M.I.T. DEPARTMENT OF EECS
| 6.033 - Computer System Engineering | DNS Hands-On Assignment |
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.
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.
Here is an example usage of dig:
athena% dig slashdot.org
; <<>> DiG 9.3.1 <<>> slashdot.org ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 997 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 3
;; QUESTION SECTION: ;slashdot.org. IN A
;; ANSWER SECTION: slashdot.org. 3600 IN A 216.34.181.45 (*)
;; AUTHORITY SECTION: slashdot.org. 86399 IN NS ns-2.ch3.sourceforge.com. slashdot.org. 86399 IN NS ns-1.ch3.sourceforge.com. slashdot.org. 86399 IN NS ns-1.sourceforge.com. ;; ADDITIONAL SECTION: ns-1.ch3.sourceforge.com. 172800 IN A 216.34.181.21 ns-1.sourceforge.com. 172800 IN A 208.122.22.23 ns-2.ch3.sourceforge.com. 172800 IN A 216.34.181.22
;; Query time: 69 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Wed Mar 11 17:32:51 2009 ;; MSG SIZE rcvd: 170
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 69 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. 3600 IN A 216.34.181.45
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 216.34.181.45. The
expiry time field "3600" indicates that this
record/entry is valid for 3600 seconds (1 hour). 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 three DNS servers
(ns-1.ch3.sourceforge.com., ns-1.sourceforge.com.
and ns-2.ch3.sourceforge.com.) 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% dig @amsterdam.lcs.mit.edu slashdot.org
; <<>> DiG 9.3.1 <<>> @amsterdam.lcs.mit.edu slashdot.org ; (1 server found) ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 1988 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 3
;; QUESTION SECTION: ;slashdot.org. IN A
;; ANSWER SECTION: slashdot.org. 3600 IN A 216.34.181.45 ...[output truncated]
We can also see that these queries are resulting in the recursive
searches described in section 4.1.1 of the class 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% dig @a.ROOT-SERVERS.NET www.slashdot.org +norecurs
; <<>> DiG 9.3.1 <<>> @a.ROOT-SERVERS.NET www.slashdot.org +norecurs ;; (1 server found) ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 1888 ;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 6, ADDITIONAL: 12
;; QUESTION SECTION: ;www.slashdot.org. IN A
;; AUTHORITY SECTION: org. 172800 IN NS B0.ORG.AFILIAS-NST.org. org. 172800 IN NS A0.ORG.AFILIAS-NST.INFO. org. 172800 IN NS A2.ORG.AFILIAS-NST.INFO. org. 172800 IN NS D0.ORG.AFILIAS-NST.org. org. 172800 IN NS C0.ORG.AFILIAS-NST.INFO. org. 172800 IN NS B2.ORG.AFILIAS-NST.org. ;; ADDITIONAL SECTION: A0.ORG.AFILIAS-NST.INFO. 172800 IN A 199.19.56.1 A0.ORG.AFILIAS-NST.INFO. 172800 IN AAAA 2001:500:e::1 A2.ORG.AFILIAS-NST.INFO. 172800 IN A 199.249.112.1 A2.ORG.AFILIAS-NST.INFO. 172800 IN AAAA 2001:500:40::1 B0.ORG.AFILIAS-NST.org. 172800 IN A 199.19.54.1 B0.ORG.AFILIAS-NST.org. 172800 IN AAAA 2001:500:c::1 B2.ORG.AFILIAS-NST.org. 172800 IN A 199.249.120.1 B2.ORG.AFILIAS-NST.org. 172800 IN AAAA 2001:500:48::1 C0.ORG.AFILIAS-NST.INFO. 172800 IN A 199.19.53.1 C0.ORG.AFILIAS-NST.INFO. 172800 IN AAAA 2001:500:b::1 D0.ORG.AFILIAS-NST.org. 172800 IN A 199.19.57.1 D0.ORG.AFILIAS-NST.org. 172800 IN AAAA 2001:500:f::1 ;; Query time: 84 msec ;; SERVER: 198.41.0.4#53(198.41.0.4) ;; WHEN: Wed Mar 11 17:45:41 2009 ;; MSG SIZE rcvd: 436As 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. 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.
dig, find the IP address
of thyme.lcs.mit.edu. What is the IP address?
dig answer for thyme
includes a record of type CNAME.
In the terminology of chapter 4, what does CNAME mean?
ai
and ai. (note the dot at the end) respectively? Use the host command for this question. host is a simpler tool that
performs a DNS lookup and prints only the answer.
/etc/resolv.conf file, what can you say
about the context of DNS searches for ai
and ai.?
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
dig to ask one of the root servers the
address of lirone.csail.mit.edu, without
recursion. What command do you use to do this?
lirone.csail.mit.edu. What commands did you use to do this, and what is the IP address?
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.
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