e v called by the inetd wrapper, which we can use to view traffic and information on
users connecting to wrapped daemons, however the most common use which we will explore here is
using it to redirect to and from /bin/bash or similar shell, for both good and bad.

One method could be this:

adam@adamp:~$ nc -v -e '/bin/bash' -l -p 1234 –tlistening on [any] 1234 ...
connect to [] from localhost [] 51210

In one window, and a simple telnet localhost 1234 in another window:

adam@adamp:~$ telnet 1234
Connected to
Escape character is '^]'
echo Test


The scanning features of netcat can be used against yours or your friends networks to get useful
information about which hosts have certain ports open. You can also send a precompiled data file to
each. For example:

Echo EXIT | nc -w 1 20-250 500-600 5990-7000

Will scan on ports 20-250, 500-600 and 5990-7000. Every port that it finds is open, it will
pipe the output of echo "EXIT" being the word "EXIT" to that port.
The results are as follows:

(For the sanity of my server, I have blocked out a number of parts from certain service banners.)
And now with UDP scanning: nc -v -w 1 u 20-250 500-600 5990-7000 we receive:

adam@adamp:~$ nc -u -v -w 1 20-250 500-600 5990-7000localhost [] 250 (?) openadam@adamp:~$

-v was to put netcat into verbose mode, and u was telling netcat to fall into UDP mode.


"Your TCP spoofing possibilities are mostly limited to destinations you can source-route to, while
locally bound to your phony address.

Many sites block source-routed packets these days for precisely this reason.

If your kernel does oddball things when sending source-routed packets, try moving the pointer
around with -G. You may also have to fiddle with the routing on your own
machine before you start receiving packets back.

Warning: some machines still send out traffic using the source address of the outbound interface,
regardless of your binding, especially in the case of localhost.

Check first. If you can open a connection but then get no data back from it, the target host is probably
killing the IP options on its end [this is an option inside TCP wrappers and several other packages],
which happens after the 3-way handshake is completed.

If you send some data and observe the "send-q" side of "netstat" for that connection increasing but
never getting sent, that's another symptom. Beware: if Sendmail 8.7.x detects a source-routed SMTP
connection, it extracts the hop list and sticks it in the Received: header!"

Spoofing is a useful technique, as is source routing.

Source routing is almost obsolete now, and the majority of routers filter out source routed packets.
Source routing in a nutshell is basically setting the route that the packet will take at the source, and
storing that information along with the packet.

Normally, each router makes its own mind up as to where a packet will get routed, and follows its
predefined routing tables. If we have access to all routers between our device and the target device
(which can be one machine if youre talking about your local LAN server), then we are able to modify
the routing entries on those devices, bind a phoney address to our machine and source route packets to
the intended destination.

Spoofing is where we modify the source address of a packet so that the recipient believes it came from
a different address. There are two problems with this;

A number of clever ISP routers will drop packets with incorrect source addresses.

If the destination host does get to receive your spoofed packet, it will send data back to the
spoofed address (instead of ours). This does have a number of uses however in the example of
ICMP ping flooding a host and spoofing the source address to (as a theoretical

Simple Response Service

echo -e "GET HTTP/1.0\n\n" | nc w 80

We make a connection to on port 80 (Web server port), and put in an HTTP request for At this point, we are presented with the HTML spurted out by the web server.
We can pipe this to "| less" or similar or even our favourite HTML interpreter.

Take a look at this example, and you will see what we have done here. In one instance we have created
an HTML file webfrontend and we now pipe that HTML to any incoming connection to netcat on port
1111. We then make a connection on the larger window, using lynx and we have
made ourselves a tiny http server, possibly could be used as a holding page server or something similar.

Advanced Uses

Now we'll set up a server netcat to listen on port 1111. We'll also set up a client netcat to talk to the real
web server on port 81. By getting them to pass all data they receive to each other, together they form a
proxy; something that sits in the middle of a network connection. Here are the commands we use:

mknod backpipe p

nc -l -p 1111 0backpipe

Because bash pipes only carry data in one direction, we need to provide a way to carry the responses as
well. We can create a pipe on the local filesystem to carry the data in the backwards direction with the
mknod command; this only needs to be run once.

Requests coming into the proxy from the client arrive at the first nc, listening on port 1111. They get
handed off to the "tee" command, which logs them to the inflow file, then continue on to the second nc
command which hands them off to the real web server. When a response comes back from the server, it
arrives back at the second nc command, gets logged in the second tee command to the outflow file, and
then gets pushed into the backpipe pipe on the local filesystem. Since the first netcat is listening to that
pipe, these responses get handed to that first netcat, which then dutifully gives them back to the original

While the above example is for watching tcp streams going to and from a web server, the above
technique is useful for watching any tcp connection. In fact, since nc also works with udp packets
something telnet can't do - it should be possible to even set up udp proxies this way.

Windows Command Shell

As we can see from the image above, we have started netcat with options of l p 1234 e
"c:\windows\system32\cmd.exe". These are the same options as with the Unix shell, and this should
theoretically start a cmd.exe shell listening in on port 1234:

As you see from above, this has succeeded. Netcat and program execution for Windows can be used in
exactly the same way.

Unauthorised Proxying

Assume you are an administrator of a Linux router. Using the methods above, as well as your iptables
software, you can proxy a users outgoing connection through your nc proxy. Using iptables with the j
DNAT target and the j REDIRECT target, you can transparently proxy outgoing connections through
to any other ports you want, and what better to use than your nc proxy?


Cryptcat can be found at: and is the ultimate companion for
Netcat. It includes a lightweight version of Netcat, featuring encrypted transport properties. (Just for
those superbly paranoid!). Useful for encrypting communications out of a network.

Final Thoughts

If I was given one tool on a freshly installed PC, I would ask for Netcat. Due to its versatility and its
huge range of uses, it can be used as a transfer tool, a scanning tool, a server, a proxy and so much
more. I have put down everything useful I can think of, and welcome any further suggestions directed

Command Cheat Sheet

The following are the most useful uses of netcat:

For windows nc d can be used to detach from the console.

nc -l -p [port] will create a simple listening tcp port. Add u to put into UDP mode.
nc -e [program] To redirect stdin/stdout from program.
nc -w [timeout] To set a timeout before netcat automatically quits. (Used within a loop usually)
program | nc To pipe output of program to netcat
nc | program To pipe output of netcat to program
nc -h Help sheet
nc -v To put into verbose mode, or use v v to put into ultra-verbose mode!
nc -g or nc G Source routing flags
nc -t Use telnet negotiation (If performing telnet negotiation)
nc -o [file] Hex dump traffic to file
nc -z No I/O (Used for scanning ports)