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2. Networking

Basic tools:

ip address

ip addr

ip a

ip addr show lo

ip route

ip r

ping -c 3 8.8.8.8

dig linuxcommand.org

dig linuxcommand.org +short

ping -c 3 linuxcommand.org

traceroute linuxcommand.org

tracepath linuxcommand.org

For downloading files we can use wget or curl:

wget http://linuxcommand.org/index.php

less index.php

wget -O index.html 'http://linuxcommand.org/index.php'

less index.html

curl http://linuxcommand.org/index.php

curl http://linuxcommand.org/index.php > index.html

Netcat is a simple tool for network communication.

Let's use it to listen to the port 12345:
```
nc -l 12345
```
Open another terminal and connect to the same port like this:
```
nc localhost 12345    # on the second terminal
```
Now, any line that you type here is sent and displayed to the other terminal:
```
Hello network
```
```
The quick brown fox jumped over the internet
```
Check the other terminal.

Interrupt them with Ctrl-c.

This may not seem very impressive, but instead of localhost we could have used a real server name or IP and connect to it remotely. It may be used to check that the TCP port 12345 on the server is accessible from the client (in case that there is a firewall, for example).

For checking a UDP port we can add the option -u to both of these commands.

It can also be used as a simple tool for file transfer:
```
nc -l 12345 > file.txt    # on the first terminal
```
```
nc -w 3 localhost 12345 < /etc/passwd    # on the second terminal
```
```
ls file.txt    # on the first terminal
```
```
cat file.txt    # on the first terminal
```
As another example, let's combine it with tar to transfer a whole directory:
```
mkdir cptest
```
```
cd cptest
```
```
nc -l 12345 | tar xzpf -
```
Switch to the second terminal.
```
cd testdir
ls
```
```
tar czpf - . | nc -w 3 localhost 12345
```
Switch to the first terminal.
```
ls
```
```
cd ..
```
```
rm -rf cptest
```

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