1 1 00:00:00,230 --> 00:00:03,450 DNS, Domain Name System. 2 2 00:00:03,450 --> 00:00:05,230 In this lesson, we're going to talk about 3 3 00:00:05,230 --> 00:00:08,020 the Domain Name System, or DNS. 4 4 00:00:08,020 --> 00:00:10,810 The DNS protocol is used to help your network clients 5 5 00:00:10,810 --> 00:00:13,530 find a website using human-readable host names 6 6 00:00:13,530 --> 00:00:15,820 instead of numeric IP addresses. 7 7 00:00:15,820 --> 00:00:18,470 For example, if I told you to visit my website, 8 8 00:00:18,470 --> 00:00:21,860 I can simply say, "Hey, go over to diontraining.com," 9 9 00:00:21,860 --> 00:00:24,010 and that's a lot easier for you to remember 10 10 00:00:24,010 --> 00:00:29,010 than having to remember it's 66.123.45.237, 11 11 00:00:29,380 --> 00:00:32,790 or whatever the IP address of my web server happens to be. 12 12 00:00:32,790 --> 00:00:35,110 After all, saying all those numbers 13 13 00:00:35,110 --> 00:00:36,640 as part of a TV commercial, 14 14 00:00:36,640 --> 00:00:38,440 isn't quite as catchy or memorable 15 15 00:00:38,440 --> 00:00:40,980 as telling your consumer to visit diontraining.com, 16 16 00:00:40,980 --> 00:00:44,860 or coca-cola.com or microsoft.com, right? 17 17 00:00:44,860 --> 00:00:46,800 So, how does the computer know how to find 18 18 00:00:46,800 --> 00:00:48,380 the web server's IP address 19 19 00:00:48,380 --> 00:00:50,320 from these different domain names? 20 20 00:00:50,320 --> 00:00:54,820 Well, that's the purpose of DNS, the Domain Name System. 21 21 00:00:54,820 --> 00:00:57,620 The way DNS works is that the user's computer 22 22 00:00:57,620 --> 00:01:00,780 gets told to go to someplace like diontraining.com. 23 23 00:01:00,780 --> 00:01:02,917 And so, it reaches out to a DNS server and says, 24 24 00:01:02,917 --> 00:01:05,680 "Hey, who is diontraining.com?" 25 25 00:01:05,680 --> 00:01:07,807 And the DNS server is going to then reply back and say, 26 26 00:01:07,807 --> 00:01:10,250 "Oh, I know diontraining.com. 27 27 00:01:10,250 --> 00:01:13,030 Its IP addresses is 66 dot something, 28 28 00:01:13,030 --> 00:01:14,730 dot something, dot something." 29 29 00:01:14,730 --> 00:01:17,610 Then, the client gets redirected to a web server 30 30 00:01:17,610 --> 00:01:20,490 using their router, and their way in connection 31 31 00:01:20,490 --> 00:01:22,290 since they now know the right IP address 32 32 00:01:22,290 --> 00:01:23,920 to use as their destination. 33 33 00:01:23,920 --> 00:01:25,730 And all this happens in the background 34 34 00:01:25,730 --> 00:01:27,480 for your users and your computer, 35 35 00:01:27,480 --> 00:01:29,840 without anyone having to actually ask for it, 36 36 00:01:29,840 --> 00:01:31,940 because DNS is such an embedded part 37 37 00:01:31,940 --> 00:01:33,770 of our networks and systems. 38 38 00:01:33,770 --> 00:01:35,760 Now, most of us as home users, 39 39 00:01:35,760 --> 00:01:37,960 won't be running our own DNS servers, 40 40 00:01:37,960 --> 00:01:40,490 but instead, we rely on our internet service providers 41 41 00:01:40,490 --> 00:01:41,930 to do this for us. 42 42 00:01:41,930 --> 00:01:43,710 But if you're running your own websites 43 43 00:01:43,710 --> 00:01:45,420 or a large corporate network, 44 44 00:01:45,420 --> 00:01:48,750 you might also have your own DNS server inside your network 45 45 00:01:48,750 --> 00:01:50,260 and you'll be responsible for setting up 46 46 00:01:50,260 --> 00:01:53,300 your own DNS records that dictate what servers are located 47 47 00:01:53,300 --> 00:01:56,710 at what IP addresses and for what purposes. 48 48 00:01:56,710 --> 00:01:58,720 This allows you to run your own domain name 49 49 00:01:58,720 --> 00:01:59,990 and host resolution, 50 50 00:01:59,990 --> 00:02:03,230 which will convert those domain names to IP addresses. 51 51 00:02:03,230 --> 00:02:04,500 If you want to think of it like this, 52 52 00:02:04,500 --> 00:02:07,550 it's similar to having a contact list on your phone. 53 53 00:02:07,550 --> 00:02:10,470 Nowadays, how many phone numbers do you have memorized? 54 54 00:02:10,470 --> 00:02:12,230 Probably not a whole lot of 'em, right? 55 55 00:02:12,230 --> 00:02:14,610 Because normally you're going to pull out your smartphone, 56 56 00:02:14,610 --> 00:02:16,880 scroll to the person's name and hit their name 57 57 00:02:16,880 --> 00:02:19,470 with your finger and then the phone dials them up. 58 58 00:02:19,470 --> 00:02:22,590 For example, if I want to call my wife, I scroll to her name, 59 59 00:02:22,590 --> 00:02:24,600 I push a picture of her face on my phone 60 60 00:02:24,600 --> 00:02:27,710 and then immediately my phone dials her number. 61 61 00:02:27,710 --> 00:02:30,500 I don't have to memorize all 10 digits of her phone number 62 62 00:02:30,500 --> 00:02:32,510 because my phone does it for me. 63 63 00:02:32,510 --> 00:02:34,160 That's because as people, 64 64 00:02:34,160 --> 00:02:37,270 we have a harder time remembering numbers than we do names. 65 65 00:02:37,270 --> 00:02:39,380 And so we do this face or name 66 66 00:02:39,380 --> 00:02:42,110 to number conversion using our contact list. 67 67 00:02:42,110 --> 00:02:43,890 It's the same thing for computers 68 68 00:02:43,890 --> 00:02:46,330 except computers actually like numbers 69 69 00:02:46,330 --> 00:02:47,860 a lot better than names. 70 70 00:02:47,860 --> 00:02:49,940 So we want to convert the domain names 71 71 00:02:49,940 --> 00:02:52,520 that are easier for us into IP addresses 72 72 00:02:52,520 --> 00:02:55,310 that are easier for computers to do their routing with. 73 73 00:02:55,310 --> 00:02:57,440 And that's all DNS really does for us. 74 74 00:02:57,440 --> 00:03:00,640 It converts names to numbers and numbers to names. 75 75 00:03:00,640 --> 00:03:03,350 Now, one of the concepts with DNS that we need to talk about 76 76 00:03:03,350 --> 00:03:07,710 is what's known as a fully qualified domain name or FQDN. 77 77 00:03:07,710 --> 00:03:11,140 This is when a domain name is under a top-level provider. 78 78 00:03:11,140 --> 00:03:14,780 For example, the most common top-level of provider is .com, 79 79 00:03:14,780 --> 00:03:19,300 but we also have things like .mil, .edu, .org and .net. 80 80 00:03:19,300 --> 00:03:21,480 Let's use the example of Dion Training. 81 81 00:03:21,480 --> 00:03:24,340 At Dion Training, we have lots of different servers. 82 82 00:03:24,340 --> 00:03:26,300 One of our servers is our web server 83 83 00:03:26,300 --> 00:03:28,927 and it's located at www.diontraining.com. 84 84 00:03:30,860 --> 00:03:33,770 Now the top-level domain here is .com. 85 85 00:03:33,770 --> 00:03:36,530 The domain name that I use is Dion Training. 86 86 00:03:36,530 --> 00:03:40,770 To be fully qualified, I have to add the www in front of it. 87 87 00:03:40,770 --> 00:03:45,030 This makes it www.diontraining.com. 88 88 00:03:45,030 --> 00:03:46,890 Now, if you want to go to my web server, 89 89 00:03:46,890 --> 00:03:48,680 that's what you're going to type into your browser, 90 90 00:03:48,680 --> 00:03:51,670 www.diontraining.com. 91 91 00:03:51,670 --> 00:03:54,220 And now you're going to get redirected to my web server 92 92 00:03:54,220 --> 00:03:57,250 because DNS knows how it should revolve the IP address 93 93 00:03:57,250 --> 00:04:01,030 of my web server using that fully qualified domain name. 94 94 00:04:01,030 --> 00:04:03,410 Now, if you wanted to go to my file server instead, 95 95 00:04:03,410 --> 00:04:06,780 you're going to have to type in ftp.diontraining.com 96 96 00:04:06,780 --> 00:04:08,950 because that's the server I'm running there. 97 97 00:04:08,950 --> 00:04:10,400 If you want to go to my mail server, 98 98 00:04:10,400 --> 00:04:13,250 you might type mail.diontraining.com. 99 99 00:04:13,250 --> 00:04:14,750 All three of these are examples 100 100 00:04:14,750 --> 00:04:18,020 of fully qualified domain names or FQDNs. 101 101 00:04:18,020 --> 00:04:20,910 Essentially there's a service, a dot, 102 102 00:04:20,910 --> 00:04:24,330 a domain name, a dot, and a top-level domain. 103 103 00:04:24,330 --> 00:04:25,610 And this works the same way, 104 104 00:04:25,610 --> 00:04:27,140 no matter which domain you're looking at 105 105 00:04:27,140 --> 00:04:28,790 across the internet. 106 106 00:04:28,790 --> 00:04:31,940 Now, DNS is going to be set up as a hierarchy. 107 107 00:04:31,940 --> 00:04:34,180 This occurs at five different levels. 108 108 00:04:34,180 --> 00:04:36,860 We have the root level, the top-level domain, 109 109 00:04:36,860 --> 00:04:40,390 the second-level domains, subdomains, and the host. 110 110 00:04:40,390 --> 00:04:42,170 The root level is the highest level 111 111 00:04:42,170 --> 00:04:43,820 in the DNS hierarchy tree. 112 112 00:04:43,820 --> 00:04:46,050 And the root name server is going to answer requests 113 113 00:04:46,050 --> 00:04:47,640 in the root zone. 114 114 00:04:47,640 --> 00:04:49,450 These servers contain the global list 115 115 00:04:49,450 --> 00:04:53,797 of all the top-level domains, things like .com, .net, 116 116 00:04:53,797 --> 00:04:56,300 .org, .mil and others. 117 117 00:04:56,300 --> 00:04:59,800 The second level down is going to be the top-level domains. 118 118 00:04:59,800 --> 00:05:02,200 These are broken up into two categories, 119 119 00:05:02,200 --> 00:05:05,927 organizational hierarchies, such as .com, .net, 120 120 00:05:05,927 --> 00:05:09,100 .org and others, and then the geographical hierarchy 121 121 00:05:09,100 --> 00:05:13,477 such as .uk for the United kingdom, .fr for France, 122 122 00:05:13,477 --> 00:05:16,730 .it for Italy and other countries like that. 123 123 00:05:16,730 --> 00:05:19,820 The third level down is known as second-level domains. 124 124 00:05:19,820 --> 00:05:23,260 These domains sit directly below the top-level domain. 125 125 00:05:23,260 --> 00:05:26,050 For example, my domain, Dion Training 126 126 00:05:26,050 --> 00:05:30,600 is a second-level domain under the .com top-level domain. 127 127 00:05:30,600 --> 00:05:34,010 The .com sits underneath the root domain. 128 128 00:05:34,010 --> 00:05:36,400 This is the way these things level up together 129 129 00:05:36,400 --> 00:05:38,080 as part of the hierarchy. 130 130 00:05:38,080 --> 00:05:40,930 The fourth level down is known as the subdomain. 131 131 00:05:40,930 --> 00:05:42,840 So if I wanted to create a new server 132 132 00:05:42,840 --> 00:05:46,260 underneath my second level domain of diontraining.com, 133 133 00:05:46,260 --> 00:05:48,430 I can do that using a subdomain. 134 134 00:05:48,430 --> 00:05:50,720 In my case, I have a lot of different subdomains 135 135 00:05:50,720 --> 00:05:51,990 for different purposes. 136 136 00:05:51,990 --> 00:05:55,690 I have my main website hosted at the www subdomain. 137 137 00:05:55,690 --> 00:05:59,140 So you type in www.diontraining.com 138 138 00:05:59,140 --> 00:06:00,960 and that takes you to my web server, 139 139 00:06:00,960 --> 00:06:03,010 but I also have one called support. 140 140 00:06:03,010 --> 00:06:04,860 Now, the support subdomain is located 141 141 00:06:04,860 --> 00:06:06,260 at support.diontraining.com. 142 142 00:06:07,440 --> 00:06:10,700 I have another one for mail, mail.diontraining.com. 143 143 00:06:10,700 --> 00:06:12,760 These are all different subdomains. 144 144 00:06:12,760 --> 00:06:15,840 Now, the fifth and final level down is the host level. 145 145 00:06:15,840 --> 00:06:17,860 This is the lowest and the most detailed level 146 146 00:06:17,860 --> 00:06:21,390 inside of the DNS hierarchy and refers to a specific machine 147 147 00:06:21,390 --> 00:06:23,370 or server on the network. 148 148 00:06:23,370 --> 00:06:25,000 Now, when we think of DNS, though, 149 149 00:06:25,000 --> 00:06:26,740 most of us are just going to think of something 150 150 00:06:26,740 --> 00:06:28,700 like a fully qualified domain name, 151 151 00:06:28,700 --> 00:06:31,810 something like www.diontraining.com, 152 152 00:06:31,810 --> 00:06:33,170 which contains a subdomain, 153 153 00:06:33,170 --> 00:06:36,050 a second-level domain, and a top-level domain. 154 154 00:06:36,050 --> 00:06:38,060 Now, if I wanted to take it a step further, 155 155 00:06:38,060 --> 00:06:40,300 I can look at it from a URL perspective, 156 156 00:06:40,300 --> 00:06:43,240 which is known as a Uniform Resource Locator. 157 157 00:06:43,240 --> 00:06:46,070 Again, let's take my web server as our example here, 158 158 00:06:46,070 --> 00:06:48,630 www.diontraining.com. 159 159 00:06:48,630 --> 00:06:50,770 That's my fully qualified domain name, 160 160 00:06:50,770 --> 00:06:53,580 but it doesn't tell you how to access it. 161 161 00:06:53,580 --> 00:06:56,210 Do you want to do this securely or insecurely? 162 162 00:06:56,210 --> 00:06:59,740 Well, you're going to have to tell that by doing the URL. 163 163 00:06:59,740 --> 00:07:02,070 Well, if you want to give me your username and password, 164 164 00:07:02,070 --> 00:07:03,810 you should do this securely. 165 165 00:07:03,810 --> 00:07:07,780 So you're going to add the HTTPS colon slash slash 166 166 00:07:07,780 --> 00:07:11,020 in front of www.diontraining.com. 167 167 00:07:11,020 --> 00:07:14,740 And that becomes a URL, a Uniform Resource Locator, 168 168 00:07:14,740 --> 00:07:16,600 because it tells you how to access 169 169 00:07:16,600 --> 00:07:19,320 diontraining.com's web server. 170 170 00:07:19,320 --> 00:07:22,300 That's why we have that HTTPS at the beginning. 171 171 00:07:22,300 --> 00:07:25,140 It is the Hypertext Transfer Protocol Secure, 172 172 00:07:25,140 --> 00:07:27,290 and that's the method of accessing it. 173 173 00:07:27,290 --> 00:07:28,770 Now, if you wanted to access my website 174 174 00:07:28,770 --> 00:07:30,080 and do it insecurely, 175 175 00:07:30,080 --> 00:07:33,740 you could do that by adding HTTP colon slash slash 176 176 00:07:33,740 --> 00:07:35,520 to be the beginning of the web address. 177 177 00:07:35,520 --> 00:07:37,450 Now, if you want to connect using FTP, 178 178 00:07:37,450 --> 00:07:40,160 you would use FTP colon slash slash, 179 179 00:07:40,160 --> 00:07:43,360 ftp.diontraining.com as your URL. 180 180 00:07:43,360 --> 00:07:45,120 And so there's lots of different ways to do this 181 181 00:07:45,120 --> 00:07:46,960 as you tell the system what to do. 182 182 00:07:46,960 --> 00:07:48,580 And that's the difference between a URL 183 183 00:07:48,580 --> 00:07:50,770 and a fully qualified domain name. 184 184 00:07:50,770 --> 00:07:52,590 Next, we need to talk about the different types 185 185 00:07:52,590 --> 00:07:56,190 of DNS records that exist within a DNS server. 186 186 00:07:56,190 --> 00:07:57,640 Inside your DNS server, 187 187 00:07:57,640 --> 00:07:59,220 you're going to create different records 188 188 00:07:59,220 --> 00:08:01,290 that hold different types of information 189 189 00:08:01,290 --> 00:08:02,870 based on your use case. 190 190 00:08:02,870 --> 00:08:04,717 These are known as A records, 191 191 00:08:04,717 --> 00:08:07,590 AAAA records, CNAME records, 192 192 00:08:07,590 --> 00:08:12,590 MX records, SOA records, PTR records, TXT records, 193 193 00:08:12,900 --> 00:08:15,310 SRV records, and NS records. 194 194 00:08:15,310 --> 00:08:16,410 Let's take a quick look 195 195 00:08:16,410 --> 00:08:18,400 at each of these different record types. 196 196 00:08:18,400 --> 00:08:19,970 First, we have an A record 197 197 00:08:19,970 --> 00:08:22,000 which stands for an Address record, 198 198 00:08:22,000 --> 00:08:24,416 and A record is used to link a host name 199 199 00:08:24,416 --> 00:08:26,150 to an IPv4 address. 200 200 00:08:26,150 --> 00:08:30,720 For example, there's an A record for www.diontraining.com 201 201 00:08:30,720 --> 00:08:34,250 and it links it to the IP address of 45.79.184.180. 202 202 00:08:37,510 --> 00:08:39,180 In addition to this, you may also set 203 203 00:08:39,180 --> 00:08:41,080 an A record for the at host 204 204 00:08:41,080 --> 00:08:43,710 and this indicates a record for the root domain. 205 205 00:08:43,710 --> 00:08:45,180 In the example of Dion Training 206 206 00:08:45,180 --> 00:08:48,220 are at record signifies the diontraining.com, 207 207 00:08:48,220 --> 00:08:50,780 which is the root of our domain is going to be listed 208 208 00:08:50,780 --> 00:08:52,300 at a certain IP. 209 209 00:08:52,300 --> 00:08:54,380 This way, our users can find our website 210 210 00:08:54,380 --> 00:08:57,950 by going the subdomain of www.diontraining.com 211 211 00:08:57,950 --> 00:09:00,840 or just by going to diontraining.com 212 212 00:09:00,840 --> 00:09:03,610 by using that A at record. 213 213 00:09:03,610 --> 00:09:06,990 Now, A records only work for IPv4 addresses, 214 214 00:09:06,990 --> 00:09:11,190 but a lot of modern websites are also supporting IPv6. 215 215 00:09:11,190 --> 00:09:13,940 To map a domain name to an IPv6 address, 216 216 00:09:13,940 --> 00:09:16,660 you're going to use an AAAA record. 217 217 00:09:16,660 --> 00:09:19,720 So using the Dion Training site as the example, 218 218 00:09:19,720 --> 00:09:24,720 I could set the AAAA record to 2400:cb00:2049:1::a29f:1804 219 219 00:09:33,180 --> 00:09:36,520 if that was the IPv6 address for my web server. 220 220 00:09:36,520 --> 00:09:39,740 As you could see, IPv6 addresses are a lot more complicated 221 221 00:09:39,740 --> 00:09:43,100 than IPv4 and really hard for us humans to memorize, 222 222 00:09:43,100 --> 00:09:46,430 which is why we like to use these AAAA records 223 223 00:09:46,430 --> 00:09:48,360 or the four A records. 224 224 00:09:48,360 --> 00:09:50,010 The next type of record we can use 225 225 00:09:50,010 --> 00:09:53,420 is known as a CNAME record, or Canonical Name record. 226 226 00:09:53,420 --> 00:09:54,850 Now a CNAME record is used 227 227 00:09:54,850 --> 00:09:57,740 instead of an A record or an AAAA record 228 228 00:09:57,740 --> 00:10:00,140 if you want to point a domain to another domain name 229 229 00:10:00,140 --> 00:10:01,830 or a subdomain name. 230 230 00:10:01,830 --> 00:10:04,370 For example, I own a lot of different domain names 231 231 00:10:04,370 --> 00:10:06,920 out there in addition to diontraining.com. 232 232 00:10:06,920 --> 00:10:09,010 Some of these are for former companies I've had 233 233 00:10:09,010 --> 00:10:10,440 or projects I used to run, 234 234 00:10:10,440 --> 00:10:13,240 and some are ones I plan to use someday in the future. 235 235 00:10:13,240 --> 00:10:16,590 But in the meantime, I have them set with a CNAME record 236 236 00:10:16,590 --> 00:10:19,090 that resolves back to diontraining.com. 237 237 00:10:19,090 --> 00:10:21,773 For example, I used to run a website called itil4exam.com, 238 238 00:10:22,750 --> 00:10:25,360 but I've since stopped using that domain name 239 239 00:10:25,360 --> 00:10:27,220 and I've merged all those courses 240 240 00:10:27,220 --> 00:10:29,940 into my own diontraining.com website. 241 241 00:10:29,940 --> 00:10:32,210 So when I shut down that old web server, 242 242 00:10:32,210 --> 00:10:35,550 I still want to do allow people to type an itil4exam.com 243 243 00:10:35,550 --> 00:10:38,440 and reach something instead of just an error page. 244 244 00:10:38,440 --> 00:10:40,290 So I use the CNAME record 245 245 00:10:40,290 --> 00:10:42,980 to point it directly to diontraining.com. 246 246 00:10:42,980 --> 00:10:44,790 Now, why would I want to do that? 247 247 00:10:44,790 --> 00:10:46,980 Well, we were in the other site for a few years 248 248 00:10:46,980 --> 00:10:49,030 in parallel to diontraining.com 249 249 00:10:49,030 --> 00:10:51,850 and many of the links posted there are all over the internet 250 250 00:10:51,850 --> 00:10:54,180 with recommendations to our courses that were hosted 251 251 00:10:54,180 --> 00:10:56,910 on that itil4exam.com website. 252 252 00:10:56,910 --> 00:10:58,880 One of our most popular courses from that site 253 253 00:10:58,880 --> 00:11:00,750 was our ITIL 4 Foundation course, 254 254 00:11:00,750 --> 00:11:05,750 and it was located itil4exam.com/itil-4-foundation. 255 255 00:11:06,240 --> 00:11:08,460 Now, if you type that into your web browser, 256 256 00:11:08,460 --> 00:11:11,260 it will resolve the itil4exam.com part 257 257 00:11:11,260 --> 00:11:13,920 to diontraining.com and take you directly 258 258 00:11:13,920 --> 00:11:18,330 to diontraining.com/itil-4-foundation. 259 259 00:11:18,330 --> 00:11:20,280 This means that even if you're using a link 260 260 00:11:20,280 --> 00:11:22,290 that you found on Reddit that recommended our course 261 261 00:11:22,290 --> 00:11:24,570 from a few years ago, it's still going to work 262 262 00:11:24,570 --> 00:11:26,430 and take you to our sales page, 263 263 00:11:26,430 --> 00:11:29,180 even though the original itil4exam.com server 264 264 00:11:29,180 --> 00:11:32,750 no longer is even in service and has been taken offline. 265 265 00:11:32,750 --> 00:11:35,040 Another use case for using a CNAME record 266 266 00:11:35,040 --> 00:11:37,430 is when you're using a software as a service offering 267 267 00:11:37,430 --> 00:11:40,020 that provides you with a subdomain on their server. 268 268 00:11:40,020 --> 00:11:43,160 For example, I use a service desk ticket tracking software 269 269 00:11:43,160 --> 00:11:46,600 known as Freshdesk, which it comes at freshdesk.com 270 270 00:11:46,600 --> 00:11:48,300 and they gave us a rather difficult to remember 271 271 00:11:48,300 --> 00:11:49,760 subdomain for their service, 272 272 00:11:49,760 --> 00:11:53,960 something like fdus-143-d15.freshdesk.com. 273 273 00:11:55,830 --> 00:11:58,950 So to make it easier for my staff to find our support desk, 274 274 00:11:58,950 --> 00:12:01,630 we create a CNAME record called Support, 275 275 00:12:01,630 --> 00:12:04,560 and we pointed that to this difficult to remember subdomain. 276 276 00:12:04,560 --> 00:12:08,000 So if my staff enter support.diontraining.com, 277 277 00:12:08,000 --> 00:12:10,130 it takes them right to our support system, 278 278 00:12:10,130 --> 00:12:12,970 which is actually redirecting us into this cloud instance, 279 279 00:12:12,970 --> 00:12:16,260 that's provided by the subdomain issued by Freshdesk. 280 280 00:12:16,260 --> 00:12:18,980 Remember, CNAME records cannot be used 281 281 00:12:18,980 --> 00:12:20,510 to point to an IP address. 282 282 00:12:20,510 --> 00:12:23,120 It can only be used to point to another domain name 283 283 00:12:23,120 --> 00:12:24,730 or subdomain name. 284 284 00:12:24,730 --> 00:12:26,690 Next, we have an MX record, 285 285 00:12:26,690 --> 00:12:28,740 which stands for mail exchange record, 286 286 00:12:28,740 --> 00:12:30,460 and it does what you think it would. 287 287 00:12:30,460 --> 00:12:33,180 It helps you figure out where an email server is. 288 288 00:12:33,180 --> 00:12:36,950 An MX record is used to direct emails to your mail server. 289 289 00:12:36,950 --> 00:12:38,670 The MX record is going to be used to indicate 290 290 00:12:38,670 --> 00:12:41,580 how email messages should be routed when they're using 291 291 00:12:41,580 --> 00:12:46,470 the SMTP or Simple Mail Transfer Protocol over port 25. 292 292 00:12:46,470 --> 00:12:49,500 MX records like CNAME records can only be used 293 293 00:12:49,500 --> 00:12:52,870 to point to another domain and not an IP address. 294 294 00:12:52,870 --> 00:12:54,430 When you create your MX records, 295 295 00:12:54,430 --> 00:12:56,500 you're also going to be able to provide the priority 296 296 00:12:56,500 --> 00:12:58,110 for each of these records. 297 297 00:12:58,110 --> 00:13:00,970 This lets you indicate your preference for which server 298 298 00:13:00,970 --> 00:13:03,290 the email should try to use first. 299 299 00:13:03,290 --> 00:13:05,000 When it comes to setting the priority, 300 300 00:13:05,000 --> 00:13:06,590 the lower the number you enter, 301 301 00:13:06,590 --> 00:13:08,220 the higher the priority it is. 302 302 00:13:08,220 --> 00:13:10,330 It's essentially golf rules. 303 303 00:13:10,330 --> 00:13:13,680 So if we have mail1.diontraining.com set at 10 304 304 00:13:13,680 --> 00:13:17,210 and mail2.diontraining.com set at 20 for their priorities, 305 305 00:13:17,210 --> 00:13:20,270 the emails are going to attempt to use mail one first. 306 306 00:13:20,270 --> 00:13:21,770 If it fails to reach mail one, 307 307 00:13:21,770 --> 00:13:23,950 then it will try to reach mail two. 308 308 00:13:23,950 --> 00:13:26,040 Now, if you want to load balance your email 309 309 00:13:26,040 --> 00:13:27,730 across multiple servers, 310 310 00:13:27,730 --> 00:13:30,840 you simply need to set their priorities to the same value. 311 311 00:13:30,840 --> 00:13:33,740 So if I created records for mail one and mail two, 312 312 00:13:33,740 --> 00:13:35,860 and they both had a priority of 10, 313 313 00:13:35,860 --> 00:13:38,060 all incoming emails are going to alternate 314 314 00:13:38,060 --> 00:13:40,340 and load balanced equally between these servers. 315 315 00:13:40,340 --> 00:13:41,600 The first one goes to one. 316 316 00:13:41,600 --> 00:13:44,130 The second one goes to two. The third one goes to one. 317 317 00:13:44,130 --> 00:13:46,660 The fourth one goes to two, and so on. 318 318 00:13:46,660 --> 00:13:48,610 Next, we have an SOA record, 319 319 00:13:48,610 --> 00:13:50,810 which stands for Start of Authority. 320 320 00:13:50,810 --> 00:13:52,700 Now, a Start of Authority record is used 321 321 00:13:52,700 --> 00:13:56,060 to store important information about a domain or a zone. 322 322 00:13:56,060 --> 00:13:57,560 This information can include things 323 323 00:13:57,560 --> 00:13:59,650 like when the domain was last updated, 324 324 00:13:59,650 --> 00:14:01,910 the email address of the administrator of the domain, 325 325 00:14:01,910 --> 00:14:03,860 or even how long the server should wait 326 326 00:14:03,860 --> 00:14:05,410 prior to sending an update for all 327 327 00:14:05,410 --> 00:14:08,050 of its DNS records to other zones. 328 328 00:14:08,050 --> 00:14:10,870 These Start of Authority records are critically important 329 329 00:14:10,870 --> 00:14:12,750 whenever a DNS server is attempting 330 330 00:14:12,750 --> 00:14:14,770 to conduct a zone transfer. 331 331 00:14:14,770 --> 00:14:17,770 Now, you may be wondering what is a zone transfer? 332 332 00:14:17,770 --> 00:14:20,730 Well, a DNS zone transfer is simply the process 333 333 00:14:20,730 --> 00:14:23,570 of sending your DNS records from the primary name server 334 334 00:14:23,570 --> 00:14:25,320 to a secondary name server. 335 335 00:14:25,320 --> 00:14:28,140 When this occurs, the first record that's transferred over 336 336 00:14:28,140 --> 00:14:31,000 is the SOA record, that Start of Authority. 337 337 00:14:31,000 --> 00:14:33,150 And this is used by the secondary server 338 338 00:14:33,150 --> 00:14:35,560 to see if the other records need to be updated 339 339 00:14:35,560 --> 00:14:37,230 because the Start of Authority record 340 340 00:14:37,230 --> 00:14:40,250 contains a serial number that acts as a type of versioning. 341 341 00:14:40,250 --> 00:14:41,640 When a zone transfer occurs, 342 342 00:14:41,640 --> 00:14:45,180 this is going to use the TCP protocol to do the data transfer. 343 343 00:14:45,180 --> 00:14:47,920 This ensures the data's going to be sent over successfully 344 344 00:14:47,920 --> 00:14:50,350 from the primary server to the secondary server 345 345 00:14:50,350 --> 00:14:52,970 and verify it has been received there. 346 346 00:14:52,970 --> 00:14:56,430 Next, we have a PTR record or a Pointer record. 347 347 00:14:56,430 --> 00:14:58,230 Now, a Pointer record is used to correlate 348 348 00:14:58,230 --> 00:15:00,300 an IP address with a domain name. 349 349 00:15:00,300 --> 00:15:03,740 Essentially a Pointer record is the opposite of an A record. 350 350 00:15:03,740 --> 00:15:06,030 These Pointer records are used to conduct what is known 351 351 00:15:06,030 --> 00:15:07,970 as a reverse DNS lookup. 352 352 00:15:07,970 --> 00:15:10,870 So if a user is trying to determine if a domain name 353 353 00:15:10,870 --> 00:15:12,960 is going to be used for a given IP, 354 354 00:15:12,960 --> 00:15:15,430 that query can be made against the Pointer records 355 355 00:15:15,430 --> 00:15:17,200 instead of the A records. 356 356 00:15:17,200 --> 00:15:20,780 This lets you go from an IP address to a domain name. 357 357 00:15:20,780 --> 00:15:23,050 Now, you may be wondering why do I even need 358 358 00:15:23,050 --> 00:15:25,890 a Pointer record if I already have an A record? 359 359 00:15:25,890 --> 00:15:28,410 Well, honestly, you don't always need them, 360 360 00:15:28,410 --> 00:15:30,580 but they can be helpful if you're trying to prove 361 361 00:15:30,580 --> 00:15:32,850 your domain is not associated with spam, 362 362 00:15:32,850 --> 00:15:35,360 if you're trying to troubleshoot an email delivery issue, 363 363 00:15:35,360 --> 00:15:36,930 or if you're trying to help create 364 364 00:15:36,930 --> 00:15:40,180 a better logging environment by converting your IP addresses 365 365 00:15:40,180 --> 00:15:42,000 back into domain names. 366 366 00:15:42,000 --> 00:15:44,810 Also, when you store an IP address in a Pointer record, 367 367 00:15:44,810 --> 00:15:49,780 it's going to be reversed and in-addr.arpa 368 368 00:15:49,780 --> 00:15:51,550 is added to it at the end. 369 369 00:15:51,550 --> 00:15:55,053 For example, if your IP was 66.55.44.33, 370 370 00:15:57,060 --> 00:15:59,000 and you're storing it in a Pointer record, 371 371 00:15:59,000 --> 00:16:03,197 it's actually going to be stored as 33.44.55.66.in-addr.arpa. 372 372 00:16:08,300 --> 00:16:10,100 Now, you might be wondering 373 373 00:16:10,100 --> 00:16:13,650 why is the top-level domain .arpa being used here, 374 374 00:16:13,650 --> 00:16:15,610 and what the heck does that mean? 375 375 00:16:15,610 --> 00:16:18,220 Well, the internet started out as the ARPANET, 376 376 00:16:18,220 --> 00:16:19,830 which stands for the Advanced Research 377 377 00:16:19,830 --> 00:16:21,550 Projects Agency Network. 378 378 00:16:21,550 --> 00:16:24,230 This was the first wide area packet switch network, 379 379 00:16:24,230 --> 00:16:26,220 and it was developed by the US military 380 380 00:16:26,220 --> 00:16:28,550 to connect a bunch of military defense researchers 381 381 00:16:28,550 --> 00:16:31,610 at universities located all over America. 382 382 00:16:31,610 --> 00:16:34,150 Now, the ARPA top-level domain was actually created 383 383 00:16:34,150 --> 00:16:36,690 as the first top-level domain that was defined 384 384 00:16:36,690 --> 00:16:40,370 for what would become the internet later on and as such, 385 385 00:16:40,370 --> 00:16:43,240 it was used for managing network infrastructure. 386 386 00:16:43,240 --> 00:16:47,090 This is why you see the .arpa as the top-level domain 387 387 00:16:47,090 --> 00:16:48,860 use by all Pointer records. 388 388 00:16:48,860 --> 00:16:51,310 It's just a relic of history and it's still there 389 389 00:16:51,310 --> 00:16:54,090 because we built the internet on top of ARPANET. 390 390 00:16:54,090 --> 00:16:57,020 For the exam, you don't need to know about ARPA 391 391 00:16:57,020 --> 00:16:59,480 or this internet history that I just talked about. 392 392 00:16:59,480 --> 00:17:00,860 It's just something that I have a lot of students 393 393 00:17:00,860 --> 00:17:04,480 ask me about, why do Pointer records end .arpa? 394 394 00:17:04,480 --> 00:17:05,770 And that's the reason. 395 395 00:17:05,770 --> 00:17:08,210 So when you conduct this type of lookup, 396 396 00:17:08,210 --> 00:17:09,790 you're trying to determine the host name 397 397 00:17:09,790 --> 00:17:11,340 based on a given IP. 398 398 00:17:11,340 --> 00:17:13,840 And this process is known as reverse DNS 399 399 00:17:13,840 --> 00:17:15,680 or a reverse lookup. 400 400 00:17:15,680 --> 00:17:19,000 Normally, when you use DNS, you do a forward lookup, 401 401 00:17:19,000 --> 00:17:22,270 which goes from a domain name to an IP address. 402 402 00:17:22,270 --> 00:17:25,320 Instead we're going from an IP address to a domain name 403 403 00:17:25,320 --> 00:17:27,400 and that's a reverse lookup. 404 404 00:17:27,400 --> 00:17:30,510 Next, we have TXT records or text records. 405 405 00:17:30,510 --> 00:17:32,910 Now, a text record is used by domain administrators 406 406 00:17:32,910 --> 00:17:36,610 to add texts into the Domain Name System or DNS. 407 407 00:17:36,610 --> 00:17:38,610 Originally, text records were designed 408 408 00:17:38,610 --> 00:17:40,750 as a way for us to add human-readable notes 409 409 00:17:40,750 --> 00:17:43,670 into our DNS records and over time, 410 410 00:17:43,670 --> 00:17:45,650 these things had more and more things 411 411 00:17:45,650 --> 00:17:47,660 starting to get added to these text records. 412 412 00:17:47,660 --> 00:17:50,800 And eventually, we started adding machine-readable data 413 413 00:17:50,800 --> 00:17:52,510 into these text records as well. 414 414 00:17:52,510 --> 00:17:54,790 And that's what you're going to see mostly these days. 415 415 00:17:54,790 --> 00:17:56,860 Your domain can have a lot of different text records 416 416 00:17:56,860 --> 00:17:59,440 as well, you're not limited to just one of them. 417 417 00:17:59,440 --> 00:18:01,620 Most of the time, you're going to see text records 418 418 00:18:01,620 --> 00:18:03,810 used to prove domain ownership through adding 419 419 00:18:03,810 --> 00:18:06,270 some machine-readable code for verification 420 420 00:18:06,270 --> 00:18:08,340 and to provide email spam prevention 421 421 00:18:08,340 --> 00:18:11,020 again, by adding specific machine readable code 422 422 00:18:11,020 --> 00:18:12,780 to a TXT record. 423 423 00:18:12,780 --> 00:18:15,130 For example, at diontraining.com, 424 424 00:18:15,130 --> 00:18:18,730 we have a text record with the name fdkey.support 425 425 00:18:18,730 --> 00:18:21,640 and a text string of 32 hexadecimal digits 426 426 00:18:21,640 --> 00:18:23,770 inside of our DNS records. 427 427 00:18:23,770 --> 00:18:26,800 This allows our support system Freshdesk to verify 428 428 00:18:26,800 --> 00:18:29,480 that we own the domain name, diontraining.com. 429 429 00:18:29,480 --> 00:18:31,940 So they're authorized to send out emails on our behalf 430 430 00:18:31,940 --> 00:18:33,960 to our students when our team replies 431 431 00:18:33,960 --> 00:18:36,660 inside their system to a support ticket. 432 432 00:18:36,660 --> 00:18:39,460 This is a form of domain ownership verification, 433 433 00:18:39,460 --> 00:18:41,970 because their system can query our DNS records 434 434 00:18:41,970 --> 00:18:43,890 and see that we entered this unique series 435 435 00:18:43,890 --> 00:18:48,180 of 32 hexadecimal digits into our DNS TXT record. 436 436 00:18:48,180 --> 00:18:50,187 Essentially, it's working like a password to say, 437 437 00:18:50,187 --> 00:18:52,690 "Hey, I own this domain." 438 438 00:18:52,690 --> 00:18:56,550 Next, let's talk about SRV records or Service records. 439 439 00:18:56,550 --> 00:18:59,430 Now, a Service record is used to specify a host and a port 440 440 00:18:59,430 --> 00:19:02,530 for a specific service such as voiceover IP, 441 441 00:19:02,530 --> 00:19:05,290 instant messaging or other services like that. 442 442 00:19:05,290 --> 00:19:09,270 Remember, we normally use an A record or an AAAA record 443 443 00:19:09,270 --> 00:19:11,220 when we're just associating a domain name 444 444 00:19:11,220 --> 00:19:14,900 with an IP address, either with IPv4 for an A record 445 445 00:19:14,900 --> 00:19:17,860 or IPv6 with an AAAA record. 446 446 00:19:17,860 --> 00:19:20,560 Now, we can use a CNAME record if we want to associate 447 447 00:19:20,560 --> 00:19:23,770 a domain name with a subdomain or another domain name, 448 448 00:19:23,770 --> 00:19:27,140 but none of these allowed us to specify a port, 449 449 00:19:27,140 --> 00:19:28,910 but with a Service record, 450 450 00:19:28,910 --> 00:19:32,160 we can specify a port along with our IP address. 451 451 00:19:32,160 --> 00:19:35,520 For example, if I wanted to run an XMPP chat server 452 452 00:19:35,520 --> 00:19:37,970 and link it to port 5223, 453 453 00:19:37,970 --> 00:19:41,690 I could do that with a Service record that looks like this, 454 454 00:19:41,690 --> 00:19:46,690 _xmpp._tcp.diontraining.com. Space 86400 space IN space SRV 455 455 00:19:52,560 --> 00:19:57,560 space 10 space 5 space 5223 space chat.diontraining.com. 456 456 00:19:58,960 --> 00:20:00,400 Whew! That's a mouthful. 457 457 00:20:00,400 --> 00:20:05,240 Now, that is a single SRV record in a DNS server. 458 458 00:20:05,240 --> 00:20:07,930 This would say that I'm using the XMPP service 459 459 00:20:07,930 --> 00:20:09,790 with the TCP protocol. 460 460 00:20:09,790 --> 00:20:12,360 The time to live for the record is going to be updated 461 461 00:20:12,360 --> 00:20:16,650 every 86,400 seconds, which is 24 hours. 462 462 00:20:16,650 --> 00:20:19,930 The class is set to IN, which stands for Internet 463 463 00:20:19,930 --> 00:20:22,580 and it's an SRV or Service record. 464 464 00:20:22,580 --> 00:20:25,290 The priority is 10 and the weight is five. 465 465 00:20:25,290 --> 00:20:27,530 These two settings are used for prioritization 466 466 00:20:27,530 --> 00:20:30,310 and load balancing across multiple servers. 467 467 00:20:30,310 --> 00:20:32,440 Finally, we see the port number 5223, 468 468 00:20:33,310 --> 00:20:37,150 and the target server of chat.diontraining.com. 469 469 00:20:37,150 --> 00:20:39,060 This is the port and server we're mapping 470 470 00:20:39,060 --> 00:20:43,250 the XMPP service to within this particular Service record. 471 471 00:20:43,250 --> 00:20:45,370 Finally, we have an NS record 472 472 00:20:45,370 --> 00:20:47,650 which stands for name server record. 473 473 00:20:47,650 --> 00:20:49,760 Now, a name server record is used to indicate 474 474 00:20:49,760 --> 00:20:51,960 which DNS name server in the world 475 475 00:20:51,960 --> 00:20:55,220 is going be the authoritative one for that domain. 476 476 00:20:55,220 --> 00:20:58,190 This is important because DNS uses that hierarchal model 477 477 00:20:58,190 --> 00:21:00,550 we talked about and all the servers need to know 478 478 00:21:00,550 --> 00:21:02,900 who owns that record and is authorized 479 479 00:21:02,900 --> 00:21:04,550 to make the changes to it. 480 480 00:21:04,550 --> 00:21:07,330 Now, a name server is a type of DNS server 481 481 00:21:07,330 --> 00:21:10,490 that stores all the DNS records for a given domain, 482 482 00:21:10,490 --> 00:21:12,850 including all the types we've already discussed, 483 483 00:21:12,850 --> 00:21:16,950 like A record, AAAA records, Canonical Name records, 484 484 00:21:16,950 --> 00:21:20,630 MX, mail exchange records, TXT, text records, 485 485 00:21:20,630 --> 00:21:24,340 PTR, Pointer records, and SRV, Service records. 486 486 00:21:24,340 --> 00:21:26,340 There are often more than one name server 487 487 00:21:26,340 --> 00:21:28,600 for domain as well, so you can have a primary 488 488 00:21:28,600 --> 00:21:30,520 and a backup name server. 489 489 00:21:30,520 --> 00:21:32,310 Additionally, you don't always have to host 490 490 00:21:32,310 --> 00:21:33,500 your own name servers. 491 491 00:21:33,500 --> 00:21:35,250 In the case of diontraining.com, 492 492 00:21:35,250 --> 00:21:36,960 we don't host our own name servers, 493 493 00:21:36,960 --> 00:21:39,120 instead we rely on Cloudflare, 494 494 00:21:39,120 --> 00:21:42,080 which is a cloud service provider that does this for us. 495 495 00:21:42,080 --> 00:21:45,440 So if you look up the DNS records for diontraining.com, 496 496 00:21:45,440 --> 00:21:46,990 the authoritative source for that 497 497 00:21:46,990 --> 00:21:49,530 is to of Cloudflare's name servers. 498 498 00:21:49,530 --> 00:21:52,020 Now, up to this point, I've talked about DNS 499 499 00:21:52,020 --> 00:21:54,310 from a perspective of hosting a publicly available 500 500 00:21:54,310 --> 00:21:57,280 DNS server that anyone in the world can access, 501 501 00:21:57,280 --> 00:22:01,160 but DNS can actually be used internally or externally. 502 502 00:22:01,160 --> 00:22:02,900 Everything I've talked about so far 503 503 00:22:02,900 --> 00:22:04,610 is talking about externally. 504 504 00:22:04,610 --> 00:22:07,490 But let's talk a little bit about internally. 505 505 00:22:07,490 --> 00:22:09,200 These days with cloud computing, 506 506 00:22:09,200 --> 00:22:12,640 it is very common to set up an internal DNS servers as well, 507 507 00:22:12,640 --> 00:22:15,380 that lets your cloud instances within the same network 508 508 00:22:15,380 --> 00:22:19,290 or private cloud access each other using internal DNS names, 509 509 00:22:19,290 --> 00:22:22,000 instead of having to use their IP addresses. 510 510 00:22:22,000 --> 00:22:24,020 To do this internal A records are created, 511 511 00:22:24,020 --> 00:22:26,210 an internal Pointer records are also created 512 512 00:22:26,210 --> 00:22:27,780 in the reverse zone. 513 513 00:22:27,780 --> 00:22:31,040 Luckily, most cloud providers will automatically create, 514 514 00:22:31,040 --> 00:22:33,870 update, and remove these internal DNS records for you 515 515 00:22:33,870 --> 00:22:36,250 as you create and remove different virtual machines 516 516 00:22:36,250 --> 00:22:38,950 and other instances in your private cloud. 517 517 00:22:38,950 --> 00:22:40,970 External DNS is what most of us 518 518 00:22:40,970 --> 00:22:42,740 are going to be more familiar with though. 519 519 00:22:42,740 --> 00:22:44,360 These are the records that are created around 520 520 00:22:44,360 --> 00:22:47,010 the domain names that we purchase from a central authority 521 521 00:22:47,010 --> 00:22:49,180 and that we use on the public internet. 522 522 00:22:49,180 --> 00:22:50,760 Now, for each DNS record, 523 523 00:22:50,760 --> 00:22:54,360 we also have what's known as a TTL or time to live 524 524 00:22:54,360 --> 00:22:55,870 that's associated with it. 525 525 00:22:55,870 --> 00:22:58,150 Now, inside of each of our DNS records, 526 526 00:22:58,150 --> 00:23:00,590 whether internal or external, we're going to create 527 527 00:23:00,590 --> 00:23:03,320 what's known as a TTL or time to live 528 528 00:23:03,320 --> 00:23:04,860 that's associated with it. 529 529 00:23:04,860 --> 00:23:07,830 The time to live is a setting that tells the DNS resolver 530 530 00:23:07,830 --> 00:23:11,330 how long it can cache a query before requesting a new one. 531 531 00:23:11,330 --> 00:23:14,210 So if my DNS records are set with a time to live 532 532 00:23:14,210 --> 00:23:18,160 of 86,400 seconds, which is usually the default, 533 533 00:23:18,160 --> 00:23:21,180 that means my computer will resolve that DNS record 534 534 00:23:21,180 --> 00:23:23,510 and it will remember it for 24 hours 535 535 00:23:23,510 --> 00:23:25,870 before it has to go back out to the DNS server 536 536 00:23:25,870 --> 00:23:27,950 and ask for that information again. 537 537 00:23:27,950 --> 00:23:31,010 This DNS resolver also known as a DNS cache 538 538 00:23:31,010 --> 00:23:33,210 is located on your individual hosts. 539 539 00:23:33,210 --> 00:23:35,670 So if you're running windows 10, for example, 540 540 00:23:35,670 --> 00:23:38,830 your computer is making a local copy of every DNS entry 541 541 00:23:38,830 --> 00:23:41,100 it resolves as you connect to websites 542 542 00:23:41,100 --> 00:23:42,480 all over the internet. 543 543 00:23:42,480 --> 00:23:44,730 This temporary database remembers the answers 544 544 00:23:44,730 --> 00:23:46,750 that received from the DNS server. 545 545 00:23:46,750 --> 00:23:48,780 So if you go to diontraining.com, 546 546 00:23:48,780 --> 00:23:50,420 the first time you do that today, 547 547 00:23:50,420 --> 00:23:53,500 your computer has to ask the DNS server for that IP address, 548 548 00:23:53,500 --> 00:23:55,350 but now it knows where it is, 549 549 00:23:55,350 --> 00:23:58,160 and it remembers that IP for the next 24 hours. 550 550 00:23:58,160 --> 00:24:00,400 If you visit my website five times today, 551 551 00:24:00,400 --> 00:24:02,910 you only had to look up that IP address once. 552 552 00:24:02,910 --> 00:24:05,450 This helps speed up the entire process for us. 553 553 00:24:05,450 --> 00:24:06,820 Now, if you try again tomorrow, 554 554 00:24:06,820 --> 00:24:09,380 your computer's going to first check it's DNS cache, 555 555 00:24:09,380 --> 00:24:11,080 and it's going to see there's a record there. 556 556 00:24:11,080 --> 00:24:13,320 But if that time to live is already passed, 557 557 00:24:13,320 --> 00:24:14,840 it's going to invalidate that record 558 558 00:24:14,840 --> 00:24:16,710 and perform another lookup. 559 559 00:24:16,710 --> 00:24:18,080 The final thing we need to cover 560 560 00:24:18,080 --> 00:24:20,470 is the concept of a recursive lookup. 561 561 00:24:20,470 --> 00:24:22,830 You see, when your computer wants to find a given website 562 562 00:24:22,830 --> 00:24:24,350 like diontraining.com, 563 563 00:24:24,350 --> 00:24:27,610 it first has to ask its DNS server where it's located. 564 564 00:24:27,610 --> 00:24:29,010 So if you're sitting at home 565 565 00:24:29,010 --> 00:24:31,600 and using a Verizon Fios connection, for example, 566 566 00:24:31,600 --> 00:24:34,393 you're going to ask their DNS server, who's diontraining.com? 567 567 00:24:35,300 --> 00:24:38,530 Now, Verizon's DNS may or may not know the IP address 568 568 00:24:38,530 --> 00:24:40,030 for diontraining.com. 569 569 00:24:40,030 --> 00:24:41,990 After all, there are millions and millions 570 570 00:24:41,990 --> 00:24:43,160 of websites out there. 571 571 00:24:43,160 --> 00:24:45,880 If they had to rethink our records every 24 hours, 572 572 00:24:45,880 --> 00:24:47,700 that would take a long time. 573 573 00:24:47,700 --> 00:24:51,150 So instead DNS uses this recursive strategy 574 574 00:24:51,150 --> 00:24:52,380 to perform the lookup. 575 575 00:24:52,380 --> 00:24:55,640 So you ask Verizon, and if they don't know the answer, 576 576 00:24:55,640 --> 00:24:58,590 they're going to go up a level and ask the next DNS server. 577 577 00:24:58,590 --> 00:25:00,010 If that server doesn't know the answer, 578 578 00:25:00,010 --> 00:25:01,290 it will go up another level 579 579 00:25:01,290 --> 00:25:04,110 and we'll keep this process going until it finds somebody 580 580 00:25:04,110 --> 00:25:07,180 who knows the IP address of diontraining.com. 581 581 00:25:07,180 --> 00:25:08,550 Now, if during this recursion, 582 582 00:25:08,550 --> 00:25:11,200 it reaches all the way up to the .com or the root domain 583 583 00:25:11,200 --> 00:25:14,960 for diontraining.com, then it can ask the .com root server, 584 584 00:25:14,960 --> 00:25:18,330 which DNS server is authoritative for diontraining.com 585 585 00:25:18,330 --> 00:25:21,130 and get the authoritative answer directly from them. 586 586 00:25:21,130 --> 00:25:22,880 Essentially with a recursive lookup, 587 587 00:25:22,880 --> 00:25:24,577 your DNS resolver saying, 588 588 00:25:24,577 --> 00:25:26,660 "I don't know what this domain's IP is, 589 589 00:25:26,660 --> 00:25:28,600 but I'm going to ask my DNS server 590 590 00:25:28,600 --> 00:25:31,060 and that server will hunt it down until it finds it. 591 591 00:25:31,060 --> 00:25:32,930 And then tell me that IP." 592 592 00:25:32,930 --> 00:25:34,550 Now, there's another method that can be used, 593 593 00:25:34,550 --> 00:25:36,410 which is known as an iterative lookup. 594 594 00:25:36,410 --> 00:25:39,320 With an iterative lookup, it's similar to recursive lookup, 595 595 00:25:39,320 --> 00:25:41,770 except that the DNS server is not going to continue 596 596 00:25:41,770 --> 00:25:44,620 to look up the information for you and send you the result 597 597 00:25:44,620 --> 00:25:46,520 instead in an iterative lookup, 598 598 00:25:46,520 --> 00:25:49,350 your DNS resolver is going to ask the DNS server 599 599 00:25:49,350 --> 00:25:51,100 what the IP for the domain is. 600 600 00:25:51,100 --> 00:25:53,160 And if the DNS server doesn't know, 601 601 00:25:53,160 --> 00:25:56,150 it's going to tell your resolver to ask the next DNS server, 602 602 00:25:56,150 --> 00:25:58,620 and that server will provide its IP. 603 603 00:25:58,620 --> 00:26:00,580 Now, with the recursive lookup, 604 604 00:26:00,580 --> 00:26:01,870 the DNS server would hunt it down 605 605 00:26:01,870 --> 00:26:03,470 and report back to resolver. 606 606 00:26:03,470 --> 00:26:05,180 But with an iterative lookup, 607 607 00:26:05,180 --> 00:26:08,320 your DNS resolver is going to continually doing this query 608 608 00:26:08,320 --> 00:26:10,150 all the way up through this recursion 609 609 00:26:10,150 --> 00:26:13,110 until it finds the one with the IP for that domain. 610 610 00:26:13,110 --> 00:26:15,130 So it really is just a matter of who's doing 611 611 00:26:15,130 --> 00:26:17,070 the hunting for this information. 612 612 00:26:17,070 --> 00:26:17,903 All right. 613 613 00:26:17,903 --> 00:26:20,320 We have covered a lot of information in this lesson. 614 614 00:26:20,320 --> 00:26:22,020 So as a quick summary, 615 615 00:26:22,020 --> 00:26:24,090 what do you need to know for the exam? 616 616 00:26:24,090 --> 00:26:26,650 Well, you need to understand how DNS works 617 617 00:26:26,650 --> 00:26:29,490 by using its various record types to convert domain names 618 618 00:26:29,490 --> 00:26:32,650 to IP addresses and IP addresses to domain names. 619 619 00:26:32,650 --> 00:26:33,970 You should remember that A records 620 620 00:26:33,970 --> 00:26:36,860 are used for domain names and IPv4 addresses, 621 621 00:26:36,860 --> 00:26:39,660 while AAAA records are used for domain names 622 622 00:26:39,660 --> 00:26:41,610 with IPv6 addresses. 623 623 00:26:41,610 --> 00:26:43,840 CNAME records are used to map domain names 624 624 00:26:43,840 --> 00:26:45,520 to other domain names. 625 625 00:26:45,520 --> 00:26:47,250 MX records are used for email 626 626 00:26:47,250 --> 00:26:49,690 and NS records are used for name servers, 627 627 00:26:49,690 --> 00:26:52,500 TXT records store texts, either as human-readable 628 628 00:26:52,500 --> 00:26:54,170 or machine-readable data. 629 629 00:26:54,170 --> 00:26:56,500 Pointer records are used to match up an IP address 630 630 00:26:56,500 --> 00:26:58,950 with a domain name for reverse DNS lookups 631 631 00:26:58,950 --> 00:27:02,350 and Service records are used to map both a port and an IP 632 632 00:27:02,350 --> 00:27:04,360 to a domain name for a service. 633 633 00:27:04,360 --> 00:27:06,640 If you remember that summary, you should be able to answer 634 634 00:27:06,640 --> 00:27:08,550 pretty much most of the DNS questions 635 635 00:27:08,550 --> 00:27:10,000 you're going to see on the exam.