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+date = '2024-10-31T22:00:53-03:00'
+title = 'Netcat'
+description = "Thanwer's Blog Post about netcat tool"
++++
+
+I heard too many times a work coleague, or a third-party vendor telling me "there is something wrong on the network here", or "This port isn't open!".
+
+You can use websites to try to test for open ports, a quick Google search should yield you a couple good results, but sometimes that is not enough.
+
+
+## The problem
+Let's suppose you followed my other post about [restricting source address with iptables]({{< ref "/posts/iptables-basics" >}} "Iptables basics"), in this case those random web tools won't help you, so you use nmap!
+
+I will make a post about nmap in the future, for now let's use it in a very simple way to check for an open port:
+
+```bash
+$ sudo nmap -sS -p 80 example.com
+Password:
+Starting Nmap 7.95 ( https://nmap.org ) at 2024-10-31 22:08 -03
+Nmap scan report for example.com (192.0.0.4)
+Host is up (0.18s latency).
+Other addresses for example.com (not scanned): 2001:db8::192:0:0:4
+
+PORT   STATE SERVICE
+80/tcp open  http
+
+Nmap done: 1 IP address (1 host up) scanned in 0.39 seconds
+```
+
+As you can see, the important part is -p 80, which specifies that we want to test port 80, and the last argument is the host we are scanning.
+
+Now let's suppose that another person which is setting up a PABX, which is a classical example... I have a lot of histories about telephony people... For example, he may have set a simple wrong configuration, like the gateway or subnet mask, and that causes his service to not work. He does the same tests I showed above, but instead of "open" that says the port is "filtered".
+
+That is enough argument for him, now he will blame your network for that. How can we prove that our firewall rules are correct and working?
+
+The simplest answer is using netcat!
+
+## netcat (nc)
+
+Netcat is a very simple, but very powerful tool for those cases. With that, you can setup a listener on any TCP or UDP port!
+
+### Installing
+
+The instalation is very simple! On Debian systens you can do it with the command:
+```bash
+$ sudo apt install netcat
+```
+
+### Using the tool
+
+I invite you to read the [nc man page](https://linux.die.net/man/1/nc).
+
+The main options we need to use are:
+
+* -p : This specifies which port should we listen to.
+* -k : This makes netcat to keep listening after a client disconects.
+* -u : Use UDP instead of TCP
+
+### Use cases
+
+#### Check for network comunication
+
+Listen on port 2000:
+```bash
+$ nc -l 2000
+
+```
+This starts a server which listens on port 5000.
+
+Now let's open another terminal and connect a client to this port:
+```bash
+$ nc localhost 2000
+
+```
+
+On both terminals you should see as a chat app working, this is being transmited over the network!
+
+See an example of this working:
+
+![Example of netcat usage](/images/nc-example1.gif)
+
+#### File transfer
+
+Netcat can be used to send and receive files between two hosts over a network. This can be particularly useful in environments without FTP or SCP access.
+
+On the receiver’s end:
+
+```bash
+$ nc -l -p 1234 > received_file.txt
+```
+On the sender’s end, assuming that 192.0.0.2 is the receiving server:
+
+```bash
+$ nc 192.0.0.2 -p 1234 < file_to_send.txt
+```
+
+This can be useful for transferring a log file for example.
+
+#### Port scan
+
+nc can be used as a simpler alternative to nmap:
+```bash
+$ nc -z host.example.com 20-30
+Connection to host.example.com 22 port [tcp/ssh] succeeded!
+Connection to host.example.com 25 port [tcp/smtp] succeeded!
+```
+
+#### Directly talking to servers
+
+nc can be used as an alternative to telnet for troubleshooting.
+
+For example, you can talk to a SMTP server directly over port 25:
+
+```bash
+$ nc localhost 25 << EOF
+HELO host.example.com
+MAIL FROM: <user@host.example.com>
+RCPT TO: <user2@host.example.com>
+DATA
+Body of email.
+.
+QUIT
+EOF
+```
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