When working with network applications, it comes in handy to be able to see open ports and connections, and to be able to interact with ports on different hosts for testing purposes. In this lesson, we will be looking at some basic commands for networking, and in what situations they might come in handy. The first command is netstat. This will show all open ports on our host. You see here, we only have one open port on a default boon to desktop application, which is Port 53. We can look this up in the special file services.

And this file contains all basic port numbers for programs and protocols. We see here Port 53 is the DNS server. Just by analyzing the output we cannot determine which program is listening on this port. That is because this process is not owned by our current user. That's why A bid slash program name column is empty. If you run the same command again with sudo we see that this process is named DNS mask.

And if you want more information, we can look it up in the main page. It's a lightweight DHCP and caching DNS server. other useful information we get from this command are the program protocol in this case DHCP total bytes not copied total bytes not acknowledged, local and framed address and port. Getting the port is the main reason we are using this command. This is also important for determining if the port is open just on localhost or if it's listening for incoming connections on the network, the state of the port. Usually this is listen, the bid and program name which helps us identify which program is listening what port now if To run a program that is supposed to be listening on a certain port, and we don't know if it's working, we can find out with nested.

Let's open the most basic HTTP server by running this command. As you can see from the output, it's listening on port one dot 8000. On interface, localhost. If we open a new pane and run the netstat command, we will see the open port and the P slash Nate. You probably already know this. But just to be on the safe side, we will be looking at different host names as static DNS entries on our machine.

This is helpful when developing applications that need to connect to servers and the server's change their IP address or when you want to emulate a remote server on the local machine. For this we type the following command. You can quickly understand the format of the file from the existing content. Let's add on aliases for our local hosts so that we can see it under a different name. We recommend using non existing top level domain names for local host, such as local or Dev. And this is to avoid overwriting any existing address because at the CO hosts takes precedence in DNS resolution.

Now, if we open the address in the browser on port 8000, we will see our local Python server running and serving content. The next command is the map. As you can see, it's not installed on default on a boon to so let's go ahead and install it by typing sudo apt install a new map. new map is a command used for checking all open ports on a remote host, also known as a port scanner. If you run them up on our network gateway, which in our case is this IP Get all of the open ports on the gateway. As you can see, there is again the DNS port open the HTTP and HTTPS servers, which are used as a web page for configuring the router and port for the 9152, which at this time is not specific to any common protocol.

And that's why it is marked as unknown. numa does not know for sure that those specific programs are actually running on the host. All it does is verify what ports are open and write the default application that usually runs on that port. If you are not sure what server we need to connect to, or if we want to know how many servers are in our current network, we can run new MX up on the local network address specifying the network mask as the destination that we get this information from if config if our IP address Is this one and our network mask is this one that means the command will look like this for the slash 24 which means that basically the network will have IPS ranging for 192 dot 168 dot zero dot zero to 192 dot what 168 dot zero dot 255. We see here we have three active hosts and it even gives us the latency so we can determine which host is closer.

Nima is helpful when developing client server applications. For example, when you want to see what ports are accessible on the server, however, new map might miss application specific ports, which are non standard to actually connect to a given port we will be using telnet which comes pre installed on a boon to test to see if a particular port accept connections just type the hostname followed by the port. If the port is listening and accepts connection telnet will output the message like this one. This means that you can also connect from your application. So if you are having difficulties connecting, it's usually a client problem, the server is working fine to get out of telnet hit this command followed by Ctrl D. Also, in some cases, we need to get the IP address of a particular host name. The simplest way to do this is to use the host command host ubuntu.com.

Today, we've learned only the basics the minimum elements you need in order to start working with host names and ports. For a deeper understanding of networks and package traffic. We recommend checking out courses on penetration testing and network traffic analyzing tools such as Wireshark