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Hello everybody and welcome to this tutorial. Today, I will show you how to schedule tasks to run upon rebooting your system, or at a certain point of time. In this particular tutorial, we will create a script or a command to change your MAC address each time you restart your Linux system. So, every time you reset your computer, each time you turn off your computer and turn it back on, not only does it give you a random address that anonymizes you further on wireless networks, but it will make sure that you do not share your permanent MAC address. You might think to yourself, well, I can just change the MAC address every time I need to change it. Well, you will forget, trust me. It’s far simpler to set a cronjob once, and it will run each time you reboot your computer. If we take a look in ifconfig, I don’t actually have any wireless interfaces configured here, mostly because this is a virtual machine. We will be doing some of the things on my other machine, on my Fedora host machine, but for the time being we will use eth0, my wired interface, as an example. You can just as easily do this with any other interface that you have. The only thing that should vary is the name of interface. For example, this one is eth0, wireless1 would be eth1. On my Fedora system the wireless interface is wlp2s0.
In any case, what we need to do is clear the screen. There is is a Linux software utility called cron, it should be included with nearly every Linux distribution available, and this is what you use in order to schedule tasks to run for you in an automated fashion. Let’s type in crontab –help, press Enter, and there we go. Let’s take a look at the help menu for this command. It’s fairly small, there aren’t that many options, the tool is fairly simple. You use -e to edit user’s crontab, you use -u to specify the user for which you wish to edit the crontab. That’s helpful if you have multiple users on a single system, but by default it will edit the cronjobs for the current user. You can also list the user’s crontabs, delete the user’s crontabs, very important, and you can also always use -i to prompt before deleting. Remember, once you delete something in Linux it’s gone. There are some methods for retrieving deleted information, but more likely than not it’s permanently gone. You will find it very difficult to retrieve information that you have deleted through the usage of terminal. Anyway, enough about that, let’s go ahead and start editing our scripts that will run on startup. So, type in crontab -e, press ENTER, and there we go. We are prompted with this file. This is opened with VI text editor, so we will need to go through some of the basics. Here it says edit this file to introduce tasks to run by cron. I strongly advise reading through this entire file. It’s very small. Just read it. I’m 99% sure that you will be able to understand pretty much everything that is written here. Over here, the last line is actually the format. You have minutes, you have day of the month, you have day of the week, you have a command for the meaning of each of these separate sections. You can read this file here. So, mon is month, the dom is day of the month, this is hour, this is minute, and so on and so forth. Go through this file, read through it, and you will understand this format to the fullest extent.
For the time being, here is the command that I wish to use. In order for you to edit pretty much anything in VI, VI is a Linux text editor, you need to open a file and then press I. Just go ahead and press I on your keyboard, letter I, and it says you are currently in the insert mode. You can see how I’m highlighting it, now it says insert, and now you can actually type things in. What we want to type in is @reboot, and then type in the command macchanger -r, eth0. That is the command that we use in order to randomize the MAC address of our eth0 network interface. Here you have at reboot. So when do we want this to happen? We want it to happen when the system reboots. When we reboot our machine execute this command. If you wanted to run a script you would just need to specify a path to that script, and then a command to execute it. So just press escape now to exit insert mode, and press : on your keyboard. So just press colon on your keyboard and you will be prompted with a command line here. In the command line you need to type in “w” for write, so write these changes to the file, and then in addition to w you also need to write “q”, quit. After you’ve written to the file I want you to quit the file. If this doesn’t work you can also add an additional argument which is an exclamation mark to forcibly do it, but there is no need in this example, it’s going to work. So quit, there we go. This says installing new crontab, excellent! So we’ll just go ahead and clear the screen, type in macchanger -s to show the MAC address of my interface, eth0, press Enter, and you see it says permanent MAC, this one, current MAC that we are using, same one. Let’s go ahead and reboot the computer, and after the reboot we will see a different current MAC address listed. 08 were the first two places that were occupied in the MAC address. Once we reboot the system I will start up the ifconfig, actually not ifconfig, I will use macchanger from now on to actually verify the MAC addresses of my interfaces. Even though, even though, that is not a good thing. You should honestly be using ifconfig because it is the universal way of checking a MAC address on pretty much any Linux distro out there. While on the other hand, macchanger is most likely only installed by default on Kali Linux, and pretty much any other distro does not have it installed by default. Let’s go ahead and use ifconfig. We’re being logged in at the moment, sorry for any delays. This is a virtual machine so it’s bound to have some slight delays, but surprisingly I have found it very efficient. It’s working at a relatively fast pace. I’ll just go ahead and open up our terminal, type in ifconfig, and you can see that the hardware address has been changed. This one is no longer 08. So, to confirm this further, we’ll just go ahead and type in macchanger -s eth0, excellent! So you can see the permanent MAC is this, current Mac is this. It’s of an unknown type, unknown manufacturer that is. You can take a look at the macchanger help menu to specify the type of a MAC address that you want it to assign to your interface. Feel free to play around with that. Again, if you have any questions in regards to that, feel free to post them, I will be more than happy to help you out in any way that I can. In any case, I bid you farewell, and I hope to see you in the next tutorial.
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