6.033: Hands-on Assignment (DNS)

M.I.T. DEPARTMENT OF EECS

6.033 - Computer System Engineering

DNS Hands-On Assignment

Hands-on 4: Internet Domain Name System

Today's hands-on exercise is designed to give you a quick introduction to the Internet's Domain Name System. This is an example of a naming system which all of you use on a daily basis --- in fact you used it to get to this web-page! To prepare for this assignment, please read Appendix 4-A of the class notes, titled "Case study of the Internet Domain Name System". This should give you a good general idea of how the DNS works.

Introduction

In order to help explore the domain name system, there is a tool called dig, short for Domain Information Groper. We will be making use of dig in this assignment. dig should be available on all recent Athena workstations. It should work by default, but if it does not, please try running add watchmaker first. If that still does not work, try an Athena Sun workstation. Be sure and explicitly use /usr/athena/bin/dig when running dig; the dig we will use in this assignment is 9.3.1

Here is an example usage of dig:

athena% /usr/athena/bin/dig slashdot.org

; <<>> DiG 9.3.1 <<>> slashdot.org
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51894
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 5

;; QUESTION SECTION:
;slashdot.org. IN A

;; ANSWER SECTION:
slashdot.org.           6988    IN      A       66.35.250.150             (*)

;; AUTHORITY SECTION:
slashdot.org.           6988    IN      NS      ns2.ostg.com.
slashdot.org.           6988    IN      NS      ns2.vasoftware.com.
slashdot.org.           6988    IN      NS      ns3.vasoftware.com.
slashdot.org.           6988    IN      NS      ns1.ostg.com.
slashdot.org.           6988    IN      NS      ns1.vasoftware.com.

;; ADDITIONAL SECTION:
ns1.ostg.com.           19140   IN      A       66.35.250.10
ns1.vasoftware.com.     170637  IN      A       12.152.184.135
ns2.ostg.com.           19140   IN      A       66.35.250.11
ns2.vasoftware.com.     170637  IN      A       12.152.184.136
ns3.vasoftware.com.     170637  IN      A       66.35.250.12

;; Query time: 2 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Mar 21 01:46:51 2006
;; MSG SIZE rcvd: 235

The output tells us a lot of information about our DNS request and the response to it. At the bottom, we can see that the query was sent to our default server (127.0.0.1), and that it took roughly 2 msecs to respond. Most of the information we are interested in is in the ANSWER section, marked with a (*) above. Let's examine that section more closely:

        ;; ANSWER SECTION:
        slashdot.org.    6988    IN      A    66.35.250.150 
          name         expire   class  type     data (IP)

We can see that this result is of type A, an address record: it is telling us that the IP address for the name "slashdot.org" is 66.35.250.150. The expiry time field "6988" indicates that this record/entry is valid for 6988 seconds (slightly less than 2 hours). You can ignore the "class" field; this is nearly always IN for Internet.

The authority section contains records of type NS, which give the names of DNS servers that have name records for a particular domain. Here, we can see that five DNS servers (ns2.ostg.com., ns2.vasoftware.com., etc.) are responsible for answering requests for names in the slashdot.org domain. (Note that in all of these examples, the exact results you get may be slightly different.)

We can ask a specific server (instead of the default) for information about a host by using the following syntax:

athena% /usr/athena/bin/dig @redlab.lcs.mit.edu slashdot.org

; <<>> DiG 9.3.1 <<>> @redlab.lcs.mit.edu slashdot.org
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35814
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 5

;; QUESTION SECTION:
;slashdot.org. IN A

;; ANSWER SECTION:
slashdot.org. 3936 IN A 66.35.250.150


...[output truncated]

We can also see that these queries are resulting in the recursive searches described in section 1 of appendix 4-A of the notes by examining the flags line. The rd (recursion desired) flag indicates that dig requested a recursive lookup, and the ra (recursion available) flag indicates that the server permits recursive lookups (some do not).

dig only shows us the final result of the recursive search. One way for us to mimic the individual steps of a recursive search is to send a request to a particular DNS server and ask for no recursion. For the former, we can give an @server argument to dig. For the latter, we can pass the +norecurs flag. For example, to send a non-recursive query to one of the root servers:

athena% /usr/athena/bin/dig @a.ROOT-SERVERS.NET www.slashdot.org +norecurs

; <<>> DiG 9.3.1 <<>> @a.ROOT-SERVERS.NET www.slashdot.org +norecurs
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62049
;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 6, ADDITIONAL: 6

;; QUESTION SECTION:
;www.slashdot.org. IN A

;; AUTHORITY SECTION:
org.                    172800  IN      NS      TLD1.ULTRADNS.NET.
org.                    172800  IN      NS      TLD2.ULTRADNS.NET.
org.                    172800  IN      NS      TLD3.ULTRADNS.org.
org.                    172800  IN      NS      TLD4.ULTRADNS.org.
org.                    172800  IN      NS      TLD5.ULTRADNS.INFO.
org.                    172800  IN      NS      TLD6.ULTRADNS.CO.UK.

;; ADDITIONAL SECTION:
TLD1.ULTRADNS.NET.      172800  IN      A       204.74.112.1
TLD2.ULTRADNS.NET.      172800  IN      A       204.74.113.1
TLD3.ULTRADNS.org.      172800  IN      A       199.7.66.1
TLD4.ULTRADNS.org.      172800  IN      A       199.7.67.1
TLD5.ULTRADNS.INFO.     172800  IN      A       192.100.59.11
TLD6.ULTRADNS.CO.UK.    172800  IN      A       198.133.199.11

;; Query time: 14 msec
;; SERVER: 198.41.0.4#53(198.41.0.4)
;; WHEN: Tue Mar 21 22:00:51 2006
;; MSG SIZE  rcvd: 288

As you can see, the server does not know the answer and instead provides information about the servers most likely to be able to provide authoritative information. In this case, the best the root server knows is the identities of the servers for the org. domain.

With this in mind, let's do some simple exercises. This hands-on should be doable from any workstation that has dig or an equivalent command, but if you try it anywhere other than on an Athena workstation, and you run into something unexpected, the teaching staff may not be able to help you figure out what is going on.

Please turn in answers to the questions below for Tuesday's recitation. You should submit answers only to the questions asked. In particular, please do not include pages of output from dig unless specifically requested. Your TA will thank you.

I. Getting started

(1) Using dig, find the IP address of thyme.lcs.mit.edu. What is the IP address?
(2) What arguments to dig did you use to find this address?
(3) What is the expiration time for this record?
(4) The dig answer for thyme includes a record of type CNAME. In the terminology of chapter 4, what does CNAME mean?

(5) What are the IP addresses for ai and ai. (note the dot at the end) respectively? To answer this question, use the host command instead of dig. host is a simpler tool that performs a DNS lookup and prints only the answer.
(6) Why are the results different? Examining the local machine's /etc/resolv.conf file, what can you say about the context of DNS searches for ai and ai.?

II. Understanding hierarchy

For this problem, you will go through the steps of resolving a particular hostname, by iterating through a series of servers, just like a regular server might. Assuming it knows nothing else about a name, a DNS resolver will ask a well-known root server. The root servers on the Internet are in the domain root-servers.net. One way to get a list of them is with the command:

athena% dig . ns

(7) Why does this particular command return the names of the root nameservers?

(8) Use dig to ask one of the root servers the address of amsterdam.lcs.mit.edu without recursion. What command do you use to do this?
(9) It is unlikely that these servers actually know the answer so they will refer you to a server (or list of servers) that might know more. Go through the hierarchy from the root without recursion and following the referrals manually until you have found the address of amsterdam.lcs.mit.edu. What is the address?
(10) What is the record returned in the ANSWER SECTION of the final command?
(11) How many iterations did it take to find the IP address? What commands did you use for each one?

III. Understanding caching

These few queries should show you how your local machine's DNS cache works.

(12) Ask your default server for information, without recursion, about the host www.dmoz.org. What command did you use? Does it have the answer in its cache? How do you know? How long did this query take? If this information was cached, please find some other host name that is not cached and do this section with that other host.
(13) Now, ask your default server this same query but with recursion. It should return an answer for you. How long did this take?
(14) Finally, ask your default server again without recursion. How long does this request take? Has the cache served its purpose?

(15) How long did it take you to do this assignment?

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