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Hey everyone and welcome back. So previously we had just walked through the basic configuration

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for a DHCP server. Now I just talked about how we configured DHCP services for a particular

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interface in our case it was ENP0S8 which happened to reside on the 10110-24 network. Now as it

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transpires anyone within the same network segment that is someone who would be attached via a switch

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they could ultimately make requests to this particular server all via a layer 2 connection if

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you recall we talked about layer 2 in the LPIC1 certification. So anyone attached here can reach

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this server therefore they can make the necessary broadcast requests and can ultimately receive IPs

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from this address pool but again let's try to imagine a larger network so let's redraw this

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once more. We have our server here this is our Ubuntu machine we'll just say Ubuntu DHCP and we have

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our interface card here which is our original ENP0S3 and this has got an IP address of 192.1680.44

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in my case and we also added this second one here which is 10113 so we can imagine that we are connected

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to another client here okay so over here in fact let me change the color let's say that we have

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another machine right here this could be another Ubuntu box for whatever we know let's call this an

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Ubuntu client so this Ubuntu client here let's say they've got a network interface here and this

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interface here has been manually configured or it did a DHCP discover message and received an IP

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address whatever it may be the IP address is within this particular range 1100 so let's just say

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the IP address here is 101121 let's say okay cool so we have this connection here now let's add

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another server into the mix now let's say over here we have another machine once again and we'll

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just call this Ubuntu 3 and it's got a network interface card right here as does this one here

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this one also has another interface card and let's just imagine this one was configured I don't know

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for let's say 172 16 1.15 let's say with a slash 24 mask so let's say that this device is suddenly

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switched on and this device wants to get an IP address for this interface here so this device

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sends out that DHCP discover message which we know is a broadcast message so it broadcasts that out to

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anyone on the segment now you may recall from our lpik one studies that broadcasts are limited to

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the same network they cannot traverse into another network meaning that this broadcast message here

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it cannot make its way into this network to ultimately reach this interface here which means

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that as it stands right now our poor Ubuntu machine cannot talk to the DHCP server therefore it cannot

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be allocated an IP address which is really quite sad for that machine so we absolutely want to fix

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this problem so how on earth could we fix this then well let's have a look in fact let me just

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connect these together like it's an ethernet connection here is the deal what we want to do

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is we want this device here as an intermediary between the potential client and the actual DHCP

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server we need this intermediary here to play a role and that role is to really the request that

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it will hear on this segment here in fact let me change my color this segment here and it wants to

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take that and really that request to the DHCP server so here's what's going to happen then first on

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our DHCP server we want to configure another pool so like I say we already had a pool for the 10

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110 network and we're also now going to have a pool for this network up here which is going to be

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172 16 1.0 slash 24 okay so those two pools will be configured that means we have to add an additional

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subnet directive on the server right here that is only one step because like I say we still have

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this issue of the broadcast from this device here being terminated right here so like I hinted at

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this device here also has a role to play meaning that we want to install some particular software

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that is going to transform this server into a DHCP really and then we're going to have a

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configuration file on this server here and within that configuration file we can actually specify

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where about we should really request to IE we would specify the IP address of this interface

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which is the IP address of this server here so pretty much when anyone sends a DHCP request

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we know we can relate to this device here or this interface on this device so now here is what is

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going to happen this device here sends a broadcast message shouting to anyone on the segments hey I'm

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looking for a DHCP server this reaches this interface here and this client recognizes this client goes

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hey on my interface here which is configured on the 172 16 1.0 slash 24 network I am receiving

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a broadcast message for DHCP services I am configured as a DHCP really so I know where to

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send that request I send it to this IP address here so now that broadcast message is being transformed

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into a unicast message directly to this IP address and it's received right here so now this particular

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server has some interesting information it knows it's getting a DHCP request and the DHCP request

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is coming from this IP address range this network here okay because it came in on this interface here

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now the DHCP server checks its DHCP pools and it notices that it does actually have IP addresses

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within this range remember the DHCP server is configured for this range which doesn't match

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but the second range where it's configured for the second pool this one does indeed match so we can

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now say hey by the way here you go let me pass this on this DHCP offer which is going to be relayed

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once again through the relay server and back to this client all the way over here and this client

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can just as you expect it can accept to decline this offer so it can send a request so the request

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can be relayed and then everything can be acknowledged just as the way it was so even though this server

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is ultimately isolated in a completely different network this intermediary right here is just allowing

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us to relay that information to the server and as long as a server has that address pool it can

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actually provide services to devices which are crying out for broadcasts within this network

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segment right here so this can definitely be a little bit confusing when you first hear about

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it but this is ultimately the process in action so what I want to show you is the general configurations

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that we would go through to make this happen if we happen to be configuring this on a real live

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network as opposed to a virtual simulation here so what I have to do of course is I have to go on

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to my DHCP server I have to add in this additional pool which is not on my network segment here it

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is a completely different pool to anything we are connected to and then on my client right here I'm

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going to install my DHCP relay service and I think you will be surprised despite the perhaps complex

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logic or theory in play here the actual configurations really are quite simple so with that said now

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that we know what is happening under the hood let's see it in practice then shall we okay so let's go

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back onto our server right now and I will do sudo nano I'll go into the xe directory my DHCP

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directory and again I will go into DHCP dot conf so I'll type in my password so let's scroll on

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down and let's add another network so let's say subnet so the network we're going to serve is 172

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16 1.0 and the network mask is going to be for a slash 24 and the range let's just give it a range

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from between 172 16 let's say 1.25 maybe all the way to 172 16 1.200 okay and we'll have our semicolon

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and we'll say option Ritters we can add a default gateway let's just say the default gateway is going

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to be 172 16 1.15 and we'll just do the same configurations default least time 2000 max least

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time 5000 add the domain name and domain name servers this one can maybe be 8.8.8.8 add our

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semicolon and same here and we'll close this off so now we have that configured let's just save

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this right here meaning that if we happen to get a request sourced from anyone within this network

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range we're going to actually pull from this address pull right here that will be our offer from

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within these ranges with this additional information as we can see here okay so let's save you and what

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I'm now going to do is I'm going to go on to another machine here so if I just minimize this I'll just

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use another Zubuntu machine here which can act as my relay agent in this scenario okay so let's go to

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the terminal emulator let's do an ip adder so I have my enp0 s3 with an automatically configured

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ip address via virtual box let me just change this for now I will say sudo ip add and I will add

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let's use those numbers in the description I did 10 11 21 slash 24 and make this on dev

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enp0 s3 type in my password to an ip adder and let's remove the 192 168 0.70 so let's check this now

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cool so we only have this ip address here let's configure the enp0 s8 interface so I'll say sudo

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ip add and I'll add 172 I think I said 16 1.15 slash 24 and I'll make this on enp0 s8 to clear the

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screen and I'll say ip adder so just so you can visualize so let's pretend this is our relay

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server right here and here is our DHCP server pretending these are connected via ethernet

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this interface here can be the enp0 s3 which is this one here that's on the 10 110 network and

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this interface here is going to be this one here enp0 s8 and that's being configured for 172

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16 1.15 which is on the 172 16 1.0 slash 24 network so we have these interfaces both configured

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and we are imagining some type of new server happens to connect looking for an address it's

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going to send out this message here a broadcast looking for DHCP servers now we want to install

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the DHCP relay server so what we'll do here is I will say really quite simply sudo apt install

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isc DHCP hyphen really if I hit enter and I say yes open I've just realized because I changed the

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ip address configuration I actually can't reach the internet anymore so let me just quickly change

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that back so let me just add this back once again there we go I'll just do the app update I can now

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reach the internet once again apologies about that and now I'll try installing that once again

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so I'll say yes and as we can see here it's going to ask us to enter the ip address of the DHCP

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server that we should actually send these requests to so I believe the ip address we said in the example

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was 10 113 let's say okay and the interfaces that should be listening that will be enp0 s8

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and I will say okay and if we want to add additional options we can do so but for now I'll leave that

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blank now that should be our configuration in play if we wanted to modify this configuration we could

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go into the etsy directory and then into the default directory and then isc DHCP relay if I hit enter

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type in my passwords we can see here which server we should actually send the relay to this will be

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the ip address of my DHCP server on my original Ubuntu box and this is the interface on my machine

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right here for where I am listening for those DHCP broadcast messages so if we wanted to change

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these server addresses or these interfaces you could easily modify this within this configuration

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file right here so now with that said if that client happened to send out a broadcast message it would

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be heard on this interface and forwarded to this address on my server and here is my server right

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here with this ip address and as we know within my DHCP configuration we would actually have a

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suitable range for that request so I know that was a lot to deal with but it really is important to

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understand the fundamentals of what is actually going on under the hood with respect to our DHCP

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relays so if you find it a little bit confusing then definitely I would advise you to watch the video

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one two three more times just to solidify the concepts within your head but for now that is

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us for DHCP relays I hope this has been informative for you and I'd like to thank you for viewing