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In this video, you'll learn about system D sockets. System D sockets are used to listen

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for incoming traffic on the socket. And when that occurs, start the matching service. As

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you remember from the last lesson, a socket is typically a combination between an IP address

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and a port. So you can define a socket to listen on port 22. And if any traffic is coming

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in, you start the corresponding service, which obviously would typically be SSH. Like with

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timers, the name of sockets must match the names of the services that they are starting.

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And for an overview of them, you can use systemctl listunitfiles-dsockets. And when sockets are

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used, it's a socket that is enabled and not the matching service. Let's go check this

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out. So let's use systemctl listunitfiles-dsockets, which will list all the units of the type

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socket. And as you can see, there's quite a few of them. Now, we have sshd.socket. And

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I think this is a nice one for a demo. So currently, we have what? We have systemctl

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status sshd.service, which means that sshd.service is running. It's enabled. And the preset is

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enabled as well. Let's change it. I'm going to use systemctl disable minus minus now sshd.service.

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Then I'm going to check systemctl cat sshd.socket. I want to know what's going on in this socket.

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As you can see, it's a pretty simple configuration where it defines the socket. We have a listen

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stream, which refers to a TCP port. The alternative is listen datagram, which refers to a UDP

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port. And accept is yes. We'll make sure that it accepts incoming traffic. No further configuration

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because sockets by default match on the basis of the first name. So I'm going to use systemctl

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enable minus minus now sshd.socket. And then when I use systemctl status on sshd.socket,

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then we can see that it is active and it is listening. It's listening on port 22.

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And that is looking all right. And how about systemctl status sshd? There we can see that

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sshd is loaded, but it is inactive and it is dead. So PSAUX

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pipegrab ssh is not showing any ssh service at the moment. But when I'm going to use ssh

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student at localhost, then what is going to happen? I am getting an answer anyway.

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So I need to stay in this student ssh shell and open a sudo minus i root shell. And I'm using

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systemctl status sshd. And I'm using systemctl status sshd, where we can see it's disabled

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and it is inactive. And that is kind of funny, but it was started anyway. And PSAUX pipegrab

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ssh is showing that the ssh process is currently in the list. And that proves how a socket is

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working. Now I'm getting out of my socket connection. So I'm back in my ordinary root

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shell. And PSAUX pipegrab ssh is showing me nothing. So if the traffic disappears on the

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port that the socket is listening for, the corresponding service is stopped immediately.