1 00:00:00,120 --> 00:00:05,310 In this video, we get introduced to DNS in Linux for the absolute beginners. 2 00:00:05,730 --> 00:00:11,670 We will discuss the basic concepts and view some commands that will help us explore DNS configuration 3 00:00:11,670 --> 00:00:14,040 on hosts, specifically Linux. 4 00:00:14,040 --> 00:00:19,320 Hosted at the end of this section, you will go through a practice test where you're given a set of 5 00:00:19,320 --> 00:00:26,550 challenges related to DNS and are asked to solve them on a life practical Hands-On lab environment right 6 00:00:26,550 --> 00:00:27,360 in your browser. 7 00:00:28,770 --> 00:00:33,510 My name is Charlotte Monomyth and this is called Cloud before we get started. 8 00:00:33,870 --> 00:00:37,020 Don't forget to subscribe to my channel for more videos like this. 9 00:00:38,250 --> 00:00:44,370 We have two computers, A and B, both part of the same network, and they've been assigned with IP 10 00:00:44,370 --> 00:00:50,400 addresses, 190 to 168, one or ten and one, not 11, you're able to ping one computer from the other 11 00:00:50,400 --> 00:00:52,400 using the other computer's IP address. 12 00:00:53,220 --> 00:00:56,230 You know that System B has database services on them. 13 00:00:56,340 --> 00:01:02,130 So instead of having to remember the IP address of the system, B, you decide to give it a name DB 14 00:01:02,970 --> 00:01:09,330 going forward, you would like to ping System B using the name DB instead of its IP address. 15 00:01:09,720 --> 00:01:12,960 If you try to ping DB now, you would see that host. 16 00:01:12,960 --> 00:01:16,010 It is unaware of a host named DB. 17 00:01:16,830 --> 00:01:17,890 So how do you fix that? 18 00:01:18,390 --> 00:01:25,920 Basically you want to tell system error that System B at IP address 192 168 one node 11 has a name DB 19 00:01:26,310 --> 00:01:32,250 we want to tell system is that when I say DB, I mean the IP 192 161 dot 11. 20 00:01:32,790 --> 00:01:36,680 You can do that by adding an entry into the ETSI host file on system. 21 00:01:36,710 --> 00:01:43,590 He mentioned the IP address and the name you want your host to see System B as with all system E, that 22 00:01:43,590 --> 00:01:51,300 the IP at 190 to 160 at one note eleven is a host named DB pings to be now get sent to the correct IP 23 00:01:51,300 --> 00:01:52,140 and are successful. 24 00:01:52,830 --> 00:01:59,850 Now there is an important point to note here with all system is that the IP at 100 to 168 one dot 11 25 00:01:59,850 --> 00:02:02,480 is a host named Be Host. 26 00:02:02,490 --> 00:02:03,870 It takes that for granted. 27 00:02:04,260 --> 00:02:09,120 Whatever we put in the ATC hosts file is the source of truth for host it. 28 00:02:09,840 --> 00:02:11,370 But that may not be the truth. 29 00:02:11,580 --> 00:02:18,690 Host It does not check to make sure if systemis actual name is be, for instance, running a hostname 30 00:02:18,690 --> 00:02:24,530 command on System B reveals that it is named host to but host. 31 00:02:24,540 --> 00:02:25,550 It doesn't care. 32 00:02:25,770 --> 00:02:27,930 It goes by what's in the host file. 33 00:02:29,700 --> 00:02:36,240 You can even fool system into believing that System B is Google, just add an entry into the House file 34 00:02:36,240 --> 00:02:42,990 with an IP mapping to Google dot com, then ping Google and you will get a response from System B, 35 00:02:43,680 --> 00:02:48,990 so we have two names pointing to the same system, one as CDB and another as Google. 36 00:02:49,350 --> 00:02:51,870 And we can use either names to read System B. 37 00:02:53,450 --> 00:03:00,170 You can have as many names as you want for as many servers as you want in the ETSI hosts file every 38 00:03:00,170 --> 00:03:05,750 time we reference another hosted by its name from host A through a PIN command or a Central Command 39 00:03:05,750 --> 00:03:09,200 or through any of the applications or tools within the system. 40 00:03:09,500 --> 00:03:14,360 It looks into its ETSI hosts file to find out the IP address of that host. 41 00:03:14,990 --> 00:03:20,060 Translating Hostname to IP address this way is known as named resolution. 42 00:03:21,140 --> 00:03:27,650 Within a small network of few systems, you can easily get away with the entries in the seahorse file 43 00:03:28,100 --> 00:03:34,220 on each system, I specify which are the other systems in the environment, and that's how it was done 44 00:03:34,220 --> 00:03:35,060 in the past. 45 00:03:36,740 --> 00:03:42,290 Until the environment grew and these files got filled with too many entries and managing these became 46 00:03:42,290 --> 00:03:42,870 too hard. 47 00:03:43,370 --> 00:03:48,940 If one of the servers IP changed, you would need to modify the entries in all of these hosts. 48 00:03:49,250 --> 00:03:56,290 And that's where we decided to move all these entries into a single server who will manage essentially 49 00:03:56,720 --> 00:04:03,530 we call that our DNS server and then we point all hosts to look up that server if they need to resolve 50 00:04:03,530 --> 00:04:08,330 the hostname to an IP address instead of its own ETSI host files. 51 00:04:09,050 --> 00:04:10,070 So how do we do that? 52 00:04:10,230 --> 00:04:13,460 How do we point our host to a DNS server? 53 00:04:14,120 --> 00:04:18,290 Our DNS server has the IP 190 to 160 eight one hundred. 54 00:04:18,590 --> 00:04:23,810 Every host has a DNS resolution configuration file at ETSI Resolved. 55 00:04:23,960 --> 00:04:28,970 Can't you add an entry into it specifying the address of the DNS over? 56 00:04:29,570 --> 00:04:35,690 We say name server and point it to one idea to that 168 at one hundred and that should be it. 57 00:04:36,170 --> 00:04:42,500 Once this is configured on all of your host, every time a host comes up across a hostname that it does 58 00:04:42,500 --> 00:04:45,830 not know about, it looks it up from the DNS over. 59 00:04:46,460 --> 00:04:53,150 If the IP of any of the host was to change, simply update the DNS over and all host should resolve 60 00:04:53,150 --> 00:04:55,220 the new IP address going forward. 61 00:04:55,730 --> 00:05:02,300 You no longer need any entries in the XY hosts file in any of the hosts, but that does not mean you 62 00:05:02,300 --> 00:05:04,250 can't have entries in hosts file. 63 00:05:04,640 --> 00:05:10,370 You still can, for example, say you were to provision a test server for your own needs. 64 00:05:11,210 --> 00:05:15,470 You don't think others would need to resolve the server by its name, so it need not be added to the 65 00:05:15,830 --> 00:05:16,250 server. 66 00:05:16,490 --> 00:05:22,900 In that case, you can add an entry into your host at C hosts file to resolve the server. 67 00:05:23,330 --> 00:05:24,650 You can now resolve the server. 68 00:05:24,810 --> 00:05:27,620 However, no other system will be able to do that. 69 00:05:27,840 --> 00:05:34,100 So a system is able to use hostname to IP mapping from the ETSI host file locally as well as from a 70 00:05:34,100 --> 00:05:35,340 remote DNS server. 71 00:05:36,140 --> 00:05:42,680 What if you have an entry in both places, one in your ETSI house file and another in DNS? 72 00:05:43,070 --> 00:05:49,250 I have an entry in my local files set to 100 to 168, one door one one five and someone added an entry 73 00:05:49,250 --> 00:05:53,920 for the same host to one two one six eight one one one six on the DNS over. 74 00:05:54,440 --> 00:06:01,070 In that case, the host first looks in the local ETSI hosts file and then looks at the name server. 75 00:06:01,250 --> 00:06:05,840 So if it finds the entry in the local ATC host file, it uses that. 76 00:06:06,050 --> 00:06:09,240 If not, it looks for that host in the DNS over. 77 00:06:09,950 --> 00:06:11,660 But that order can be changed. 78 00:06:11,930 --> 00:06:16,520 The order is defined by an entry in the file at the switch. 79 00:06:16,520 --> 00:06:19,550 Dot switched off the line with the hosts entry. 80 00:06:19,820 --> 00:06:27,410 As you can see, the order is first filed and then followed by DNS files, refers to the host file and 81 00:06:27,410 --> 00:06:29,480 DNS refers to the DNS over. 82 00:06:29,810 --> 00:06:36,080 So for every hostname, the host first looks into the ETSI hosts file and if it cannot find it there, 83 00:06:36,290 --> 00:06:38,090 it then looks at the DNS over. 84 00:06:38,600 --> 00:06:42,500 This order can be modified by editing this entry in the file. 85 00:06:43,310 --> 00:06:49,160 As part of this order, our host would resolve the test server to one only to one sixty eight one one 86 00:06:49,160 --> 00:06:49,760 one five. 87 00:06:50,420 --> 00:06:53,810 What if you try to bring a server that is not in either list? 88 00:06:53,870 --> 00:06:57,650 For example, I try and pin it on Facebook dot com. 89 00:06:57,830 --> 00:07:02,500 I don't have Facebook dot com in my Etsy host file and I don't have it in my DNS over either. 90 00:07:02,750 --> 00:07:04,310 So in that case it will fail. 91 00:07:04,850 --> 00:07:10,610 You can add another entry into your result or confirm to point to a name server that knows Facebook. 92 00:07:10,790 --> 00:07:16,670 For example, eight eight eight eight eight is a common, well known public name server available on 93 00:07:16,670 --> 00:07:21,350 the Internet hosted by Google that knows about all the websites on the Internet. 94 00:07:21,920 --> 00:07:26,630 You can have multiple name servers like this configured on your host, but then you have to configure 95 00:07:26,630 --> 00:07:28,760 that on all your hosts in your network. 96 00:07:29,150 --> 00:07:32,840 You already have a name server within your network configuration, all the hosts. 97 00:07:33,200 --> 00:07:39,530 So in that case, you can configure the server itself to forward any unknown host names to the public 98 00:07:39,530 --> 00:07:41,000 name server on the Internet. 99 00:07:41,810 --> 00:07:45,410 You should not be able to pin external sites such as Facebook dot com. 100 00:07:46,040 --> 00:07:51,410 Until now, we've been just trying to reach systems with their names like Web, DB, inforce, etc. 101 00:07:51,800 --> 00:07:55,910 But we just tried to pin Facebook at Facebook dot com. 102 00:07:56,450 --> 00:07:59,660 What is this name with a W W W and dot com at the end. 103 00:07:59,840 --> 00:08:05,450 It's called a domain name and it is how IPS translate to names that we can remember on the public internet, 104 00:08:05,450 --> 00:08:07,030 just like how we did for our hosts. 105 00:08:07,640 --> 00:08:12,290 Now the reason they are in this format, separated by dots, is to group like things together. 106 00:08:12,980 --> 00:08:18,830 The last portion of the domain name, the dot coms, the dot net start edu, dot org, etc. are the 107 00:08:18,830 --> 00:08:24,530 top level domains that represent the intent of the website dot com for commercial or general purpose, 108 00:08:24,950 --> 00:08:31,910 dot net for network, dot edu for educational organizations and dot org for non-profit organizations. 109 00:08:32,890 --> 00:08:38,830 Let's look at one in particular, in Google's case, the dot is the route, that's where everything 110 00:08:38,830 --> 00:08:39,280 starts. 111 00:08:39,520 --> 00:08:41,810 Dot com is a top level domain. 112 00:08:42,100 --> 00:08:50,050 Google is the domain name assigned to Google and w w W is a subdomain, the subdomains helping further 113 00:08:50,050 --> 00:08:51,190 grouping things together. 114 00:08:51,190 --> 00:08:56,130 Under Google, for example, Google's map service is available at Maps dot Google dot com. 115 00:08:56,620 --> 00:09:01,060 So Maps is a subdomain Google storage service is available at trial. 116 00:09:01,060 --> 00:09:05,350 Dot Google dot com mobile apps are available at Appstore, Google dot com. 117 00:09:05,740 --> 00:09:09,310 Google's email service are available at mail dot com. 118 00:09:09,790 --> 00:09:14,640 You can further divide each of these into as many subdomains based on your needs. 119 00:09:15,070 --> 00:09:17,560 So you begin to see a tree structure forming. 120 00:09:19,560 --> 00:09:25,560 When you try to reach any of these domain names, say, about dot Google dot com from within your organization, 121 00:09:25,860 --> 00:09:29,750 your request first hits your organization's internal DNS server. 122 00:09:30,270 --> 00:09:32,760 It doesn't know who apps or Google is. 123 00:09:33,100 --> 00:09:38,670 It forwards your request to the Internet on the Internet, the IP address of the server serving Appstore, 124 00:09:38,680 --> 00:09:44,760 Google dot com, maybe resolve with the help of multiple DNS servers, a routine server looks at your 125 00:09:44,760 --> 00:09:49,200 request and points you to a server serving dot coms dot com. 126 00:09:49,210 --> 00:09:54,720 The server looks at your request and forwards you to Google and Google's China server provides you the 127 00:09:54,720 --> 00:09:58,320 IP of the server serving the app's applications. 128 00:09:59,040 --> 00:10:06,240 In order to speed up all future results, your organization's DNS server may choose to cache this IP 129 00:10:06,360 --> 00:10:10,590 for a period of time, typically a few seconds up to few minutes. 130 00:10:11,320 --> 00:10:15,180 That way it doesn't have to go through the whole process again each time. 131 00:10:16,290 --> 00:10:18,060 So that was out in the public. 132 00:10:18,570 --> 00:10:20,040 What about your organization? 133 00:10:20,460 --> 00:10:25,290 Your organization can have a similar structure to, for example, your organization could be called 134 00:10:25,290 --> 00:10:32,160 as my company come and have multiple subdomains for each purpose for external facing website. 135 00:10:32,520 --> 00:10:37,890 Moeldoko, my company larcombe for accessing your organization's mail drive, for accessing storage 136 00:10:38,220 --> 00:10:43,470 paid or company account, for accessing the payroll application, hitcher, for accessing our application, 137 00:10:43,470 --> 00:10:43,980 etc.. 138 00:10:44,280 --> 00:10:49,260 All of these are configured in your organization's internal DNS server. 139 00:10:50,510 --> 00:10:57,140 The reason we discussed all of this is to understand another entry in the ETSI Resolved file, remember, 140 00:10:57,140 --> 00:11:00,980 this is the file where we configure the DNS server to be used for our host. 141 00:11:01,220 --> 00:11:06,120 With that, we were able to resolve servers in your organization with just their names like Web. 142 00:11:06,440 --> 00:11:12,110 We have now introduced more standard domain names like Web, dot, my company, dot com or DB that my 143 00:11:12,110 --> 00:11:13,450 company, dot com, etc.. 144 00:11:13,820 --> 00:11:16,650 Now when you ping where you can no longer get a response. 145 00:11:16,670 --> 00:11:21,950 Of course, this is because we are trying to ping web, but there is no record for by the name web on 146 00:11:21,950 --> 00:11:22,920 my server. 147 00:11:23,240 --> 00:11:25,650 Instead it is web dot, my company dot com. 148 00:11:25,670 --> 00:11:28,440 So you have to use web dot my company dot com. 149 00:11:29,240 --> 00:11:34,370 Now I can understand if someone outside of a company wants to access our web server, he would have 150 00:11:34,370 --> 00:11:36,130 to use web, got my company dot com. 151 00:11:36,470 --> 00:11:42,320 But within our company, your own company, you want to simply address the Web server by its first name, 152 00:11:42,320 --> 00:11:42,740 web. 153 00:11:43,040 --> 00:11:47,690 Just like how you address other members in your family simply by their first names, which is not the 154 00:11:47,690 --> 00:11:52,010 case when someone outside your family addresses them using their full names. 155 00:11:52,430 --> 00:11:57,830 So what do you do to configure web to resolve my web dot my company dot com? 156 00:11:58,590 --> 00:12:05,840 You want to say when I say web, I mean when my company dot com for that you make an entry into your 157 00:12:05,840 --> 00:12:12,800 house, Etsy and resolve the conflict, call search and specify the domain name you want to append. 158 00:12:13,400 --> 00:12:18,980 Next time you try to ping web, you will see it actually tries web up my company dot com. 159 00:12:19,130 --> 00:12:22,760 Now your host is intelligent enough to exclude the search domain. 160 00:12:22,760 --> 00:12:28,700 If you specified a domain in your query like this, you may also provide additional search domains like 161 00:12:28,700 --> 00:12:29,120 this. 162 00:12:29,120 --> 00:12:35,660 So it would mean when I say web, I mean Web, dot my company, dot com or web or prod, dot my company 163 00:12:35,660 --> 00:12:36,190 dot com. 164 00:12:36,920 --> 00:12:41,520 So your host would try searching all of these domain names when you look for a hostname. 165 00:12:42,410 --> 00:12:44,750 Finally, a word about a record types. 166 00:12:45,080 --> 00:12:47,600 So how are the records stored in the DNS? 167 00:12:47,610 --> 00:12:52,730 So we know that it stores IP to host names that's known as air records. 168 00:12:52,910 --> 00:12:57,110 Storing IPV six to host names is known as Quadir Records. 169 00:12:57,590 --> 00:13:02,390 Mapping one name to another name is called C Name Records, for example. 170 00:13:02,390 --> 00:13:08,240 You may have multiple aliases for the same application, like a food delivery service may also be reached 171 00:13:08,240 --> 00:13:09,830 at eat or hungry. 172 00:13:10,370 --> 00:13:12,680 That's where a C name record is used. 173 00:13:12,950 --> 00:13:14,240 Name to name mapping. 174 00:13:14,930 --> 00:13:17,600 There are many more, but that's all we're going to look at for now. 175 00:13:18,440 --> 00:13:23,300 Now, Paing may not always be the right tool to test DNS resolution. 176 00:13:23,750 --> 00:13:26,630 There are a few other tools as well, such as A. Up. 177 00:13:26,990 --> 00:13:31,280 You can use an S lookup to query a hostname from a DNS server. 178 00:13:31,850 --> 00:13:37,390 But remember, in this lookup does not consider the entries in the local ETSI hosts file. 179 00:13:37,760 --> 00:13:43,380 So if you add an entry into the local SC hosts file for your web application and if you try to do an 180 00:13:43,380 --> 00:13:47,060 annex, look up for that web application, it is not going to find it. 181 00:13:47,870 --> 00:13:54,830 The entry for your Web application has to be present in your DNS server and its lookup only queries 182 00:13:54,830 --> 00:13:56,000 the DNS server. 183 00:13:56,910 --> 00:14:03,060 The same goes with Dick Dick is another useful tool to test the enemy's name resolution, it returns 184 00:14:03,060 --> 00:14:09,560 more details in a similar form as is stored on the server in the upcoming Practice Exercises section. 185 00:14:09,660 --> 00:14:15,720 It will practice between configuring and troubleshooting DNS in a lab environment on actual systems 186 00:14:16,080 --> 00:14:18,240 through some fun and challenging exercises. 187 00:14:18,930 --> 00:14:20,430 Well, that's it for this lecture. 188 00:14:20,640 --> 00:14:26,720 And in the next lecture, we look at how to set up an actual DNS server on a system where we will use 189 00:14:26,720 --> 00:14:28,860 accordionists as our DNS solution. 190 00:14:29,430 --> 00:14:31,140 Well, thank you so much for watching. 191 00:14:31,440 --> 00:14:34,700 And don't forget to subscribe to my channel for more videos like this.