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Hey guys and welcome back. So what I want to talk to you about in this skill right here

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is revolving around the concept of troubleshooting, network issues and if you happen to be a

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network engineer or a systems administrator you will know that we do happen to get a lot

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of issues on the networks. We've been able to troubleshoot those issues is obviously

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going to be very very helpful. It's going to make your life much much easier. So definitely

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a very useful concept to have under your belt. So with that said let's get to it then. Now

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the very first thing that I want to talk to you about in this nugget right here specifically

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relates to network configuration files. Now you may be aware that with respect to Linux

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we can manage many of our configurations directly via such text files and this is exactly what

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I'm talking about right here. We're going to manipulate particular text files with

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respect to our network settings. Now for the purposes of the LPIC2 examination there are

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some particular configuration files that we do want to be aware of. So let me just highlight

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what those are, give you a general description of what they do and we might even get to look at a

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few. So let's get to it then. Now the very first one which I want to talk to you about is a

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configuration file within the Etsy directory and this is the Etsy hosts file. Now what this

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configuration file does is it's going to allow us to associate particular host names to IP addresses.

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Now you may be thinking that this sounds pretty much like what DNS does. This is really what we

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are talking about here. The difference here is that this particular file is going to be used for

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DNS resolution on your local network. So if you happen to have a particular address on your local

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network let's say the address is 192.168.0.50 and maybe this is a web server whatever it may be.

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Instead you could just type in something like I don't know let's just maybe say my server.cbtn.com

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whatever it may be. The point is is that as opposed to having to send a particular request to an

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external DNS server to look up this host name and resolve it to that particular IP address we could

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just reference particular local files on our system that will do this binding for us. So when we

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happen to type in this address it's going to be recognized as a host name within our what's it

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called the Etsy hosts file and instead of doing that DNS look up we can just resolve it directly

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to the IP address we bind it to within this file right here. Now of course you absolutely can set

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up a DNS server for your local network but quite honestly this can be cumbersome it can be additional

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complexity that you perhaps may want to avoid especially if you are just dealing with a private

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local network this can be a great solution for you. So let me show you what this configuration

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file looks like then shall I. Okay so what I will say is nano and I'm going to the Etsy directory

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and then the host file notice that if I do not use super user privileges and I enter this file

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it's going to tell me that this is unwriteable we know why because it is in the Etsy directory this

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is going to be system wide configurations therefore we need super user privileges. Now we can see here

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we already have some configurations here now let me just show you what some of these are so say for

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example here we have this address 127.0.0.1 now you may recall this happens to be the IPv4

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look back address this is super super common you're going to see it everywhere not just my machine

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it'll be the same for you too. Now what is happening here is that on the left hand side

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let's split this here on this side as the IP addresses and on this side are the host names

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so what this actually means here is that if I just happen to type in the word local host that is

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going to be resolved to the IP address 127.0.0.1 you see that similarly if I just say IPv0 that is

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going to be resolved to the IP address 127.0.1.1 now a little bit below this might look a little

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bit more confusing but the reality is it is not confusing at all this is just simply IPv6 configurations

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and what we're looking at are hexadecimal based addresses it's still the same concept that on

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the left hand side right here we have our IP addresses just IPv6 addresses and on the right

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we can see what we are resolving this to so if I would just happen to go and let's say for example

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ping one of these host names let's just say ping and I'll just say local host what my machine is

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going to do it's going to see this as a host name it's not an actual IP address and it's going to

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look in this configuration file right here it's a host it's going to look for this particular entry

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in this case it finds it right here and it's going to say what IP address is associated with this name

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and it's going to find this value right here 127.0.0.1 in essence that is the address I will ping so

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let's just try this out then so if I just escape out here and I just say ping local host and I

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hit enter check this out we're actually getting a ping and within the brackets if I just press

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control Z to stop this you can see the address indeed is 127.0.0.1 similarly if I just say

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ping IPv0 look at this we are also just pinging this address right here so check this out I also

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have this other Linux machine here it's actually a windows machine running windows subsystem for

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Linux but you can just treat it as a separate Linux machine entirely so I'll just minimize mine

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right now and I'll open up my Linux here okay so if I open up this machine and I just say IP adder

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and I just grep to filter the results and I will grep for 192.168.0.0 so we can see here this is the

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address right here 192.168.0.39 on this machine whereas my main Linux machine if I open it up

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we can see this is 0.65 now I can actually ping this machine ping 192.168.0.39 because it is on

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the same local network and if I hit this now we can see this pings quite the thing so this means

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we have communication between this device here on the right and this device here on the left but

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again if I wanted to associate a particular hostname so that I didn't have to maybe say remember the

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IP address I could just make a modification in my etsy host files let me just do that right now

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what I will do here I'll just full screen this and of course I'm going to have to use super

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user privileges to modify this and type in my passwords we'll tell if I typed in the correct

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password there we go okay so on the left hand side let me just get some space what I can see here is

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the IP address 192.168.0.39 and all I will do in the right here is associate that with a particular

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hostname let me just call this trevor box I should imagine this is my good friend trevor's machine

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and he's on the same network so I'll just save this right now and I'll escape now I can just say

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ping trevor box and to enter and look at this we are successfully pinging this device and we are

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pinging 192.168.0.39 we have been able to resolve this by making the modification directly within

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this etsy host file really try to remember that now one thing I will say is that when we go in here

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we can actually just like we see right here we actually see two entries you see them on the left

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we have this here ip6-localhost and ip6-lookback what is happening here is the use of an alias

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so what I could do here is I can modify this let's maybe also call this machine2 let's say okay so

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that means if I ping machine2 it will resolve to this IP address or alternatively I could refer to

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it as trevor box so if I save this and escape out and I just ping machine2 notice this that we're

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actually getting a response back now the response actually is saying the IP address here but the

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device is called trevor box since that is the first name associated with the IP address but we still

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have the ability to refer to this as an alias okay and again we could add as many aliases as we so

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wish I could also say for example pizza box and say ping pizza box is still going to get a response

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from trevor box at 192.1680.39 so that is one of the big configuration files we have to be aware of

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the next one I want to talk to you about is etsy once again same big location and it is the resolve.conf

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configuration file and notice this is resolve without an e on the end okay just remember the

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spelling here within this configuration file this is well no surprise is the resolver configuration

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file and what this is going to allow us to do is going to allow us to specify dns servers as well as

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the search domains for the particular host so let me actually show you what this configuration

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file will look like then so we will clear the screen if I say sudo nano etsy resolve.conf

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let me go up there we go go in here check this out what we're actually seeing here is the ip address

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of my name server i.e my dns server so we use the name server keyword here and then the ip address

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now this is just going to be a local host lookup you can tell this because it is a 127 based ip

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address we're thinking lookbacks here now we have this keyword here called options this allows us to

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specify additional options now some options you may see say for example you might see a timeout

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option so that would mean that if you happen to specify multiple name servers as opposed to just

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one right here you listed maybe say two different domain name servers what you could do with the

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timeout option is you could specify the time and seconds that you should wait for a particular

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response from a dns server before attempting to go to the next server so what i'm seeing here is if

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we happen to use two different servers let's just say we had one at 192 168 1.1 and then maybe we're

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also using google's public dns server of 8.8.8.8 we could say a timeout and the options parameter

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and set the timeout to 2 that would mean if we happen to send a dns request we would first use

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this particular dns server but if we did not get a response back within the two seconds timeout

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we would then abandon this and begin to use this to resolve the query now the options parameter is

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going to allow us to specify other things as opposed to the timeout things such as the maximum

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amount of attempts that we would want to issue i.e the amount of queries before a failure occurs

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so let me just modify this configuration file right now if i go in here and i happen to type in

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a let's just say a bad address 192 168 0.145 let's just maybe say and i save you an escape if i try

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to ping google.com we're not going to be able to get a particular response that is because

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we can see here we have a failure in name resolution what is happening is that we're trying to send

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this request google.com and have it be resolved quite frankly by a server which does not exist

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there is nothing on my local network with this ip address now what i could do is i could in fact

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specify google's public dns server this is a very well known dns server of 8.8.8.8 if i save this

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this happens to be a real dns server out on the internet so if i now resend this request notice

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here but actually now resolving this to a particular ip address we can see this indeed

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right here so within this configuration file this is where we specify which dns server we want to use

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and we can specify additional options i.e which server to use next after a particular time it

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happens or after a amount of failed attempts whatever it may be you can control these parameters

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within this configuration file now another configuration file we have to be aware of

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this one is debian based specific i.e this is a configuration file you will find on debian systems

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and this one is again within the etsy directory not surprising you here and it is in the network

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folder and the file is called interfaces so this particular configuration file is the primary

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file on debian based systems whereby you can make interface configuration changes so you may recall

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we have concepts such as dhcp this is how an interface can dynamically and automatically get an ip

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address and alternatively we can statically configure an ip address i.e we actually type the ip

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address in ourselves manually if we want to be able to do such things for maybe say a particular

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interface say for example if i do an if config we can see this interface right here enp0s3 if we

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wanted to make a modification to its ip address such that it would be manually changed or request an

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ip address via dhcp the location whereby we could do such a thing again is in the etsy network

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interfaces file now on red hat based systems every single interface on your machine say for

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example you had eth0 and eth1 and maybe a different look back address whatever it may be each interface

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would actually have its own separate configuration file now each of these configuration files will

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be found within a particular directory and this is going to be in the etsy directory it will be in

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the sysconfig directory and within this it's going to be in a directory called network-corrupts

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i'm kind of running out of space here but this directory here is going to have multiple configuration

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files for every individual interface as opposed to debian whereby we have one file whereby we

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can specify the configurations for all interfaces again just like with the one we saw before we

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can make changes to things such as dhcp configuration settings as well as static ip address allocation

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just a different way of handling this thing depending on the family of operating system

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you're using debian versus red hats now the last configuration file that we do have to be aware of

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once this is again in the etsy directory easy to remember that point and it is called the ns

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switch configuration file this is the name service switch config file and what this is used to do

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it's used to determine which source you should get your dns from and in which order to remember

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we had our etsy hosts file this is a configuration file and we would also have our name server which

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we found in our resolve.conf file now the question is if i happen to make a request for a name resolution

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such as google.com what should i consult first should i actually look in the configuration

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file for my local host or should i just go directly to the name server and make a dns request to the

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server itself well we can choose our priority by specifying this within xanss switch let me just

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show you to do nano xanss switch.conf am i going here we can see the actual process here so look

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at this see for this right here host we can see that for looking up hosts we would first consult

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our files first and then if we could not resolve that particular host name from our local files we

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would then use the dns query now what i actually could do here is i could invert this such that

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and of course i'm not actually going to be able to save this because i'm not using super user

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privileges but i could just remove this right here and say dns and then files that would mean that

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even if we had an entry within the xanss host file configuration it would actually not be looked up

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instead we would just send a request to the dns server first and only if that happened to fail and

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we couldn't get a response then would we fall back and use our local files and again the order of

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which is specified within xanss switch.conf now there actually is more things as you can probably

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gather that you can do within this particular file but with respect to the scope of the lpik2

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examination this here is really what we are focusing in on okay doc so i do know that was a lot to

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take in for these particular configuration files but like i say understanding what these files

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actually are and what they can do as well as their particular location is going to give you a lot of

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granular control over how your network operates and by having this control and understanding

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of the configuration files this is going to go a long way for you to be able to resolve and

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troubleshoot particular network issues whether it's ip address allocation or say for example

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dns lookups okay doc so i hope this has been informative for you and i'd like to thank you for viewing