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Hey guys and welcome back.

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So to wrap up this skill on network troubleshooting, there are a couple more things I want to

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talk to you about.

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The first one relates to host name configuration.

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Now this could very well have fallen under the topic of network configuration files because

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you can actually change the configuration directly via a particular configuration file.

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However there are actually different ways to change this particular value.

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So let's begin with that first.

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So if I just actually issue the command host name, this is going to print out well my host

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name.

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What we can actually do here is we can indeed change the name of our host name and we can

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do it directly with this command.

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We can just say host name and let's just call this maybe John's computer and if I hit enter.

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Oh of course I have to be the super user so I'll just change this to John's computer

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with super user type in my password.

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Now it might not be apparent but my host name has actually been changed.

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If I happen to say host name we can see here the name has changed.

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Now if I would happen to get a new shell i.e. by logging in once again or by going directly

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to my machine.

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So while those will close this one down I will get a new terminal.

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Notice here the name has indeed changed.

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Similarly if I happen to close my SSH session and I open a new putty session.

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If I log in as IPv0 type in my password you can see indeed just the same way the host

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name has changed to John's computer.

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Now the big thing here though is that this configuration change.

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This configuration change is not persistent meaning that if I happen to reboot my machine

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this change would just be undone i.e. we would revert back to the original host name.

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So let's just test this out.

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I can say sudo reboot type in my password that kills the connection.

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Now my machine is rebooting.

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Ok so if I log in I just full screen this.

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If I go to my terminal notice it has reverted back to IPv0 and I say host name with the

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command once again it has reverted.

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Now if I want to make this change persistent I can actually modify a particular configuration

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file.

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Now that file again is in the Etsy directory and it's called host name really easy to

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remember.

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The difference here is that if I happen to modify the host name directly within this

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configuration file that change will be persistent i.e. meaning that if I happen to reboot my

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system the new host name will still stay in place.

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So what I can do here is if I show you this particular file and I say sudo nano etsy and

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go into host name hit enter.

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We can see here the host name has been set to IPv0.

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If we want to make a modification to this particular configuration file we can actually

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do so directly via the command line.

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The difference is that we don't use the command host name we actually use the command host

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name CTL and by doing this this will actually modify that particular file.

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So I'll say sudo host name CTL and I can say set hyphen host name and change this to

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maybe say john test.

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Now if I hit enter again it appears like nothing has actually changed here but if I say host

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name now the name has indeed been changed to john test and if I go back into this configuration

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file and I hit enter notice this configuration file has been updated.

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Now like I say this here doesn't appear like it's changed within the prompt that is because

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we're still using the same shell.

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If I happen to close this down and open up a new terminal window indeed we see john test

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instead and again if I happen to say sudo reboot and reboot the system hit enter the system

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reloads the system has rebooted let's log back in we'll open up a terminal let's go in.

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Notice this time the changes though have actually survived the reboot that is because when we

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use the host name command itself we do not actually modify the etsy host name configuration

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file however when we use the host name CTL command we do indeed modify that configuration

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file and that makes all the difference with respect to the persistence of this particular

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configuration.

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So that is something we have to know for the lpik 2 examination.

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Now before we close I just want to make note of two very important files both within the

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etsy directory one is called hosts.allow and the other is hosts.deny.

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Now we actually have this concept of what is called a TCP wrapper okay.

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Now all TCP wrappers refer to are particular programs that happen to be compiled with something

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called libwrap.

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Try to remember that and all libwrap is is a very particular library.

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Now this library happens to be used by things such as say SSH because the SSH server daemon

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i.e. SSHD will use this particular library.

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Now when something like say SSHD happens to want to use libwrap in order to be able to

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authorize a particular connection it's first going to look in etsy host.allow and if from

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within this file it sees that this particular service is allowed to be made to this particular

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network or destination then that connection will indeed be allowed to happen.

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Now if no rule within this particular file happens to be found and matched the libwrap

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library is going to look at particular rules within the other file etsy host.deny and if

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we happen to get a match within this particular file then the libwrap library is going to say

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to that service this connection is not allowed and quite simply if we happen to have no match

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in either file then the libwrap library is going to tell that particular service be SSHD

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to indeed allow this access.

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But again we do want to understand the order of operations here.

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The first place it will look like I say will be in etsyhost.allow and right now this particular

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configuration is blank but you might actually see something like say the service SSHD followed

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by a colon and then you could specify a particular destination or you could give a network range

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or indeed you could say the keyword all which would mean that for the SSHD daemon all connections

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are going to be allowed or we could specify particular keywords such as accept and we

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can add in a particular host to not allow say for example 0.50 this would mean that for this

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particular daemon we would allow absolutely everyone to use SSHD except for this particular host in

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which case they would be disallowed. Now again if we do not happen to find a match within this

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particular file the next file we would go to would be an etsyhost.deny. The difference here being

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is if we go in let me just escape out and I go into hosts.deny if I happen to say SSHD and then

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specify all as opposed to allowing everyone access to SSHD instead we are denying them access because

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we are now in the deny file. Now one thing to say is that if we happen to have a conflict i.e.

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we happen to allow a particular service within hosts.allow but we have actually got a similar rule

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within hosts.deny disallowing a particular service for a particular connection the one that is going

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to take precedence and when is going to be the hosts.allow file that will override anything within

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hosts.deny the reason being here is that hosts.allow that particular file is going to be evaluated

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first so if that happens to find a match within that file the hosts.deny file will not even be

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checked. Ok doc so that can be a little bit confusing but it certainly is worthwhile to remember

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for the purposes of the examination these tcp wrappers are going to use that libwrap library

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in that particular library is going to check these particular configuration files

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when these particular services are going to be called to be used say for example our network

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connection over ssh libwrap will consult these particular files and being able to control access

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to these particular files can have a great impact on these network connections and understanding this

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is going to have a profound effect with your ability to diagnose these types of problems and

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fix these types of network connection issues so that is us for our network troubleshooting

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i hope this has been informative for you and i'd like to thank you for viewing