In this exercise you'll learn about DNS by using dig to make DNS queries. These are the primary objectives of this exercise:
- Expose the different responsibilities of different members of the DNS hierarchy.
- Explore how different DNS servers respond differently to different DNS queries.
- Familiarize yourself with the different kinds of DNS record types, and distinguish between them.
dig is a command line tool that comes installed on most unix and linux systems, or can be installed with your favorite package manager. If not, you can use this web interface -- but the user experience is much worse so I strongly suggest you use the command line instead.
This is the second part of this exercise, the first can be found here.
Make the following two request with dig
dig a ns1.google.com
This command returns the IP address of one of Google's authoritative namer servers. My results look like this:
; <<>> DiG 9.10.6 <<>> a ns1.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 33256
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1452
;; QUESTION SECTION:
;ns1.google.com. IN A
;; ANSWER SECTION:
ns1.google.com. 5209 IN A 216.239.32.10
;; Query time: 14 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Wed Jul 25 11:16:43 PDT 2018
;; MSG SIZE rcvd: 59
Your default DNS resolver (mine is CloudFlare's 1.1.1.1) responded to our request. But we can use dig to send messages directly to other DNS servers.
Now make the following two requests with dig. In the second request, replace 216.239.32.10 with whatever IP address you recieved in the last request.
dig a google.com
dig @216.239.32.10 google.com
Did you get the same IP address for both requests?
It is unlikely, but possible to get the same IP address for each of these requests.
If so, why might this be? If not, why not?
If you DID it means that the Google authoritative name server you queried this time happens to have the same IP address as the resolver you queried last time. A single domain name can have several IP addresses associated with it, and the DNS server can decide which one to send you if it knows about more than one IP address for a requested domain.
Your DNS resolver relies on (at least) one of the Google authoritative servers -- the IP address your resolver delivers depends on which authoritative server was used, when the request was made, the state of the resolvers cache, and other factors too.
If you got two different IP addresses, try using traceroute to determine if one of those two addresses is "closer" to you in terms of number of nodes on the internet. If you did not get two different IP addresses, try querying another one of Google's authoritative servers to get a new IP address for google.com
For example, I have received these IP address for google.com so far:
216.58.194.174 and 172.217.5.110 so I am going to run:
traceroute 216.58.194.174
traceroute 172.217.5.110
For me, the first IP address took 15 hops and the second took only 9. 172.217.5.110 is topologically closer to me on the internet. Which IP address was closer to you?
The DNS Hierarchy has 3 classes of servers. Root -> TLD -> Authoritative.
Root servers know about TLD servers, TLD servers know about Authoritative servers, and Authoritative servers know about specific websites and web services. In this part of the exercise we'll explore the process of resolving a DNS query the way a DNS resolver would if it didn't have any information in it's cache.
First, we will use your local resolver to find the IP address of two root server. Try the following:
dig A a.root-servers.net
dig A b.root-servers.net
What IP addresses identify these two root servers?
a.root-servers.net -> 198.41.0.4
b.rootservers.net -> 199.9.14.201
All the root servers have similar names, you can query for a-m.root-servers.net. These root servers have fixed IP address that do not change -- this is critical to the Internet's infrastructure. If the root IP address changed... who would we ask to determine the IP address of those servers?
Try using traceroute to determine which of those two root servers is closer to you
traceroute 198.41.0.4
traceroute 199.9.14.201
Using the IP address of one of the root servers make a query directly to a root server asking for an A record for google.com.
dig @198.41.0.4 A google.com
Your result should not be anything like the results from the previous section -- here's what I got:
; <<>> DiG 9.10.6 <<>> A a.root-servers.net ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12648 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 1452 ;; QUESTION SECTION: ;a.root-servers.net. IN A
;; ANSWER SECTION: a.root-servers.net. 632 IN A 198.41.0.4
;; Query time: 14 msec ;; SERVER: 1.1.1.1#53(1.1.1.1) ;; WHEN: Fri Jun 22 15:32:43 PDT 2018 ;; MSG SIZE rcvd: 63
Tylers-MacBook-Pro:dns-exercises tylerbettilyon$ dig @198.41.0.4 A google.com
; <<>> DiG 9.10.6 <<>> @198.41.0.4 A google.com ; (1 server found) ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8001 ;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 27 ;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 1472 ;; QUESTION SECTION: ;google.com. IN A
;; AUTHORITY SECTION: com. 172800 IN NS a.gtld-servers.net. com. 172800 IN NS b.gtld-servers.net. com. 172800 IN NS c.gtld-servers.net. com. 172800 IN NS d.gtld-servers.net. com. 172800 IN NS e.gtld-servers.net. com. 172800 IN NS f.gtld-servers.net. com. 172800 IN NS g.gtld-servers.net. com. 172800 IN NS h.gtld-servers.net. com. 172800 IN NS i.gtld-servers.net. com. 172800 IN NS j.gtld-servers.net. com. 172800 IN NS k.gtld-servers.net. com. 172800 IN NS l.gtld-servers.net. com. 172800 IN NS m.gtld-servers.net.
;; ADDITIONAL SECTION: a.gtld-servers.net. 172800 IN A 192.5.6.30 b.gtld-servers.net. 172800 IN A 192.33.14.30 c.gtld-servers.net. 172800 IN A 192.26.92.30 d.gtld-servers.net. 172800 IN A 192.31.80.30 e.gtld-servers.net. 172800 IN A 192.12.94.30 f.gtld-servers.net. 172800 IN A 192.35.51.30 g.gtld-servers.net. 172800 IN A 192.42.93.30 h.gtld-servers.net. 172800 IN A 192.54.112.30 i.gtld-servers.net. 172800 IN A 192.43.172.30 j.gtld-servers.net. 172800 IN A 192.48.79.30 k.gtld-servers.net. 172800 IN A 192.52.178.30 l.gtld-servers.net. 172800 IN A 192.41.162.30 m.gtld-servers.net. 172800 IN A 192.55.83.30 a.gtld-servers.net. 172800 IN AAAA 2001:503:a83e::2:30 b.gtld-servers.net. 172800 IN AAAA 2001:503:231d::2:30 c.gtld-servers.net. 172800 IN AAAA 2001:503:83eb::30 d.gtld-servers.net. 172800 IN AAAA 2001:500:856e::30 e.gtld-servers.net. 172800 IN AAAA 2001:502:1ca1::30 f.gtld-servers.net. 172800 IN AAAA 2001:503:d414::30 g.gtld-servers.net. 172800 IN AAAA 2001:503:eea3::30 h.gtld-servers.net. 172800 IN AAAA 2001:502:8cc::30 i.gtld-servers.net. 172800 IN AAAA 2001:503:39c1::30 j.gtld-servers.net. 172800 IN AAAA 2001:502:7094::30 k.gtld-servers.net. 172800 IN AAAA 2001:503:d2d::30 l.gtld-servers.net. 172800 IN AAAA 2001:500:d937::30 m.gtld-servers.net. 172800 IN AAAA 2001:501:b1f9::30
;; Query time: 30 msec ;; SERVER: 198.41.0.4#53(198.41.0.4) ;; WHEN: Fri Jun 22 15:34:10 PDT 2018 ;; MSG SIZE rcvd: 835
**What information has the the root server given us?**
> Root servers do not serve "recursive" DNS requests. This response does not have any records in the answer query which means the root server does not know the IP address for google.com. Instead, the root server has given us NS and A records for all of the "generic top level domain (GTLD) servers in the authority and additional information section. This means "I don't know the answer to your question, try asking one of these servers next."
**Which class of server in the hierarchy are a-m.gtld-servers.net?**
> These are TLD servers.
**Why didn't we get an A record for google.com like we asked for?**
> Because the root server doesn't have such a record in its database, and it does not serve recursive DNS requests.
**How can we use this information to get closer to the answer we want (an A record for google.com)**
> Next we should make the same request to one of the servers identified. The next question is an example of doing exactly this.
**What is the relationship between the records in the Authority section and the Additional section?**
> The authority section contains NS records for each of the GTLD servers, the additional information contains A records for each of these GTLD servers. This helps speed up the process instead of making the resolver ask for NS records and A records separately.
**Ask one of these TLD servers the same question, for an A record for google.com.**
> I used the IP address for m.gtld-servers.net
> ```
dig @192.55.83.30 A google.com
If you did part 1 of this exercise the response should look familiar, this is what I got:
; <<>> DiG 9.10.6 <<>> @192.55.83.30 A google.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51556
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 4, ADDITIONAL: 9
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;google.com. IN A
;; AUTHORITY SECTION:
google.com. 172800 IN NS ns2.google.com.
google.com. 172800 IN NS ns1.google.com.
google.com. 172800 IN NS ns3.google.com.
google.com. 172800 IN NS ns4.google.com.
;; ADDITIONAL SECTION:
ns2.google.com. 172800 IN AAAA 2001:4860:4802:34::a
ns2.google.com. 172800 IN A 216.239.34.10
ns1.google.com. 172800 IN AAAA 2001:4860:4802:32::a
ns1.google.com. 172800 IN A 216.239.32.10
ns3.google.com. 172800 IN AAAA 2001:4860:4802:36::a
ns3.google.com. 172800 IN A 216.239.36.10
ns4.google.com. 172800 IN AAAA 2001:4860:4802:38::a
ns4.google.com. 172800 IN A 216.239.38.10
;; Query time: 30 msec
;; SERVER: 192.55.83.30#53(192.55.83.30)
;; WHEN: Fri Jun 22 15:37:59 PDT 2018
;; MSG SIZE rcvd: 287
What did the TLD server tell us?
The TLD server told us that it also does not have the IP address for google.com and suggested we ask one of the four name servers it provided. Those four name servers are all Google owned authoritative name servers.
Once again, why didn't we get an A record for google.com?
Because the TLD server we queried did not have an A record for google.com. Instead it knew about the authoritative name servers for google.com
How does this information help us get closer to an A record for google.com?
Now we can use one of the IP addresses for a google authoritative name server. For example, the IP address I received for
ns4.google.comwas216.239.38.20. I candig @216.239.38.20 A google.comin order to get an IP address for google.com.