ncat, a modern implementation of Netcat

This post appeared originally in our sysadvent series and has been moved here following the discontinuation of the sysadvent microsite

ncat is a utility that is like the UNIX cat command but for network connections. It’s based on the original netcat and comes with a couple of more modern features.

In this short post, we’ll go through a couple of examples to see exactly what uses this tool has. I’m currently using ncat version 7.01, in Ubuntu 16.04. ncat is a part of the nmap package in Ubuntu.

Shiny new things

A couple of the features of ncat, some of which are new, are:

• IPv6 Support
• Chain multiple ncat together
• Support for SSL
• Ability to specify specific hosts to allow or deny access to in listen mode

While the new features are great, it’s important to note that ncat is not 100% reverse compatible with the original netcat.

Examples

Let’s continue with a couple of examples to get you started.

IPv4 or IPv6?

To force ncat to only use either IPv4 og IPv6, use:

• -4
• -6

..as in:

ncat -6 <server> 10100

..to connect to a server only through IPv6.

Banner grabbing

ncat -C scanme.nmap.org 80

..and type in:

GET / HTTP/1.0

..and press enter twice. The result will be something along the lines of:

...
HTTP/1.1 200 OK
Date: Fri, 25 Nov 2016 09:01:08 GMT
Server: Apache/2.4.7 (Ubuntu)
Accept-Ranges: bytes
Vary: Accept-Encoding
Connection: close
Content-Type: text/html
...

The option -C is used because it requires CRLF line endings.

Chaining

An example from nmap’s website; sending a log file from host1 to host3, by way of host2:

host3:

ncat -l > log.txt

host2:

ncat -l --sh-exec "ncat host3"

host1:

ncat --send-only host2 < log.txt

Cloning partitions over the network

One of the more useful tricks is the ability to clone partitions over the network.

On the system you’d like to clone the partition from, do:

dd if=/dev/sda | ncat -l 10100

..and on the receiving machine:

ncat <server> 10100 | dd of=/dev/sda

To speed up the process of transfer you can always throw in gzip for compression:

dd if=/dev/sda | gzip -9 | ncat -l 10100

..and:

ncat <server> 10100 | gzip -d | dd of=/dev/sda

Web server

Setting up a simple web server is also easy:

ncat -l 8080 -k --sh-exec "echo -e 'HTTP/1.1 200 OK\r\n'; cat index.html"

The option -k makes ncat keep listening and accepting more connections after the first one is finished.

File transfer with SSL

On the machine you want to send the file from:

ncat -l 10100 --ssl --send-only < secret.tar.gz

..and on the receiving end:

ncat <server> 10100 --ssl > secret.tar.gz

The option –send-only does what it says - it only sends data and ignores received.

Chat server

As far I know, the are two main ways to do this.

First way:

Start listening on a port of your choice:

ncat -l 10100

..and connect to it from another machine:

ncat <server> 10100

Type in some text and the line will appear on the other machine when you press enter. You won’t be able to see who wrote what, but hey, it’s good enough if you want to communicate with someone.

Second way:

The new fancier way of starting a chat-server is by using --chat:

ncat --chat -l 10100

Users who then want to connect to the chat:

ncat <server> 10100

The output will be something along the lines of:

<user0> Hello?
<user5> Is it me you're looking for?

The user IDs generated by ncat are based on the file descriptor for each connection and must be considered arbitrary. Also, you won’t see <userX> in front of the text you type, but others will see it. The main difference when using –chat is that you and every user connected to the server will get a <userX> tag, making it easier to see who wrote what.

Mail client

ncat also works as a mail client. Expect to type a lot:

ncat -C mail.example.com 25

..followed up by typing:

220 mail.example.com ESMTP
HELO client.example.com
250 mail.example.com Hello client.example.com
MAIL FROM:a@example.com
250 OK
RCPT TO:b@example.com
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
From: a@example.com
To: b@example.com
Subject: Greetings from ncat

This short message is brought to you by ncat.
.
250 OK
QUIT
221 mail.example.com closing connection

TCP/UDP daytime server

The daytime service, defined in RFC 867, sends a human-readable date and time string over TCP or UDP port 13. It ignores any input. So, we can use:

ncat -l 13 --keep-open --send-only --exec "/bin/date"

Add –udp to create an UDP daytime server instead.

Access control

• Allow one host, deny others:

   ncat -l --allow 10.0.0.2
  

• Deny one host, allow others:

   ncat -l --deny 10.0.0.2
  

• Allow or deny hosts from file:

   ncat -l --allowfile trusted-hosts.txt
  

Replace --allowfile with --denyfile to deny and trusted-hosts.txt with a file that contains the hosts to be denied.

These are just a few of the things that you can do with ncat. Have fun exploring the rest!