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8 Practical Linux Netcat NC Command Examples

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  11/13/2017 8 Practical Linux Netcat NC Command Examples 1/11 Like  101 ≡ MenuHomeFree eBook Start HereContactAbout 8 Practical Linux Netcat NC Command Examples  by Himanshu Arora on April 23, 2012 Tweet  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 thisutility is in the scripts where we need to deal with TCP/UDP sockets.In this ar ticle 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 :  11/13/2017 8 Practical Linux Netcat NC Command Examples 2/11 $ 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 wecan 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.  11/13/2017 8 Practical Linux Netcat NC Command Examples 3/11  NOTE : Do not use the -w flag with -l flag at the server side as in that case -w flag causes no effect and hencethe 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 IPV4address 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. Sincein 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 addressesServer : $ 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 clientside.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 normallynetcat server also terminates.  11/13/2017 8 Practical Linux Netcat NC Command Examples 4/11 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 receivesan 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 receivingEOF)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 followingnetstat command. So we see that this connection is now using the UDP protocol.
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