Netcat or nc is a networking utility for debugging and investigating the network.
This utility can be used for creating TCP/UDP connections and investigating them. The biggest use of this utility is in the scripts where we need to deal with TCP/UDP sockets.
In this article we will learn about the netcat command by some practical examples.
1. Netcat in a Server-Client Architecture
The netcat utility can be run in the server mode on a specified port listening for incoming connections.
$ nc -l 2389
Also, it can be used in client mode trying to connect on the port(2389) just opened
$ nc localhost 2389
Now, if we write some text at the client side, it reaches the server side. Here is the proof :
$ nc localhost 2389 HI, server
On the terminal where server is running :
$ nc -l 2389 HI, server
So we see that netcat utility can be used in the client server socket communication.
2. Use Netcat to Transfer Files
The netcat utility can also be used to transfer files. At the client side, suppose we have a file named ‘testfile’ containing :
$ cat testfile hello test
and at the server side we have an empty file ‘test’
Now, we run the server as :
$ nc -l 2389 > test
and run the client as :
cat testfile | nc localhost 2389
Now, when we see the ‘test’ file at the server end, we see :
$ cat test hello test
So we see that the file data was transfered from client to server.
3. Netcat Supports Timeouts
There are cases when we do not want a connection to remain open forever. In that case, through ‘-w’ switch we can specify the timeout in a connection. So after the seconds specified along with -w flag, the connection between the client and server is terminated.
Server :
nc -l 2389
Client :
$ nc -w 10 localhost 2389
The connection above would be terminated after 10 seconds.
NOTE : Do not use the -w flag with -l flag at the server side as in that case -w flag causes no effect and hence the connection remains open forever.
4. Netcat Supports IPV6 Connectivity
The flag -4 or -6 specifies that netcat utility should use which type of addresses. -4 forces nc to use IPV4 address while -6 forces nc to use IPV6 address.
Server :
$ nc -4 -l 2389
Client :
$ nc -4 localhost 2389
Now, if we run the netstat command, we see :
$ netstat | grep 2389 tcp 0 0 localhost:2389 localhost:50851 ESTABLISHED tcp 0 0 localhost:50851 localhost:2389 ESTABLISHED
The first field in the above output would contain a postfix ‘6’ in case the IPV6 addresses are being used. Since in this case it is not, so a connection between server and client is established using IPV4 addresses.
Now, If we force nc to use IPV6 addresses
Server :
$ nc -6 -l 2389
Client :
$ nc -6 localhost 2389
Now, if we run the netstat command, we see :
$ netstat | grep 2389 tcp6 0 0 localhost:2389 localhost:33234 ESTABLISHED tcp6 0 0 localhost:33234 localhost:2389 ESTABLISHED
So now a postfix ‘6’ with ‘tcp’ shows that nc is now using IPV6 addresses.
5. Disable Reading from STDIN in Netcat
This functionality can be achieved by using the flag -d. In the following example, we used this flag at the client side.
Server :
$ nc -l 2389
Client :
$ nc -d localhost 2389 Hi
The text ‘Hi’ will not be sent to the server end as using -d option the read from stdin has been disabled.
6. Force Netcat Server to Stay Up
If the netcat client is connected to the server and then after sometime the client is disconnected then normally netcat server also terminates.
Server :
$ nc -l 2389
Client :
$ nc localhost 2389 ^C
Server :
$ nc -l 2389 $
So, in the above example we see that as soon as the client got disconnected the server was also terminated.
This behavior can be controlled by using the -k flag at the server side to force the server to stay up even after the client has disconnected.
Server :
$ nc -k -l 2389
Client :
$ nc localhost 2389 ^C
Server :
$ nc -k -l 2389
So we see that by using the -k option the server remains up even if the client got disconnected.
7. Configure Netcat Client to Stay Up after EOF
Netcat client can be configured to stay up after EOF is received. In a normal scenario, if the nc client receives an EOF character then it terminates immediately but this behavior can also be controlled if the -q flag is used. This flag expects a number which depicts number of seconds to wait before client terminates (after receiving EOF)
Client should be started like :
nc -q 5 localhost 2389
Now if the client ever receives an EOF then it will wait for 5 seconds before terminating.
8. Use Netcat with UDP Protocol
By default all the sockets that nc utility creates are TCP protocols but this utility also works with UDP protocol. To enable UDP protocol the -u flag is used.
Server :
$ nc -4 -u -l 2389
Client :
$ nc -4 -u localhost 2389
Now, both the server and client are configured to use UDP protocol. This can be confirmed by the following netstat command. So we see that this connection is now using the UDP protocol.
$ netstat | grep 2389 udp 0 0 localhost:42634 localhost:2389 ESTABLISHED
My name is Ramesh Natarajan. I will be posting instruction guides, how-to, troubleshooting tips and tricks on Linux, database, hardware, security and web. My focus is to write articles that will either teach you or help you resolve a problem. Read more about
Comments on this entry are closed.
Thanks for this overview. I just encountered an “nc” snippet in a script i have to debug, and as i never used it myself i guess this page will help me with this task.
Thank you Himanshu
@Pierre B
Thanks!!
Nice post. Thanks π
Great ………………….
Always great articles on this site. Is there any way to use netcat for multicasting udp or is there a different util for that? And for either answer if there is, will you be covering examples of it please?
A good introductory article. π
Thanks, its very useful for me.
Typo in section 3:
NOTE : Do not use the -w flag with -l flag at the server side as in that case -w flag causes no AFFECT and hence the connection remains open forever.
should read
NOTE : Do not use the -w flag with -l flag at the server side as in that case -w flag causes no EFFECT and hence the connection remains open forever.
Otherwise, very pleasant read and well structured post. Thx
@Anonymous,
Thanks for pointing it out. It is fixed.
Hello, in the first one : Netcat in a Server-Client Architecture
nc -l a_port doesn’t listen a_port. You should use nc -l -p a_port
Tanky Woo: There are at least two different versions of netcat around (“traditional” and “openbsd”, both available in Ubuntu etc). The traditional one requires -p to specify the port, while it’s optional in the openbsd version. At some point, some version (some older openbsd version?) complained if you used -p.
Hi guys,
I’m trying to understand NC a little better and this is a great tutorial! I’m trying to redirect a com port via udp, but starting out, I’m just testing a server that stays alive.
The following works fine, but if I disconnect the client, The server stays live, but when I reconnect again, no data is passed and the client drops back to the shell after attempting to transmit. Any thoughts?
Sever:
nc -k -l -4 -u 2221
Client
nc -4 -u localhost 2221
wow great post !
Hello im new in linux programming..i’m trying to send streaming packet data from my raspberry pi.
My packet data is stored on /home/pi/file , then i want to send that data to my android phone..
here is my code
cat /home/pi/file | awk -F’\t’ ‘END{print}’ | nc -l 8080
is that right ?
because on my android phone when receive the packet data is success,but sometimes jammed .
Thanks for advance !
UDP doesn’t work the way you have it defined here, there is no such thing as a UDP “connection”
UDP is connection-less and only listens for data packets on a given port, and NC behaves quite differently with UDP than it does with TCP because of this.
Your output showing a “UDP Connection” appears to be fabricated. Netstat will never show you an “established state” for anything UDP because there are no connections to establish. It won’t even show you client addresses because there are no connections, just packets coming in on a given port from any source.
very good article.
thank you!
Thanks for a very useful page and examples.
Thanks for these simple clear examples…
Thanks Sir. Very nice article. But I had a problem:
When I used IP6 to connect to Server my client or Client side terminal returned me an ERROR:
nc: getaddrinfo: Name or service not known
I googled the problem but could not found satisfactory answers. It will be very helpful if you can reply the solution. Thanks very much.