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What is DNS or domain name system.
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In this video I'm going to explain it in a lot of detail but before we get there Alexa N.S. look up
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Amazon.com.
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The DNS lookout for Amazon dot com is 176 dot 32 dot 103 dot 205.
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That is an example of what a DNS does.
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It's essentially resolving a name typically a domain name to an IP address.
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N S look up a Google dot com DNS look up for Google dot com is 172 dot 217 dot 164 dot 142.
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We as humans don't communicate easily using IP addresses.
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We use domain names.
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So if I told you to go to Google's IP address you probably don't even remember what the IP address was
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but you'll remember what Google dot com is so DNS essentially resolves a human readable name such as
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Google dot com or Amazon.com to a machine readable IP address.
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Machines don't use names they use IP addresses in IP version 4 we use dotted decimal notation IP addresses
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such as 1 9 2 1 6 8 wandered 1 IP 6 uses IP addresses such as 2001 colon colon 1 2 3.
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There are many IP addresses out there and many Web sites.
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It's much easier to remember a domain name once again like Facebook dot com or Amazon.com rather than
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the IP address of a server.
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And to further complicate it like in my example depending where you're on the world a domain name may
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resolve to a different IP address for load balancing.
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So if I'm in the UK and I ping Google dot com I may get a different result to you if you in the US or
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in Singapore or somewhere else in the world.
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It's much easier to remember the domain name than it is to remember an IP address but machines use IP
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addresses and traffic is routed across the Internet using IP addresses not names.
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DNS is a fundamental building block in networks today without a DNS internet wouldn't really work very
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well because very few of us are going to remember IP addresses.
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Now as an analogy DNS is like a telephone book taking a name converting it to a telephone number.
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But in this case taking a domain name and converting it to an IP address in the bad old days I'd have
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to look up someone's number in a book and then I'd have to manually dial their telephone number.
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But I don't think any of us do that these days on a phone like an iPhone today we're not going to manually
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type a number like this and then dial it we're going to go to our contacts and search for a contact
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and then just press on the contact to call the person.
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I mean a lot of us probably don't even know our own telephone numbers these days.
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We don't know the telephone numbers of other people because we simply look them up in a directory on
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our phone.
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Now this is a local directory.
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We can do something very similar on a P.C. by using what's called a hosts file.
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That is the most basic version of so-called DNS.
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No it's not DNS but it's a local lookup so you could create your own version of DNS locally on your
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P.C. by editing the hosts file.
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Taking that a step further companies may have a local DNS server that resolves and names within the
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organization but on the public Internet we have distributed DNS systems that allow us to resolve names
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such as Google Facebook etc..
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Now it's all very good and while talking about DNS but I want to show you practically how it works I'm
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going to show you why shock captures I'm going to show you how to setup a DNS server on a Cisco rider.
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How to set it up on a country server.
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I'll show you basically how you can manipulate a DNS to do anything that you want.
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You need to be careful that you use trusted DNS servers.
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Don't just trust any DNS server out there.
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DNS can be intercepted and you can manipulate the DNS servers used by pieces to get them to go to the
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incorrect domain.
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Fortunately today a lot of browsers like Chrome have a whole list of certificates preloaded on them
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so you'll get a warning if you end up going to an incorrect domain such as Microsoft dot com or Cisco
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dot com.
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Okay so in this topology I've got a Windows 10 computer it's connected to a Cisco switch which in turn
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is connected to a Cisco rather which connects us to the Internet.
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This topology is running in Genesis 3.
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I'm hosting this entire topology on my computer so forgive me if the fan goes a bit crazy.
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It's all running locally on my Mac.
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I also have an a bunch too P.C. which will configure as a DNS server.
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Okay firstly let's have a look at the Windows computer.
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Here's my windows.
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P.S. I'll open up a seam deep prompt.
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Make this a bit bigger.
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IP config shows me that this is the IP address of the P.C. IP version 4 default gateways 10 1 1 2 5
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4 and I should at this point be able to paying my default gateway which I can default gateway.
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Is this Cisco with IP address once again 10 1 1 2 5 4.
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The switch is a lever to switch it's not really doing anything except giving connectivity in the network.
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So back on the P.C. IP config slash all shows us that the CPC has to DNS servers configured 8 8 8 8
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8 and wandered wandered wandered 1.
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In other words Google and CloudFlare are the two DNS servers configured on the P.C. so I'll start a
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wire shock capture between the P.C. and the switch so that we can see what's actually going on.
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Windows sends a lot of traffic into the network so as you can see here a bunch of traffic is being sent
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by that Windows computer out into the network.
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But I'm going to filter for DNS and then back on the P.C. what I'll do is ping a domain such as David
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Bumble dot com.
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And notice we get a reply from this IP address 2 1 7 160 0 sixty nine.
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Now the CBO spiking on my P.C. here the throughput through a Cisco switch and a Cisco broad are running
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in June 3 may be a bit slow but the point is is that I am getting replies back to that domain and if
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we have a look at the Y shock capture what you'll notice is we can see that this IP address 10 1 1 1
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cent a DNS request 2 8 8 8 8 8 for domain David Bumble dot com.
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So just to confirm on the P.C. once again IP config shows us that this is the IP address of the P.C.
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the P.C. send a request to the DNS server notice the query is for David Bumble dot com.
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It's a record a record is a domain name in IP version 4 could triple A is a domain name in IP version
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6.
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So the P.C. is asking the DNS server what the IP address is of the domain name now going back a step
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at a layer 2 in the OS model or TTP IP model if you prefer we have Ethernet to.
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That's because this network is using ethernet so it's an ethernet connection from the windows P.C. to
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the Ethernet switch the source mac addresses the P.C. destination MAC address is the broader basically
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the traffic is being switched from the P.C. to the broader because that's how it gets onto the Internet.
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So at least to source MAC address will be the P.C. destination MAC address will be the broader but at
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least three IP version 4 source IP addresses the P.C. destination IP address is Google.
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Now you may notice that this is a RF C 1918 address.
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In other words it's a private IP address it's non-refundable.
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On the internet but the router is implementing network address translation or Nat.
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This is very typical of what your right at home will be doing.
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So notice it's enacting this IP address.
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Now it is narrowing it to a another ROIC 1918 address but that's because this road is connected to a
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cloud which is actually bridging my P.C. physically.
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So this P.C. here onto my physical home network and I have an Internet router that runs this onto the
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Internet.
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So it's actually being nutted multiple times.
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But what's important to point out here is notice the protocol at least for each UDP or user data grand
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protocol source port number use this 5 2 7 4 9.
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That is what's called any femoral or random port number.
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Destination Port numbers 53 which is the well-known port number for DNS.
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When a server is configured to host multiple services it's got to serve a purpose.
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So it's a server that's acting as let's say a file server when you connect to that server it's going
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to give you a file.
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But when you connect to it using DNS it's listening on Port 53.
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If it's being configured as a DNS server so you send traffic to port 53 the server is listening on Port
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53 to running an application like which I'll show you in a moment DNS mosque which is a DNS server application
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and then it responds back to that to request on the port number that you chose.
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So if you connect to a DNS server like this piece is doing you will use a random port number or ephemeral
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port number going to a well-known port number and then it'll reply back from that well-known port number
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and we can see that here Google is replying from a source port number 53 going to the port number that
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the P.C. chose the Windows 10 piece he chose this port number.
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The Google DNS server replies back to that port number so again it's UDP destination port number as
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a source port number as this.
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Digging deeper into the DNS information we can see.
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Domain Name System.
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It's a query.
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It's a standard query for a name.
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We're trying to resolve a name the name that we resolving.
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Is David Bumble dot com and the DNS server replies back saying this is the answer.
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This domain name has this IP address 2 1 7 160 0 69.
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So back on our windows P.S. That is the IP address that we see.
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So I could copy that IP address go to a web browser.
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If I type the domain name it's going to browse to that server.
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So I'm able to connect to the domain using the domain name.
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And this depends on the server I should be able to connect to the IP address of the server.
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In this example I'm getting a form for error.
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Now some servers will not allow you to connect directly on the IP address.
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That's typically because multiple domains are hosted on a single IP address Okay I'll stop the Y shock
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capture and what I want to show you once again is that DNS is essentially just a resolution of name
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to IP address and you can do that directly on your Windows computer.
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So in windows I'll open up notepad.
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I'll run this as an administrator
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before I open a file if I pinged brought a 1 notice we told that that domain name is not found the same
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with right of one whom dot com the ping request times out.
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I can't ping that domain name but what I could do is open a file and what I'm going to do is go to see
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Windows system 32 driver's Etsy and I'm going to open the hosts file.
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This is a file on the local Windows computer.
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Just zoom in there to make it easier to read and I can edit this so I could say 10 1 1 2 5 4 is right
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1 and 10 1 1 1 2 5 4 was write a one whom dot com and save that file.
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So I'm editing a local file that maps whose names to IP addresses.
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So now when I ping wrote a one notice that works when I ping write a one dot home dot com that also
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works.
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But if I ping wrote it to that fails because it's not in the hosts file and Google is not replying back
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with that information.
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So if I said or two like this and save that file now ping or two that resolves name got resolved to
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an IP address.
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Now in this example the networks a bit unstable sort of pings a timing out they had succeeded.
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But the important part is the domain name got resolved.
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That name got resolved to an IP address.
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If I remove these entries from the hosts file and save it
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I'll clear the screen there.
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When I ping are one now that's going to timeout because I don't have an entry for that domain name.
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That's essentially what a DNS server does.
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It takes a domain name and maps it to an IP address.
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