网络课程笔记

00:00 Hi.

00:01 So in this lecture let's understand how Dhcp option sets work and what kind of attributes it has which

00:07 affects your DNS configuration for the VPC.

00:11 So as I said every VPC comes with a default Dhcp option set.

00:16 And it provides basically a dynamic host configuration protocol attributes for the VPC resources.

00:23 So through Dhcp option set you can set what should be the private domain name for your EC2 instances.

00:31 Then what should be the domain name server which is like route 53 resolver by default.

00:37 Then additional thing like network time protocol server that is NTP and net bias node type.

00:43 But out of this I think we are interested mainly in domain name server.

00:48 Because using the Dhcp option set, we can tell your VPC that do not look at the AWS provided DNS server,

00:55 but rather you can look at some other DNS server for resolving the DNS queries.

01:00 And we will do that exercise little later in this section.

01:05 But all these configurations are managed at the VPC level using the Dhcp option sets.

01:11 So as I said, when you launch the EC2 instances, this Dhcp option set parameters are automatically

01:19 applied for the EC2 instances as you launch them.

01:23 So it has the default domain name server which points to Amazon provided DNS, which is nothing but

01:30 the route 53 DNS resolver that we talked about in the last lecture.

01:36 And it also automatically configures the internal domain name for your EC2 machines.

01:43 So the typical format is private IP of the EC2 instance, then the region name and then compute dot

01:50 internal.

01:51 And we are going to again talk about this in further detail.

01:55 But overall this is what Dhcp option set does.

01:59 So I hope that is clear.

02:01 Now just to show you this is one of the Dhcp option set which is a default one.

02:06 And you can see here that it is created in Mumbai region.

02:10 And that's where the domain name is AP South one dot compute dot internal.

02:15 And the default domain name server is Amazon provided DNS.

02:20 And this Dhcp option set is automatically associated with your VPC.

02:26 And then as you launch EC2 instances they will have corresponding private DNS name something like this.

02:34 So what it does basically it sets the host name for your EC2 instances.

02:39 Now this host names could be private host names or the public host name.

02:44 Now it really depends on the DNS attribute where your EC2 instance will also receive the public hostname

02:51 or not.

02:51 But there are always this private host names.

02:54 So it sets up the host name which is in a format like IP address, dot region, dot computing internal.

03:02 And additionally the Dhcp will also set the Resolv.conf.

03:07 If it's a Linux machine which tells your EC2 instance, what is the name server for DNS resolution?

03:14 And here if you see the cursor, it has the name server 10.10.0.2, which is nothing but VPC base address

03:22 plus two.

03:23 That means all the DNS queries from the EC2 instance are going to this name server.

03:28 And we already talked about this DNS server in detail in the earlier lecture, so I hope that is clear.

03:35 Now with that, let me take you to the AWS console and show you some of these configurations.

03:41 So I just created one VPC called demo VPC.

03:44 And if you see here it is associated with Dhcp option set which is the default one.

03:50 And if you just go to this Dhcp option set then you will see the domain name as this.

03:56 And the name server is Amazon provided DNS.

04:00 And for this VPC I did not make any changes to the default Dhcp option set.

04:05 What I did is I just went to the EC2 and I launched two instances this instance one and instance 2 in

04:13 2 different subnets.

04:15 Now instance one has a public IP because it's in a public subnet, and instance two is in a private

04:22 subnet.

04:24 Now if you look at the instance one, it has the public IP and it has a private IP.

04:29 Now along with this private IP it also has a private DNS name which is assigned by this Dhcp.

04:37 But if you notice there is no public DNS name for this EC2 instance.

04:42 Now that's for the reason that there is a setting at the VPC level, which tells whether the EC2 instance

04:49 with the public IP will receive the public DNS or not.

04:53 So if we go back to our VPC and just go here and look at this VPC attributes.

05:00 So this first parameter enabled DNS resolution tells VPC that it can resolve the DNS using the Amazon

05:07 provided DNS server.

05:08 And the second attribute which is enable DNS host names.

05:11 It basically control whether the EC2 instance will receive the public IP or not.

05:16 So let's just enable this and save it.

05:25 But now, if we launch another instance in a public subnet with the public IP, it should also receive

05:31 the public DNS.

05:33 So let's quickly do that.

05:35 And while we wait for that instance we will also see some other things.

05:40 So let me quickly launch this instance and let's make sure that it is in our VPC.

05:46 And it is launched in a public subnet and it has a public IP.

05:51 Rest of the settings I'm keeping mostly as default and launching this instance.

05:57 Okay.

05:57 So it will take some time for this instance to be launched.

06:00 Okay.

06:01 Now if we come back here and see that there is this instance one and it automatically got its public

06:07 IP.

06:07 Now because we have enabled this setting at the VPC level that is enabled DNS hostname.

06:12 And for the instance three it also got this public IP.

06:18 Now we also have instance two which is in private subnet.

06:22 And it just has a private IP.

06:24 Now what we want to quickly do is from any of these instance one or instance three.

06:29 Let's try to resolve the private DNS of instance two.

06:32 And it should resolve to its corresponding private IP.

06:35 So I'm just getting the public IP address.

06:38 Or I can use public DNS as well to connect to this instance from my putty session.

06:44 Okay.

06:44 So I'm putting this DNS name I'm loading my key and just opening this session.

06:56 And you can see I am into my session now.

06:59 From here, let's try to ping to our EC2 instance two using its private DNS.

07:06 Now for that, let's go here and let's pick this private DNS and just try that.

07:13 And you can see that it is resolving to its private IP.

07:17 So this is being resolved using the Amazon provided DNS server.

07:21 And this is going through our Dhcp configurations.

07:25 Now I also want to show you one more thing.

07:27 That is what if you try to resolve the public DNS of the EC2 instance from, you know, within the EC2

07:34 instance itself.

07:35 So let's try that.

07:37 So let's get this instance three public DNS which is this.

07:43 And let's try to ping to that.

07:46 Now you see it has resolved to its private IP.

07:49 It hasn't resolved to its public IP.

07:52 Now that's because we are making the DNS queries from within the VPC.

07:56 However, if you do the same thing from your local machine.

08:00 So let me try to go to my CMD and say Nslookup.

08:08 Then you can see that it has resolved to its public IP because we are querying that from outside of

08:15 the VPC.

08:16 So that is how public DNS resolution happens.

08:20 Now one more thing I want to show you is how that resolv.conf looks like.

08:26 Right?

08:26 So if you see Resolv.conf on this machine, the name server is 10.10.0.2, which is the VPC base address

08:35 plus two.

08:36 So all the DNS queries are being sent to this particular DNS server.

08:42 And this is the search domain for your EC2 instances.

08:46 Which means if you don't provide the complete DNS name, it will automatically look at this as a suffix

08:52 of your domain name.

08:53 Now what do I mean by that?

08:55 Instead of pinging to an private DNS which ends in this, you can just say, you know, the first part

09:02 of the DNS, which is this.

09:06 And if you do that, it still resolves because it is looking at this search domain.

09:11 Now again, not important for your exam, but something that you should know.

09:15 Right.

09:16 So I hope this is clear.

09:17 And you can definitely try all these scenarios on your own.

09:21 So with that, let's go back to our presentation.

09:24 So we saw all these settings, and we also saw how EC2 instance gets private DNS and the public DNS.

09:32 Just there is one difference to know that if EC2 instance is in North Virginia region, that is US east

09:39 one, then the syntax is little different, but otherwise the syntax is IP dash private IPv4 address,

09:45 dot region, dot compute, dot internal and similar domain name for the public address as well.

09:53 And we already talked about domain names, the private and the public DNS.

09:57 So I hope that is clear.

09:59 Now, one thing to note here is that you cannot modify the existing Dhcp option set, but rather you

10:05 would have to create a new Dhcp option set and then associate that with your existing VPC.

10:11 So something like this.

10:13 Now in here we are changing our private domain name to finance dot internal.

10:19 And once you do that the search domain for this VPC is set to finance dot internal instead of AP South

10:27 1.4. internal.

10:29 Which means now all these EC2 instances can reach to each other using the domain name, something like

10:35 IP dash, private IP address, dot finance, dot internal.

10:40 But you have to wait for a few hours.

10:42 Typically for this or otherwise, depending on your operating system, you can run some Dhcp command

10:48 to refresh your Dhcp lease.

10:51 So that's another thing that you need to note is that Dhcp option set cannot be modified.

10:57 You have to create a new one and then associate that with your VPC.

11:02 Now here we talked about changing the domain name.

11:04 But very well you can also change the name server.

11:07 So instead of using Amazon provided DNS, you can point your DNS queries to your custom domain name

11:14 server.

11:15 And we are going to see that in the hands on exercise.

11:19 So which means you can change all the parameters in Dhcp option set.

11:23 And accordingly it will affect your DNS configuration.

11:28 So I hope that is clear.

11:30 Okay.

11:30 So let's move on.

11:31 And let's again talk about the VPC DNS attributes which we already saw in our exercise.

11:37 So there are two attributes one enabled DNS support.

11:41 And the default is true.

11:42 And basically decides if the DNS resolution is supported for the VPC or not.

11:47 So if it is true, then all the DNS queries are sent to AWS DNS server at the virtual IP or say base

11:55 address plus two.

11:56 The another attribute is enabled DNS hostname, and it governs where your EC2 instance with the public

12:03 IP will receive the public DNS or not.

12:06 So by default this attribute is false, but typically you should turn it on as well.

12:12 So if true, it assigns the public hostname to the EC2 instance which has a public IP.

12:19 Now with the combination of this true, there are a lot of things that may work or may not work.

12:24 For example, if you are using Amazon Route 53 private hosted zone for managing the private DNS for

12:30 the VPC, then you should have both these attributes set to true.

12:33 If any of these is false, then it won't resolve your DNS queries.

12:38 So these are a few things which probably you should remember.

12:41 And again if you want to try, you can try this in your AWS account where you set this parameter.

12:47 And you can check that whether the EC2 instance with public IP has got the public DNS or not.

12:54 We have already seen that in our exercise.

12:57 And finally, a few things that you need to remember for your exam.

13:02 So once you create a Dhcp option set you cannot modify the existing one, but rather you can create

13:08 a new one and associate that with the VPC.

13:11 At a given time.

13:12 You can only have one DSP option set associated with the VPC.

13:17 Now you can also have the VPC and say I don't want to have any DSP option set.

13:21 But the problem is in that case, VPC doesn't know where to send the DNS queries and it cannot resolve

13:28 then any DNS, which is okay if your instance can communicate with each other directly using the private

13:35 IP addresses.

13:36 Further, if you are changing your existing Dhcp option set and associating a new one with your VPC,

13:43 it may take a few hours to take in effect.

13:45 And it depends on your Dhcp lease.

13:48 But if you want that this take into effect immediately, then you can probably run some Linux command

13:54 for example Dhcp client and it refreshes your Dhcp lease.

14:00 So this will really depend on your operating system.

14:02 So this is for the Linux and for other operating system.

14:05 There will be different commands.

14:07 So you can refresh your Dhcp lease on demand.

14:10 Otherwise it will happen over few hours.

14:13 So these are all the important things that you should know for your exam.

14:17 And with that we will see few things in action so that you understand it completely.

14:23 So thanks for watching and stay tuned.