WEBVTT 00:00.810 --> 00:06.780 In this series of lectures, we get our networking basics right. 00:06.800 --> 00:15.200 We look at basic networking concepts like Switching, Routing, Gateways etc. We then understand DNS, and 00:15.200 --> 00:18.290 then we get a basic introduction to core DNS. 00:18.680 --> 00:28.930 We look at how to configure DNS settings on a Linux system within understand basics of network namespace 00:28.930 --> 00:31.060 in Linux. 00:31.160 --> 00:36.060 I am not going to bore you with theories on OSI models or network layers 00:36.260 --> 00:42.890 We're just brushing up enough networking to understand the rest of the section in this course. 00:42.960 --> 00:48.470 Now we don't just go through the concepts we see how these are configured on our systems specifically 00:48.770 --> 00:51.020 from a Linux perspective. 00:51.020 --> 00:53.670 So there are going to be lots of commands. 00:53.870 --> 01:00.680 We look at this from a system admins and the application developers perspective and not necessarily 01:00.680 --> 01:07.390 from a network engineers that way later on when we discuss these in the context of our course. 01:07.580 --> 01:13.460 You know what we were talking about where to look for information where things are configured how to 01:13.460 --> 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 01:22.500 --> 01:29.370 already good with networking in Linux feel free to skip these lectures and go straight to the ones on 01:29.370 --> 01:30.530 Kubernetes. 01:30.660 --> 01:31.760 So let's get started. 01:33.370 --> 01:34.250 So what is a 01:34.320 --> 01:35.160 Network. 01:35.320 --> 01:41.650 We have two computers A & B – laptops, desktops, VMs on the cloud, wherever. 01:41.650 --> 01:43.870 How does system A reach B? 01:43.880 --> 01:51.070 We connect them to a switch, and the switch creates a network containing the two systems. To connect 01:51.070 --> 01:52.420 them to a switch 01:52.420 --> 02:00.100 we need an interface on each host. Physical or virtual depending on the host to see the interfaces for 02:00.100 --> 02:00.970 the host. 02:01.000 --> 02:03.730 We use the IP link command. 02:04.060 --> 02:11.270 In this case, we look at the interface named eth0 that we will be using to connect to the switch. 02:11.320 --> 02:13.960 Let's assume it's a network with the address. 02:13.960 --> 02:16.900 192.168.1.0. 02:17.020 --> 02:21.270 We then assign the systems with IP addresses on the same network. 02:21.310 --> 02:29.400 For this, we use the command ip addr. Once the links are up, and the IP addresses are assigned, 02:29.410 --> 02:37.150 The computers can now communicate with each other through the switch the switch can only enable communication 02:37.300 --> 02:44.470 within and network which means it can receive packets from a host on the network and deliver it to other 02:44.470 --> 02:47.190 systems within the same network. 02:48.740 --> 02:56.450 Say we have another network containing systems C & D at address 192.168.2.0. 02:56.450 --> 03:02.570 The Systems have IP address 192.168.2.10 and 2.11 respectively. 03:02.600 --> 03:07.430 How does a system in one network reach a system in the other. 03:07.430 --> 03:16.040 How does System B with the IP 192.168.1.11 reach system C with the IP 2.10 on the 03:16.130 --> 03:17.690 other network. 03:17.990 --> 03:27.320 That’s where a Router comes in. A Router helps connect two networks together. It is an intelligent device, 03:27.320 --> 03:34.850 so think of it as another server with many network ports. Since it connects to the two separate networks, 03:34.850 --> 03:41.000 it gets two IPs assigned. One on each network. In the first network 03:41.000 --> 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. 03:49.400 --> 03:55.260 Now we have a router connected to the two networks that can enable communication between them. 03:55.580 --> 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 04:02.960 --> 04:08.930 to send the packet through the router is just another device on the network. 04:08.930 --> 04:18.150 there could be many other such devices. That’s where we configure the systems with a gateway or a route. 04:18.960 --> 04:25.800 If the network was a room, the gateway is a door to the outside world to other networks or to the 04:25.800 --> 04:27.040 Internet. 04:27.120 --> 04:35.040 The systems need to know where that door is to go through that to see the existing routing configuration 04:35.100 --> 04:42.540 on a system run the route command. It displays the kernels routing table and within that, as you can 04:42.540 --> 04:45.290 see there are no routing configurations 04:45.300 --> 04:46.450 as of now. 04:46.450 --> 04:53.640 So in this condition your system B will not be able to reach system C it can only reach other systems 04:53.970 --> 05:03.110 within the same network in the range 192.168.1.0. To configure a gateway on system B to reach the 05:03.110 --> 05:10.750 systems on network 192.168.2.0, run the ip route add command, and specify that you can reach the 05:10.760 --> 05:19.340 192.168.2.0 network through the door or gateway at 192.168.1.11. Running the route 05:19.550 --> 05:26.690 command again shows that we have a route added to reach the 192.168.2.0 series network through the 05:26.690 --> 05:28.700 router. 05:28.710 --> 05:32.430 Now remember this has to be configured on all the systems. 05:32.440 --> 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 05:39.730 --> 05:46.340 routing table to access network 192.168.1.0 through the router configured with the IP address 05:46.350 --> 05:56.420 192.168.2.1. Now suppose these systems need access to the Internet. Say they need access to Google at 05:56.430 --> 06:02.330 172.217.194.0 network on the internet. So you connect the router to the internet. 06:02.720 --> 06:10.290 and then add a new road in your routing table to road all traffic to the network 172.217.194 06:10.610 --> 06:17.420 through your router there are so many different sites on different networks on the Internet instead 06:17.420 --> 06:23.870 of adding a routing table entry for these same routers IP address for each of those networks you can 06:23.870 --> 06:31.740 simply say for any network that you don't know a route to use this router as the default gateway. 06:32.090 --> 06:38.510 This way any request to any network outside of your existing network goes to this particular router. 06:39.470 --> 06:46.970 So in a simple setup like this, all you need is a single routing table entry with a default gateway set 06:46.970 --> 06:49.700 to the routers IP address. 06:49.700 --> 06:54.630 Remember instead of the word default you could also say 0.0.0.0.0. 06:54.830 --> 06:57.450 It means any IP destination. 06:57.680 --> 07:02.640 Both of these lines mean the same thing. A 0.0.0.0 07:02.660 --> 07:08.890 Entry in the Gateway field indicates that you don't need a gateway for example in this case for system 07:08.900 --> 07:14.370 C to access any devices in the 192.168.2.0 network. 07:14.480 --> 07:20.930 It doesn't need a gateway because it is in its own network. But say you have multiple routers in your 07:20.930 --> 07:29.750 network one for the Internet another for the internal private network then you will need to have two 07:29.750 --> 07:32.510 separate entries for each network. 07:32.510 --> 07:39.320 One entry for the internal private network and another entry with the default gateway for all other 07:39.320 --> 07:42.240 networks including public networks. 07:42.260 --> 07:45.750 So if you are having issues reaching internet from your systems, 07:46.010 --> 07:53.520 This routing table and the default gateway configuration is a good place to start let us know look at 07:53.700 --> 07:57.190 how we can set up a linux host as a rotor. 07:57.210 --> 07:58.590 Let's start with a simple setup. 07:59.650 --> 08:06.530 I have 3 hosts A, B and C. A & B are connected to a network 192.168.1 and B 08:06.530 --> 08:14.360 & C to another on 192.168.2. So host B is connected to both the networks using two interfaces eth0 08:14.420 --> 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. 08:24.550 --> 08:28.280 192.168.1.6 and 192.168.2.6. 08:28.340 --> 08:31.800 How do we get A to talk to C? Basically, 08:31.850 --> 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. 08:39.290 --> 08:45.840 Host A has no idea how to reach a network at 192.168.2. 08:45.920 --> 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 08:54.440 --> 09:03.350 adding a routing table entry. We add a route to access network 192.168.2 via the gateway 09:03.350 --> 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 09:12.130 --> 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 09:21.910 --> 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 09:29.650 --> 09:36.520 add a similar entry into Host C’s routing table. This time we say to reach network 192.168.1.0, 09:36.910 --> 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 09:46.490 --> 09:54.890 error message that means our routing entries are right but we still don't get any response back. By default 09:55.220 --> 10:02.420 in Linux, packets are not forwarded from one interface to the next for example packets received on eth0 10:02.430 --> 10:11.470 on host, B are not forwarded to elsewhere through eth 1 this is this way for security reasons. 10:11.480 --> 10:17.720 For example, if you had eth0 connected to your private network and eth1 to a public network, we 10:17.720 --> 10:23.450 don't want anyone from the public network to easily send messages to the private network unless you 10:23.540 --> 10:25.410 explicitly allow that. 10:25.700 --> 10:30.170 But in this case since we know that both are private networks and it is safe to enable communication 10:30.200 --> 10:37.810 between them we can allow host B to forward packets from one network to the other whether a host can 10:37.810 --> 10:47.190 forward packets between interfaces is governed by a setting in this system at file /proc/sys/net/ipv4/ip 10:47.190 --> 10:56.250 forward by default, the value in this file is set to 0 meaning no forward set 10:56.280 --> 10:59.830 this to 1 and you should see the pings go through. 10:59.840 --> 11:06.150 Now remember simply setting this value does not persist the changes across reboots for that you must 11:06.150 --> 11:12.020 modify the same value in the /etc/sysctl.conf file. 11:12.630 --> 11:16.080 So let's take away some key commands from this lecture. 11:16.080 --> 11:23.650 These will be handy in the upcoming lectures IP link is to list and modify interfaces on the host 11:23.670 --> 11:31.920 ip addr command is to see the ip addresses assigned to those interfaces ip addr add command is used to set 11:32.190 --> 11:34.860 IP addresses on the interfaces. 11:34.860 --> 11:40.740 Now remember changed made using these commands are only valid till a restart. If you want to persist these 11:40.740 --> 11:49.110 changes you must set them in the /etc/network/interfaces file. ip route or simply the route command 11:49.110 --> 11:56.500 is used to view the routing table. And ip route add command is used to add entries into the routing table. 11:56.550 --> 12:02.290 And finally remember the command to check if IP forwarding is enabled on a host. 12:02.310 --> 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. 12:08.500 --> 12:11.100 We'll discuss about DNS configurations.