1 1 00:00:00,510 --> 00:00:04,670 DHCP or the Dynamic Host Configuration Protocol. 2 2 00:00:04,670 --> 00:00:07,670 Initially, we used to have to manually provide an IP address 3 3 00:00:07,670 --> 00:00:09,460 to every machine on our network, 4 4 00:00:09,460 --> 00:00:11,330 which is not really a big deal in your house 5 5 00:00:11,330 --> 00:00:13,290 where you might have three or four machines, 6 6 00:00:13,290 --> 00:00:15,680 but in the large networks I've worked on, 7 7 00:00:15,680 --> 00:00:17,600 we have hundreds or thousands, 8 8 00:00:17,600 --> 00:00:20,870 or hundreds of thousands of computers and servers on them. 9 9 00:00:20,870 --> 00:00:23,370 That is a ton of labor hours that we spent 10 10 00:00:23,370 --> 00:00:25,830 just for configuring all of these devices. 11 11 00:00:25,830 --> 00:00:27,900 So, someone was really smart 12 12 00:00:27,900 --> 00:00:32,100 and create an automated way of doing it, known as DHCP. 13 13 00:00:32,100 --> 00:00:35,520 DHCP was invented to help us with this monumentous task 14 14 00:00:35,520 --> 00:00:38,510 of configuring all of these servers and workstations. 15 15 00:00:38,510 --> 00:00:41,670 Now, DHCP also can help eliminate configuration errors 16 16 00:00:41,670 --> 00:00:43,880 because when a person is entering an IP address 17 17 00:00:43,880 --> 00:00:45,570 into an end user's device, 18 18 00:00:45,570 --> 00:00:48,010 there is an opportunity for human error, 19 19 00:00:48,010 --> 00:00:50,830 where they fat finger it and type in the wrong one. 20 20 00:00:50,830 --> 00:00:53,600 Also, it can be really hard to maintain a good list 21 21 00:00:53,600 --> 00:00:56,100 of every IP that every computer is using 22 22 00:00:56,100 --> 00:00:57,470 inside of your network. 23 23 00:00:57,470 --> 00:01:01,360 So often, you can get IP conflicts by assigning the same IP 24 24 00:01:01,360 --> 00:01:04,940 to multiple machines inside the same network by accident. 25 25 00:01:04,940 --> 00:01:07,390 With DHCP, none of this will happen, 26 26 00:01:07,390 --> 00:01:09,720 because each device is automatically going to get assigned 27 27 00:01:09,720 --> 00:01:11,360 an IP from a scope. 28 28 00:01:11,360 --> 00:01:14,440 Now, a scope is simply a list of valid IP addresses 29 29 00:01:14,440 --> 00:01:16,490 that are available for assignment or lease 30 30 00:01:16,490 --> 00:01:19,894 to a computer or an endpoint on a given subnet. 31 31 00:01:19,894 --> 00:01:22,460 Now, for example, in my home network, 32 32 00:01:22,460 --> 00:01:26,870 I have 254 IPs available for devices in my scope. 33 33 00:01:26,870 --> 00:01:29,500 So, when the computer joins my home network, 34 34 00:01:29,500 --> 00:01:32,940 the DHCP server automatically picks one of these unused IPs 35 35 00:01:32,940 --> 00:01:34,290 from the scope, 192.168.1.2 36 36 00:01:36,835 --> 00:01:39,057 all the way up to 192.168.1.254 37 37 00:01:41,210 --> 00:01:44,100 And then it gives that IP to that device to use. 38 38 00:01:44,100 --> 00:01:46,000 This is known as a lease. 39 39 00:01:46,000 --> 00:01:49,130 As a network administrator, you can tell your DHCP server 40 40 00:01:49,130 --> 00:01:51,990 what IP addresses should be used in your scope 41 41 00:01:51,990 --> 00:01:54,470 and you can even reserve some of those IPs 42 42 00:01:54,470 --> 00:01:55,880 that you don't want to be handed out 43 43 00:01:55,880 --> 00:01:58,430 as part of what's known as an excluded range 44 44 00:01:58,430 --> 00:01:59,850 within your scope. 45 45 00:01:59,850 --> 00:02:01,840 For example, in my home network, 46 46 00:02:01,840 --> 00:02:03,610 I have a couple of IP addresses 47 47 00:02:03,610 --> 00:02:06,110 that have been excluded from my DHCP scope. 48 48 00:02:06,110 --> 00:02:10,910 These are IP addresses in the range of 192.168.1.2 49 49 00:02:10,910 --> 00:02:14,500 up to 192.168.1.10. 50 50 00:02:14,500 --> 00:02:16,206 These IP addresses have already been assigned 51 51 00:02:16,206 --> 00:02:19,630 to some of those things in my network manually by me. 52 52 00:02:19,630 --> 00:02:20,890 Things like my printer, 53 53 00:02:20,890 --> 00:02:23,390 my network file server, and other things like that, 54 54 00:02:23,390 --> 00:02:26,410 that I always want to have the same IP address for. 55 55 00:02:26,410 --> 00:02:28,000 Now, another way you could do this 56 56 00:02:28,000 --> 00:02:30,220 instead of statically assigning these IPs 57 57 00:02:30,220 --> 00:02:33,340 is to use something known as a DHCP reservation. 58 58 00:02:33,340 --> 00:02:36,140 This is very commonly used in large networks. 59 59 00:02:36,140 --> 00:02:38,213 Now, a DHCP reservation lets you exclude 60 60 00:02:38,213 --> 00:02:39,900 some of these IP addresses 61 61 00:02:39,900 --> 00:02:41,620 from being handed out to devices 62 62 00:02:41,620 --> 00:02:43,890 unless they meet certain conditions. 63 63 00:02:43,890 --> 00:02:46,610 For example, I could set up a DHCP reservation 64 64 00:02:46,610 --> 00:02:49,050 for my printer based on its MAC address. 65 65 00:02:49,050 --> 00:02:51,410 That way, whenever my printer joins the network, 66 66 00:02:51,410 --> 00:02:54,920 the DHCP server sees that it joins, checks its MAC address, 67 67 00:02:54,920 --> 00:02:57,110 and then assign the exact same IP 68 68 00:02:57,110 --> 00:02:59,150 every single time to that printer, 69 69 00:02:59,150 --> 00:03:01,180 because it's been listed as a reserved address 70 70 00:03:01,180 --> 00:03:03,200 inside the DHCP scope. 71 71 00:03:03,200 --> 00:03:04,570 This is a great way to do things 72 72 00:03:04,570 --> 00:03:05,820 when you have a large network, 73 73 00:03:05,820 --> 00:03:07,410 instead of having to manually configure 74 74 00:03:07,410 --> 00:03:10,690 each endpoint device with a specific IP address. 75 75 00:03:10,690 --> 00:03:13,430 This lets your DHCP server do it for you. 76 76 00:03:13,430 --> 00:03:16,294 Essentially, giving it a static address every single time, 77 77 00:03:16,294 --> 00:03:19,360 but using this automatic configuration. 78 78 00:03:19,360 --> 00:03:21,950 So, with DHCP, we can automate the process 79 79 00:03:21,950 --> 00:03:25,320 of configuring all of our devices whenever they come online. 80 80 00:03:25,320 --> 00:03:26,870 When the device joins our network, 81 81 00:03:26,870 --> 00:03:29,100 it's going to reach out to our DHCP server 82 82 00:03:29,100 --> 00:03:31,130 and it's going to do what's known as a discovery. 83 83 00:03:31,130 --> 00:03:33,650 It's going to say, hey, DHCP server, 84 84 00:03:33,650 --> 00:03:35,870 I need to discover an IP address. 85 85 00:03:35,870 --> 00:03:37,916 This means I need one to be assigned to me. 86 86 00:03:37,916 --> 00:03:40,630 The DHCP server then says, okay, 87 87 00:03:40,630 --> 00:03:42,220 does this address look okay to you? 88 88 00:03:42,220 --> 00:03:44,810 And it offers up an address from the scope. 89 89 00:03:44,810 --> 00:03:47,160 This is known as the offer phase. 90 90 00:03:47,160 --> 00:03:50,230 Then the computer's going to say, yeah, I like that address. 91 91 00:03:50,230 --> 00:03:51,760 I request to take it. 92 92 00:03:51,760 --> 00:03:54,000 And that's the DHCP request phase, 93 93 00:03:54,000 --> 00:03:56,560 which is going to be our third step in this process. 94 94 00:03:56,560 --> 00:03:58,386 Finally, our DHCP server is going to acknowledge 95 95 00:03:58,386 --> 00:04:01,110 that the IP is now being used by this client, 96 96 00:04:01,110 --> 00:04:03,210 by sending what's known as an acknowledgment 97 97 00:04:03,210 --> 00:04:05,430 and saying, okay, that's your address 98 98 00:04:05,430 --> 00:04:07,840 and you can borrow it for this amount of time. 99 99 00:04:07,840 --> 00:04:10,550 This is known as a DHCP lease. 100 100 00:04:10,550 --> 00:04:12,950 Now, when we look at this in our home networks, 101 101 00:04:12,950 --> 00:04:16,630 the default lease time is going to be about 24 hours usually. 102 102 00:04:16,630 --> 00:04:18,060 For a corporate network though, 103 103 00:04:18,060 --> 00:04:20,330 you may choose to use a longer lease time. 104 104 00:04:20,330 --> 00:04:22,700 Something like seven days or 30 days, 105 105 00:04:22,700 --> 00:04:24,590 depending on your use case. 106 106 00:04:24,590 --> 00:04:26,120 In the world of cybersecurity, 107 107 00:04:26,120 --> 00:04:28,760 having devices that are constantly changing their IPs, 108 108 00:04:28,760 --> 00:04:31,670 makes it harder for us to track down when bad things happen. 109 109 00:04:31,670 --> 00:04:36,010 So, in larger networks, we tend to use a longer lease time. 110 110 00:04:36,010 --> 00:04:39,620 Now, how are you going to remember these four steps of DHCP? 111 111 00:04:39,620 --> 00:04:42,520 Well, I have a little memory aid or mnemonic for this. 112 112 00:04:42,520 --> 00:04:44,830 I like to think of "Dora The Explorer". 113 113 00:04:44,830 --> 00:04:46,910 Dora is D-O-R-A, 114 114 00:04:46,910 --> 00:04:50,400 and it's discover, offer, request, and acknowledge. 115 115 00:04:50,400 --> 00:04:52,800 That is our four steps of configuring a device 116 116 00:04:52,800 --> 00:04:55,460 on a network using DHCP. 117 117 00:04:55,460 --> 00:04:57,320 Now, when the device gets a configuration 118 118 00:04:57,320 --> 00:04:58,700 from our DHCP server, 119 119 00:04:58,700 --> 00:05:01,130 it isn't just getting that IP address though. 120 120 00:05:01,130 --> 00:05:04,250 Instead, it's going to get four key pieces of information. 121 121 00:05:04,250 --> 00:05:06,870 This is the IP address, the subnet mask, 122 122 00:05:06,870 --> 00:05:07,990 the default gateway, 123 123 00:05:07,990 --> 00:05:09,880 which is the IP address of our router, 124 124 00:05:09,880 --> 00:05:12,400 and the IP address of the DNS server 125 125 00:05:12,400 --> 00:05:15,560 so your client knows how to do a DNS name lookup. 126 126 00:05:15,560 --> 00:05:17,420 Once your client has these four pieces 127 127 00:05:17,420 --> 00:05:20,780 of information configured, that client can now get online, 128 128 00:05:20,780 --> 00:05:23,220 get out of your network, and get onto the internet 129 129 00:05:23,220 --> 00:05:25,610 because it nows knows where it is on the network 130 130 00:05:25,610 --> 00:05:26,800 with its IP address, 131 131 00:05:26,800 --> 00:05:29,180 where the router is with that gateway address 132 132 00:05:29,180 --> 00:05:32,130 and how to convert the domain names to IP addresses 133 133 00:05:32,130 --> 00:05:34,610 using that DNS servers IP. 134 134 00:05:34,610 --> 00:05:36,690 Now, I mentioned the fact that we use DHCP 135 135 00:05:36,690 --> 00:05:39,130 to dynamically assign our configuration needed 136 136 00:05:39,130 --> 00:05:40,850 for our devices to get online. 137 137 00:05:40,850 --> 00:05:43,200 But we can also do this manually, 138 138 00:05:43,200 --> 00:05:45,470 which is known as a static assignment. 139 139 00:05:45,470 --> 00:05:47,310 You see, when we do it dynamically, 140 140 00:05:47,310 --> 00:05:50,520 we let DHCP servers do the configuration for us, 141 141 00:05:50,520 --> 00:05:53,240 but you can also statically assign the configuration 142 142 00:05:53,240 --> 00:05:55,640 if you provide all four pieces of information 143 143 00:05:55,640 --> 00:05:57,210 to your client device. 144 144 00:05:57,210 --> 00:05:59,160 Now again, in most large networks, 145 145 00:05:59,160 --> 00:06:01,550 you wouldn't want to statically assign the information, 146 146 00:06:01,550 --> 00:06:03,260 but instead you're going to want to use 147 147 00:06:03,260 --> 00:06:05,300 a DHCP reservation to do it. 148 148 00:06:05,300 --> 00:06:07,710 But in your home network or a small network, 149 149 00:06:07,710 --> 00:06:10,220 you may want to statically configure things. 150 150 00:06:10,220 --> 00:06:12,490 Now, if you need a statically configure a device, 151 151 00:06:12,490 --> 00:06:14,700 be sure you double check everything you've entered 152 152 00:06:14,700 --> 00:06:16,720 for all four pieces of information. 153 153 00:06:16,720 --> 00:06:18,730 The IP address, the subnet mask, 154 154 00:06:18,730 --> 00:06:21,545 the default gateway, and the DNS servers IP. 155 155 00:06:21,545 --> 00:06:22,766 If you're troubleshooting a device 156 156 00:06:22,766 --> 00:06:24,620 that's having connectivity problems, 157 157 00:06:24,620 --> 00:06:26,348 you should check if it was statically configured. 158 158 00:06:26,348 --> 00:06:29,350 And if it was, you need to verify the IP address 159 159 00:06:29,350 --> 00:06:31,740 and the subnet mask are configured correctly, 160 160 00:06:31,740 --> 00:06:34,160 because this is often the source of your problems. 161 161 00:06:34,160 --> 00:06:35,070 So, at this point, 162 162 00:06:35,070 --> 00:06:38,130 we've talked about dynamic assignment and static assignment. 163 163 00:06:38,130 --> 00:06:39,820 Now, when we use a dynamic assignment, 164 164 00:06:39,820 --> 00:06:43,130 what happens if a device can't reach the DHCP server 165 165 00:06:43,130 --> 00:06:45,607 or it fails to receive a proper configuration? 166 166 00:06:45,607 --> 00:06:47,370 What should that device do? 167 167 00:06:47,370 --> 00:06:48,970 Should it just keep asking? 168 168 00:06:48,970 --> 00:06:51,930 Well, no, because then it's just going to sit there forever 169 169 00:06:51,930 --> 00:06:52,970 getting hung up. 170 170 00:06:52,970 --> 00:06:56,184 So, instead, we need to have an alternate configuration. 171 171 00:06:56,184 --> 00:06:58,780 Whenever you're configuring DHCP, 172 172 00:06:58,780 --> 00:07:01,970 if DHCP is not successful and it's not able to negotiate 173 173 00:07:01,970 --> 00:07:04,830 its way through the door or process for whatever reason, 174 174 00:07:04,830 --> 00:07:06,939 it's going to default to it's alternate configuration 175 175 00:07:06,939 --> 00:07:08,920 that's set by the system administrator 176 176 00:07:08,920 --> 00:07:10,830 inside the operating system. 177 177 00:07:10,830 --> 00:07:12,920 By default, this is going to be set to use 178 178 00:07:12,920 --> 00:07:16,510 what's known as an APIPA address, A-P-I-P-A, 179 179 00:07:16,510 --> 00:07:19,590 which is Automatic Private IP Address. 180 180 00:07:19,590 --> 00:07:22,171 Now, as a network administrator or a system administrator, 181 181 00:07:22,171 --> 00:07:24,360 you can also configure your device to fall back 182 182 00:07:24,360 --> 00:07:26,800 to a known good static IP address 183 183 00:07:26,800 --> 00:07:30,250 as your alternate configuration instead if you prefer. 184 184 00:07:30,250 --> 00:07:32,520 Now, when you're configuring your DHCP server, 185 185 00:07:32,520 --> 00:07:35,339 one of the things you can configure is your scope options. 186 186 00:07:35,339 --> 00:07:38,210 This allows you to configure a bunch of different things, 187 187 00:07:38,210 --> 00:07:40,260 but the most common are the subnet mask 188 188 00:07:40,260 --> 00:07:42,130 that is going to be applied to all the devices 189 189 00:07:42,130 --> 00:07:43,770 requesting that configuration, 190 190 00:07:43,770 --> 00:07:46,531 the default router or gateway that these devices should use, 191 191 00:07:46,531 --> 00:07:50,210 and the DNS server to include the IP address configuration 192 192 00:07:50,210 --> 00:07:51,250 for those devices, 193 193 00:07:51,250 --> 00:07:53,880 as well as the least time for the IP address. 194 194 00:07:53,880 --> 00:07:55,630 Now, as you can see, we've already discussed 195 195 00:07:55,630 --> 00:07:57,580 all of these things in this lesson. 196 196 00:07:57,580 --> 00:07:58,980 But I wanted to bring this up 197 197 00:07:58,980 --> 00:08:01,166 so you're aware that you can change and configure them 198 198 00:08:01,166 --> 00:08:04,949 as part of your scope options inside your DHCP server too. 199 199 00:08:04,949 --> 00:08:07,037 Another unique configuration you need to make 200 200 00:08:07,037 --> 00:08:11,168 for DHCP or network is the use of a DHCP relay. 201 201 00:08:11,168 --> 00:08:15,110 Now, a DHCP relay is any host that forwards DHCP packets 202 202 00:08:15,110 --> 00:08:17,030 between clients and servers. 203 203 00:08:17,030 --> 00:08:20,080 Now, really the only time you're going to need a DHCP relay 204 204 00:08:20,080 --> 00:08:22,540 is when the client device and the DHCP server 205 205 00:08:22,540 --> 00:08:25,157 are not located on the same subnet or network. 206 206 00:08:25,157 --> 00:08:28,210 In this case, instead of installing a DHCP server 207 207 00:08:28,210 --> 00:08:31,340 on every subnet or mini network inside of your network, 208 208 00:08:31,340 --> 00:08:34,910 you can configure one device to act as the DHCP relay, 209 209 00:08:34,910 --> 00:08:37,670 and that way you can save yourself a lot of effort. 210 210 00:08:37,670 --> 00:08:39,840 This device will listen for discovery requests 211 211 00:08:39,840 --> 00:08:42,690 and then forward that request to the DHCP server 212 212 00:08:42,690 --> 00:08:45,110 on the other network on behalf of your client, 213 213 00:08:45,110 --> 00:08:47,450 acting essentially as a middleman. 214 214 00:08:47,450 --> 00:08:49,620 Another unique thing about DHCP 215 215 00:08:49,620 --> 00:08:53,650 is that it operates using the User Datagram Protocol or UDP. 216 216 00:08:53,650 --> 00:08:56,420 For this reason, it is fire and forget method 217 217 00:08:56,420 --> 00:08:57,810 of sending data. 218 218 00:08:57,810 --> 00:08:59,970 To help the data get to where it needs to go, 219 219 00:08:59,970 --> 00:09:01,060 you may need to configure 220 220 00:09:01,060 --> 00:09:04,290 what's known as an IP helper address on your router. 221 221 00:09:04,290 --> 00:09:05,883 The IP helper address is used to forward 222 222 00:09:05,883 --> 00:09:08,134 several different kinds of UDP broadcasts 223 223 00:09:08,134 --> 00:09:11,330 across the router and can be used in conjunction 224 224 00:09:11,330 --> 00:09:14,410 with the DHCP relay that we just talked about. 225 225 00:09:14,410 --> 00:09:16,407 Remember, if the DHCP client and server 226 226 00:09:16,407 --> 00:09:18,500 are on different network segments, 227 227 00:09:18,500 --> 00:09:20,580 the router and the client's network segment 228 228 00:09:20,580 --> 00:09:22,990 has to be configured with an IP helper address 229 229 00:09:22,990 --> 00:09:26,370 for DHCP to work properly and forward those requests over 230 230 00:09:26,370 --> 00:09:27,803 to the DHCP server.