1 00:00:00,810 --> 00:00:06,780 In this series of lectures, we get our networking basics right. 2 00:00:06,800 --> 00:00:15,200 We look at basic networking concepts like Switching, Routing, Gateways etc. We then understand DNS, and 3 00:00:15,200 --> 00:00:18,290 then we get a basic introduction to core DNS. 4 00:00:18,680 --> 00:00:28,930 We look at how to configure DNS settings on a Linux system within understand basics of network namespace 5 00:00:28,930 --> 00:00:31,060 in Linux. 6 00:00:31,160 --> 00:00:36,060 I am not going to bore you with theories on OSI models or network layers 7 00:00:36,260 --> 00:00:42,890 We're just brushing up enough networking to understand the rest of the section in this course. 8 00:00:42,960 --> 00:00:48,470 Now we don't just go through the concepts we see how these are configured on our systems specifically 9 00:00:48,770 --> 00:00:51,020 from a Linux perspective. 10 00:00:51,020 --> 00:00:53,670 So there are going to be lots of commands. 11 00:00:53,870 --> 00:01:00,680 We look at this from a system admins and the application developers perspective and not necessarily 12 00:01:00,680 --> 00:01:07,390 from a network engineers that way later on when we discuss these in the context of our course. 13 00:01:07,580 --> 00:01:13,460 You know what we were talking about where to look for information where things are configured how to 14 00:01:13,460 --> 00:01:22,480 troubleshoot etc. And of course as always if these seem to be too basic for you if you or if you're 15 00:01:22,500 --> 00:01:29,370 already good with networking in Linux feel free to skip these lectures and go straight to the ones on 16 00:01:29,370 --> 00:01:30,530 Kubernetes. 17 00:01:30,660 --> 00:01:31,760 So let's get started. 18 00:01:33,370 --> 00:01:34,250 So what is a 19 00:01:34,320 --> 00:01:35,160 Network. 20 00:01:35,320 --> 00:01:41,650 We have two computers A & B – laptops, desktops, VMs on the cloud, wherever. 21 00:01:41,650 --> 00:01:43,870 How does system A reach B? 22 00:01:43,880 --> 00:01:51,070 We connect them to a switch, and the switch creates a network containing the two systems. To connect 23 00:01:51,070 --> 00:01:52,420 them to a switch 24 00:01:52,420 --> 00:02:00,100 we need an interface on each host. Physical or virtual depending on the host to see the interfaces for 25 00:02:00,100 --> 00:02:00,970 the host. 26 00:02:01,000 --> 00:02:03,730 We use the IP link command. 27 00:02:04,060 --> 00:02:11,270 In this case, we look at the interface named eth0 that we will be using to connect to the switch. 28 00:02:11,320 --> 00:02:13,960 Let's assume it's a network with the address. 29 00:02:13,960 --> 00:02:16,900 192.168.1.0. 30 00:02:17,020 --> 00:02:21,270 We then assign the systems with IP addresses on the same network. 31 00:02:21,310 --> 00:02:29,400 For this, we use the command ip addr. Once the links are up, and the IP addresses are assigned, 32 00:02:29,410 --> 00:02:37,150 The computers can now communicate with each other through the switch the switch can only enable communication 33 00:02:37,300 --> 00:02:44,470 within and network which means it can receive packets from a host on the network and deliver it to other 34 00:02:44,470 --> 00:02:47,190 systems within the same network. 35 00:02:48,740 --> 00:02:56,450 Say we have another network containing systems C & D at address 192.168.2.0. 36 00:02:56,450 --> 00:03:02,570 The Systems have IP address 192.168.2.10 and 2.11 respectively. 37 00:03:02,600 --> 00:03:07,430 How does a system in one network reach a system in the other. 38 00:03:07,430 --> 00:03:16,040 How does System B with the IP 192.168.1.11 reach system C with the IP 2.10 on the 39 00:03:16,130 --> 00:03:17,690 other network. 40 00:03:17,990 --> 00:03:27,320 That’s where a Router comes in. A Router helps connect two networks together. It is an intelligent device, 41 00:03:27,320 --> 00:03:34,850 so think of it as another server with many network ports. Since it connects to the two separate networks, 42 00:03:34,850 --> 00:03:41,000 it gets two IPs assigned. One on each network. In the first network 43 00:03:41,000 --> 00:03:49,400 we assign it an IP address 192.168.1.1 and in the second we assign it an IP 192.168.2.1. 44 00:03:49,400 --> 00:03:55,260 Now we have a router connected to the two networks that can enable communication between them. 45 00:03:55,580 --> 00:04:02,720 Now, when system B tries to send a packet to system C, how does it know where the router is on the network 46 00:04:02,960 --> 00:04:08,930 to send the packet through the router is just another device on the network. 47 00:04:08,930 --> 00:04:18,150 there could be many other such devices. That’s where we configure the systems with a gateway or a route. 48 00:04:18,960 --> 00:04:25,800 If the network was a room, the gateway is a door to the outside world to other networks or to the 49 00:04:25,800 --> 00:04:27,040 Internet. 50 00:04:27,120 --> 00:04:35,040 The systems need to know where that door is to go through that to see the existing routing configuration 51 00:04:35,100 --> 00:04:42,540 on a system run the route command. It displays the kernels routing table and within that, as you can 52 00:04:42,540 --> 00:04:45,290 see there are no routing configurations 53 00:04:45,300 --> 00:04:46,450 as of now. 54 00:04:46,450 --> 00:04:53,640 So in this condition your system B will not be able to reach system C it can only reach other systems 55 00:04:53,970 --> 00:05:03,110 within the same network in the range 192.168.1.0. To configure a gateway on system B to reach the 56 00:05:03,110 --> 00:05:10,750 systems on network 192.168.2.0, run the ip route add command, and specify that you can reach the 57 00:05:10,760 --> 00:05:19,340 192.168.2.0 network through the door or gateway at 192.168.1.11. Running the route 58 00:05:19,550 --> 00:05:26,690 command again shows that we have a route added to reach the 192.168.2.0 series network through the 59 00:05:26,690 --> 00:05:28,700 router. 60 00:05:28,710 --> 00:05:32,430 Now remember this has to be configured on all the systems. 61 00:05:32,440 --> 00:05:39,730 For example, if the system C is to send a packet to system B, then you need to add a route on system C’s 62 00:05:39,730 --> 00:05:46,340 routing table to access network 192.168.1.0 through the router configured with the IP address 63 00:05:46,350 --> 00:05:56,420 192.168.2.1. Now suppose these systems need access to the Internet. Say they need access to Google at 64 00:05:56,430 --> 00:06:02,330 172.217.194.0 network on the internet. So you connect the router to the internet. 65 00:06:02,720 --> 00:06:10,290 and then add a new road in your routing table to road all traffic to the network 172.217.194 66 00:06:10,610 --> 00:06:17,420 through your router there are so many different sites on different networks on the Internet instead 67 00:06:17,420 --> 00:06:23,870 of adding a routing table entry for these same routers IP address for each of those networks you can 68 00:06:23,870 --> 00:06:31,740 simply say for any network that you don't know a route to use this router as the default gateway. 69 00:06:32,090 --> 00:06:38,510 This way any request to any network outside of your existing network goes to this particular router. 70 00:06:39,470 --> 00:06:46,970 So in a simple setup like this, all you need is a single routing table entry with a default gateway set 71 00:06:46,970 --> 00:06:49,700 to the routers IP address. 72 00:06:49,700 --> 00:06:54,630 Remember instead of the word default you could also say 0.0.0.0.0. 73 00:06:54,830 --> 00:06:57,450 It means any IP destination. 74 00:06:57,680 --> 00:07:02,640 Both of these lines mean the same thing. A 0.0.0.0 75 00:07:02,660 --> 00:07:08,890 Entry in the Gateway field indicates that you don't need a gateway for example in this case for system 76 00:07:08,900 --> 00:07:14,370 C to access any devices in the 192.168.2.0 network. 77 00:07:14,480 --> 00:07:20,930 It doesn't need a gateway because it is in its own network. But say you have multiple routers in your 78 00:07:20,930 --> 00:07:29,750 network one for the Internet another for the internal private network then you will need to have two 79 00:07:29,750 --> 00:07:32,510 separate entries for each network. 80 00:07:32,510 --> 00:07:39,320 One entry for the internal private network and another entry with the default gateway for all other 81 00:07:39,320 --> 00:07:42,240 networks including public networks. 82 00:07:42,260 --> 00:07:45,750 So if you are having issues reaching internet from your systems, 83 00:07:46,010 --> 00:07:53,520 This routing table and the default gateway configuration is a good place to start let us know look at 84 00:07:53,700 --> 00:07:57,190 how we can set up a linux host as a rotor. 85 00:07:57,210 --> 00:07:58,590 Let's start with a simple setup. 86 00:07:59,650 --> 00:08:06,530 I have 3 hosts A, B and C. A & B are connected to a network 192.168.1 and B 87 00:08:06,530 --> 00:08:14,360 & C to another on 192.168.2. So host B is connected to both the networks using two interfaces eth0 88 00:08:14,420 --> 00:08:24,550 A has IP 192.168.1.5, C has 192.168.2.5 and B has an IP on both the networks. 89 00:08:24,550 --> 00:08:28,280 192.168.1.6 and 192.168.2.6. 90 00:08:28,340 --> 00:08:31,800 How do we get A to talk to C? Basically, 91 00:08:31,850 --> 00:08:39,160 if I try to ping 192.168.2.5 from A, it would say Network is Unreachable. And by now we know why that is. 92 00:08:39,290 --> 00:08:45,840 Host A has no idea how to reach a network at 192.168.2. 93 00:08:45,920 --> 00:08:54,260 We need to tell host A that the door or gateway to network 2 is through host B. And we do that by adding 94 00:08:54,440 --> 00:09:03,350 adding a routing table entry. We add a route to access network 192.168.2 via the gateway 95 00:09:03,350 --> 00:09:12,130 192.168.1.6. If the packets where to get through to Host C, Host C will have to send back responses 96 00:09:12,130 --> 00:09:21,910 to Host A. When Host C tries to reach Host A at 192.168.1 network, it would face the same issue. So 97 00:09:21,910 --> 00:09:29,650 we need to let know Host C know that it can reach Host A through Host B which is acting as a router. So we 98 00:09:29,650 --> 00:09:36,520 add a similar entry into Host C’s routing table. This time we say to reach network 192.168.1.0, 99 00:09:36,910 --> 00:09:46,460 talk to Host B at 192.168.2.6. When we try to ping now, we no longer get the Network Unreachable 100 00:09:46,490 --> 00:09:54,890 error message that means our routing entries are right but we still don't get any response back. By default 101 00:09:55,220 --> 00:10:02,420 in Linux, packets are not forwarded from one interface to the next for example packets received on eth0 102 00:10:02,430 --> 00:10:11,470 on host, B are not forwarded to elsewhere through eth 1 this is this way for security reasons. 103 00:10:11,480 --> 00:10:17,720 For example, if you had eth0 connected to your private network and eth1 to a public network, we 104 00:10:17,720 --> 00:10:23,450 don't want anyone from the public network to easily send messages to the private network unless you 105 00:10:23,540 --> 00:10:25,410 explicitly allow that. 106 00:10:25,700 --> 00:10:30,170 But in this case since we know that both are private networks and it is safe to enable communication 107 00:10:30,200 --> 00:10:37,810 between them we can allow host B to forward packets from one network to the other whether a host can 108 00:10:37,810 --> 00:10:47,190 forward packets between interfaces is governed by a setting in this system at file /proc/sys/net/ipv4/ip 109 00:10:47,190 --> 00:10:56,250 forward by default, the value in this file is set to 0 meaning no forward set 110 00:10:56,280 --> 00:10:59,830 this to 1 and you should see the pings go through. 111 00:10:59,840 --> 00:11:06,150 Now remember simply setting this value does not persist the changes across reboots for that you must 112 00:11:06,150 --> 00:11:12,020 modify the same value in the /etc/sysctl.conf file. 113 00:11:12,630 --> 00:11:16,080 So let's take away some key commands from this lecture. 114 00:11:16,080 --> 00:11:23,650 These will be handy in the upcoming lectures IP link is to list and modify interfaces on the host 115 00:11:23,670 --> 00:11:31,920 ip addr command is to see the ip addresses assigned to those interfaces ip addr add command is used to set 116 00:11:32,190 --> 00:11:34,860 IP addresses on the interfaces. 117 00:11:34,860 --> 00:11:40,740 Now remember changed made using these commands are only valid till a restart. If you want to persist these 118 00:11:40,740 --> 00:11:49,110 changes you must set them in the /etc/network/interfaces file. ip route or simply the route command 119 00:11:49,110 --> 00:11:56,500 is used to view the routing table. And ip route add command is used to add entries into the routing table. 120 00:11:56,550 --> 00:12:02,290 And finally remember the command to check if IP forwarding is enabled on a host. 121 00:12:02,310 --> 00:12:08,480 If you're working with a house configured as a router Well that's it for this lecture in the next lecture. 122 00:12:08,500 --> 00:12:11,100 We'll discuss about DNS configurations.