1 00:00:00,940 --> 00:00:06,760 In this lecture we will discuss how Kubernetes implements DNS in 2 00:00:06,760 --> 00:00:07,630 the cluster. 3 00:00:07,660 --> 00:00:13,690 In the previous lecture we saw how you can address a service or pod from another pod. In this lecture 4 00:00:13,690 --> 00:00:17,020 we will see how kubernetes makes that possible. 5 00:00:17,080 --> 00:00:20,120 Say you were given two pods with two IP addresses. 6 00:00:20,140 --> 00:00:21,640 How would you do it. 7 00:00:21,790 --> 00:00:25,270 Based on what we learned in the prerequisite lectures on DNS, 8 00:00:25,300 --> 00:00:32,860 an easy way to get them to resolve each other is to add an entry into each of their /etc/hosts files. On 9 00:00:32,860 --> 00:00:33,850 the first pod 10 00:00:33,910 --> 00:00:39,330 I would say the second pod web is at 10.244.2.5 11 00:00:39,520 --> 00:00:45,520 and on the second pod I would say the first pod test is at 10.244.1.5. 12 00:00:45,670 --> 00:00:50,290 But of course, when you have 1000s of PODs in the cluster, and 100s of them being created and 13 00:00:50,290 --> 00:00:51,520 deleted every minute. 14 00:00:51,520 --> 00:00:53,890 This is not a suitable solution. 15 00:00:53,890 --> 00:01:00,430 So we move these entries into a central DNS server. We then point these PODs to the DNS server by 16 00:01:00,430 --> 00:01:07,390 adding an entry into their /etc/resolv.conf file specifying that the nameserver is at the IP address 17 00:01:07,390 --> 00:01:11,460 of the DNS server, which happens to be 10.96.0.10 18 00:01:11,470 --> 00:01:18,760 in this case. Every time a new pod is created, we add a record in the DNS server for that pod so that 19 00:01:18,880 --> 00:01:25,810 other pods can access the new POD, and configure the /etc/resolv.conf file in the POD to 20 00:01:25,810 --> 00:01:28,470 the DNS server so that the pod can resolve 21 00:01:28,570 --> 00:01:30,910 other pods in the cluster. 22 00:01:30,910 --> 00:01:37,220 This is kind of how kubernetes does it. Except that it does not create similar entries for PODs to map podname 23 00:01:37,300 --> 00:01:40,870 to its IP address as we have seen in the previous lecture. 24 00:01:40,870 --> 00:01:49,090 It does that for services. For pods it forms host names by replacing dots with dashes in the IP address 25 00:01:49,090 --> 00:01:56,260 of the pod. Kubernetes implements DNS in the same way. It deploys a DNS server within the cluster. 26 00:01:56,830 --> 00:02:04,730 Prior to version v1.12 the DNS implemented by kubernetes was known as kube-dns. With Kubernetes 27 00:02:04,840 --> 00:02:13,110 version 1.12 the recommended DNS server is CoreDNS. We took a brief look at a core DNS in one of 28 00:02:13,110 --> 00:02:15,120 the prerequisite lectures. 29 00:02:15,120 --> 00:02:18,120 So how is the core DNS setup in the cluster. 30 00:02:18,120 --> 00:02:24,330 The CoreDNS server is deployed as a POD in the kube-system namespace in the kubernetes cluster. 31 00:02:24,360 --> 00:02:29,200 Well they are deployed as two pods for redundancy, as part of a replicaset. 32 00:02:29,520 --> 00:02:35,120 they are actually a replicaset within a deployment. But it doesn’t really matter. We’ll just see 33 00:02:35,120 --> 00:02:38,280 CoreDNS as a POD in this lecture. 34 00:02:38,280 --> 00:02:45,360 This POD runs the coreDNS executable, the same executable that we ran when we deployed CoreDNS 35 00:02:45,420 --> 00:02:46,630 ourselves. 36 00:02:46,740 --> 00:02:49,880 CoreDNS requires a configuration file. In our case 37 00:02:49,920 --> 00:02:54,080 we used a file named Corefile. So does kubernetes. 38 00:02:54,150 --> 00:03:00,730 It uses a file named Corefile located at /etc/coredns. Within this file 39 00:03:00,740 --> 00:03:07,760 you have a number of plugins configured. The ones highlighted in orange. Plugins are configured for handling 40 00:03:07,820 --> 00:03:14,210 errors, reporting health, monitoring metrics, cache etc. The plugin that makes CoreDNS work with 41 00:03:14,210 --> 00:03:19,760 Kubernetes, is the kubernetes plugin. And this is where the top level domain name for the cluster 42 00:03:19,850 --> 00:03:28,340 is set. In this case cluster.local. So every record in the coredns DNS server falls under this 43 00:03:28,340 --> 00:03:30,800 domain. Within the kubernetes plugin 44 00:03:30,800 --> 00:03:32,460 there are multiple options. 45 00:03:32,620 --> 00:03:39,220 The pods option you see here, is what is responsible for creating a record for PODs in the cluster. 46 00:03:39,230 --> 00:03:44,570 Remember we talked about a record being created for each POD by converting their IPs into a dashed 47 00:03:44,570 --> 00:03:47,320 format that's disabled by default. 48 00:03:47,480 --> 00:03:50,840 But it can be enabled with this entry here. 49 00:03:50,840 --> 00:03:57,620 Any record that this DNS server can’t solve, for example say a POD tries to reach www.google.com 50 00:03:57,770 --> 00:04:04,810 it is forwarded to the nameserver specified in the coredns pods /etc/resolv.conf file. The 51 00:04:04,820 --> 00:04:09,290 /etc/resolv.conf file is set to use the nameserver from the kubernetes 52 00:04:09,290 --> 00:04:18,450 Node. Also note, that this core file is passed into the pod has a configMap object. That way if you 53 00:04:18,450 --> 00:04:23,240 need to modify this configuration you can edit the ConfigMap object. 54 00:04:23,310 --> 00:04:28,590 We now have the coredns pod up and running using the appropriate kubernetes plugin. 55 00:04:28,740 --> 00:04:36,180 It watches the kubernetes cluster for new PODs or services, and every time a pod or a service is created it 56 00:04:36,300 --> 00:04:39,510 adds a record for it in its database. 57 00:04:39,510 --> 00:04:43,530 Next step is for the pod to point to the coreDNS server. 58 00:04:43,830 --> 00:04:50,570 What address do the PODs use to reach the DNS server? When we deploy CoreDNS solution, 59 00:04:50,570 --> 00:04:55,550 It also creates a service to make it available to other components within a cluster. 60 00:04:55,550 --> 00:05:02,640 The service is named as kube-dns by default. The IP address of this service is configured as 61 00:05:02,640 --> 00:05:04,480 nameserver on the PODs. 62 00:05:04,500 --> 00:05:09,340 Now you don’t have to configure this yourself. The DNS configurations on PODs are done by kubernetes 63 00:05:09,350 --> 00:05:12,720 automatically when the PODs are created. 64 00:05:12,720 --> 00:05:17,720 Want to guess which kubernetes component is responsible for that? The kubelet. 65 00:05:17,910 --> 00:05:24,190 If you look at the config file of the kubelet you will see the IP of the DNS server and domain in it. 66 00:05:24,270 --> 00:05:30,090 Once the pods are configured with the right nameserver, you can now resolve other pods and services. 67 00:05:30,540 --> 00:05:36,130 You can access the web-service using just web-service, or web-service.default or web-service.default.svc or web-service.default.svc.cluster.local. 68 00:05:36,210 --> 00:05:41,220 You can access the web-service using just web-service, or web-service.default or web-service.default.svc or web service.default.svc.cluster.local. 69 00:05:41,460 --> 00:05:47,310 If you try to manually lookup the web-service using nslookup or the host command web-service command, it 70 00:05:47,310 --> 00:05:53,130 will return the fully qualified domain name of the web-service, which happens to be web-service.default.svc.cluster.local. 71 00:05:53,130 --> 00:05:55,440 will return the fully qualified domain name of the web-service, which happens to be web-service.default.svc.cluster.local. 72 00:05:55,740 --> 00:05:58,980 But you didn't ask for that you just set up service. 73 00:05:58,980 --> 00:06:01,520 So how did it look up for the full name. 74 00:06:01,530 --> 00:06:07,730 It so happens, the resolv.conf file also has a search entry which is set to default.svc.cluster.local 75 00:06:07,730 --> 00:06:13,770 as well as svc.cluster.local and cluster.local. 76 00:06:13,770 --> 00:06:19,590 This allows you to find the service using any name. web-service or web-service.default or web-service.default.svc. 77 00:06:19,590 --> 00:06:21,090 This allows you to find the service using any name. web-service or web-service.default or web-service.default.svc. 78 00:06:21,090 --> 00:06:27,990 However, notice that it only has search entries for service . So you won’t be able to reach a pod the same 79 00:06:27,990 --> 00:06:28,440 way. 80 00:06:28,500 --> 00:06:33,280 For that you need to specify the full FQDN of the pod. 81 00:06:33,300 --> 00:06:35,160 Well that's it for this lecture. 82 00:06:35,160 --> 00:06:41,620 Head over to the practice test and practice working with DNS on our kubernetes cluster. I will see you 83 00:06:41,860 --> 00:06:42,820 in the next lecture.