1. Chapter 2 (Section 2.5) Application Layer - DNS
Modified for GT ECE6612
by Prof. John Copeland
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Thanks and enjoy! JFK/KWR
All material copyright
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach Featuring the Internet, 5rd edition. Jim Kurose, Keith Ross Addison-Wesley, July Textbook for ECE3076.
2/25/13
06a DNS.ppt

2. DNS: Domain Name System
People: many identifiers:
SSN, name, passport #
Internet hosts, routers:
IP address (32 bit) - used for addressing datagrams
“name”, e.g., - used by humans
Q: map between IP addresses and name ?
Domain Name System:
distributed database implemented in hierarchy of many name servers
application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)
note: core Internet function, implemented as application-layer protocol
complexity at network’s “edge”
06a DNS.ppt

3. DNS DNS is Hierarchical DNS services
Why not centralize DNS?
single point of failure
traffic volume
distant centralized database
maintenance
doesn’t scale!
DNS services
Hostname to IP address translation
Host aliasing
Canonical and alias names
Mail server aliasing
Load distribution
Replicated Web servers: set of IP addresses for one canonical name
06a DNS.ppt

4. Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
org DNS servers
edu DNS servers
poly.edu
DNS servers
umass.edu
yahoo.com
amazon.com
pbs.org
Client wants IP for 1st approx:
Client queries a root server to find “com” DNS server
Client queries com DNS server to get “amazon.com” Authoritative DNS server
Client queries amazon.com DNS server to get IP address for “
06a DNS.ppt

5. DNS: Root name servers
contacted by local name server that can not resolve Top_Level name (eats in
Originally there were 7 Top-Level domains (com, org, edu, mil, gov, info, arpa) Now there are hundreds ( us, uk, cn, tv, name, ...)
ICANN assigns domain names (
13 root name servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA (and 17 other locations)
i Autonomica, Stockholm (plus 3 other locations)
k RIPE London (also Amsterdam, Frankfurt)
m WIDE Tokyo
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also Los Angeles)
d U Maryland College Park, MD
g US DoD Vienna, VA
h ARL Aberdeen, MD
j Verisign, ( 11 locations)
06a DNS.ppt

6. TLD and Authoritative Servers
Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.
Network Solutions, Inc. maintains servers for com TLD
Educause maintains servers for edu TLD
[ TLD servers share responsibilities]
Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).
Can be maintained by organization or service provider
Every “Autonomous System” (AS) must have two (backup).
Local DNS servers: organization’s DNS servers located on various subnets to provide DNS lookups for hosts on the subnet. May not be accessible from outside the subnet. Their IP addresses are part of the host's network configuration (manual or DHCP).
PC looks first at “hosts” file. DNS Hack #0, add false info to it.

7. Local Name Server Does not strictly belong to hierarchy
Each ISP (residential ISP, company, university) has one.
Also called “default name server” or “resolver”
When a host makes a DNS query, query is sent to its local DNS server
Acts as a proxy, forwards query into hierarchy.
Today a DNS proxy (resolver) is built into most DSL and cable-modem routers
DNS Hack #1: Change DNS configured IP to IP of attacker-controlled server (Windows Registry or UNIX /etc/resolv.conf)
06a DNS.ppt

8. authoritative DNS server
Example
root DNS server
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
Host sends a "recursion-requested" query request to dns.poly.edu.
[Host is doing a non-recursive search]
Local DNS server does a "recursive" search. This requires contacting several other DNS servers before the final answer is given to host. 2 3
TLD DNS server 4 5
local DNS server
dns.poly.edu 7 6 1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
$ nslookup gaia.cs.umass.edu
answer
gaia.cs.umass.edu
06a DNS.ppt

9. Non-recursive queries
root DNS server
A3.NSLTD.COM
norecurse or "iterated" query:
contacted server replies with name of server to contact
“I don’t know this name, but ask this server”
local DNS server
dns.poly.edu
authoritative DNS server
NS1.umass.edu 3 4 2 5 1 6
requesting host
cis.poly.edu
gaia.cs.umass.edu
$ nslookup -norecurse -v gaia.cs.umass.edu
$ nslookup -norecurse -v gaia.cs.umass.edu A3.NSLTD.COM
$ nslookup -norecurse -v gaia.cs.umass.edu NS1.umass.com
answer
06a DNS.ppt

10. “dig” with “+trace” will show the entire recursive lookup.
copeland$ dig +trace (may have to do from ecelinsrva.ece)
; <<>> DiG P1 <<>> +trace
;; global options: +cmd
IN NS e.root-servers.net.
IN NS c.root-servers.net.
IN NS a.root-servers.net.
(11 lines deleted)
;; Received 496 bytes from #53( ) in 13 ms
. . .
com IN NS h.gtld-servers.net.
com IN NS j.gtld-servers.net.
com IN NS e.gtld-servers.net.
; Received 504 bytes from #53( ) in 138 ms
google.com IN NS ns2.google.com.
google.com IN NS ns1.google.com.
google.com IN NS ns3.google.com.
google.com IN NS ns4.google.com.
;; Received 168 bytes from #53( ) in 56 ms
IN A
IN A
(4 lines deleted)
;; Received 128 bytes from #53( ) in 64 ms

11. DNS: caching and updating records
once (any) name server learns a mapping, it caches the mapping (Domain’s DNS = IP)
cache entries timeout (disappear) after some time (usually 20 minutes)
TLD servers typically cached longer in local name servers
Thus root name servers not often visited
update/notify mechanisms under design by IETF
RFC 2136
DNS Hack #0: Add a “name -> IP” entry in the UNIX /etc/hosts file, or Windows Registry file.
06a DNS.ppt

12. RR format: (name, value, type, ttl)
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)
Type=A (AAAA for IPv6)
name is hostname
value is IP address
Type=CNAME
name is alias name for some “canonical” (the real) name
is really
servereast.backup2.ibm.com
value is canonical name
Type=NS
name is domain (e.g. foo.com)
value is hostname of authoritative name server for this domain
Type=MX
value is name of mailserver associated with name
06a DNS.ppt

13. DNS protocol, messages
DNS protocol : query and reply messages, both with same message format
msg header
identification: 16 bit # for query, reply to query uses same #
flags:
query or reply
recursion desired
recursion available
reply is authoritative
06a DNS.ppt

14. DNS protocol, messages * Name, type fields for a query RRs in response
to query
records for
authoritative servers
additional “helpful”
info that may be used

15. Inserting records into DNS
Example: just created startup “Network Utopia”
Register name networkuptopia.com at a registrar (e.g., Network Solutions)
Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary)
Registrar inserts two RRs into the .com TLD server:
(networkutopia.com, dns1.acme.com, NS)
(dns1.acme.com, , A)
Put in authoritative server Type A record for and Type MX record for networkutopia.com into dns1.acme.com (DNS service)
DNS Hack #2 Register a domain like “myserver.ru”, have links in like “
This can hide the real domain. A unique subnet can id the local resolver, and recipient or Web page viewer.
06a DNS.ppt

16. >dig anyhost.nt.com +norecurse -q=any ; # NO NS SPECIFIED
;; res options: init defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 11306
;; flags: qr ra; QUERY: 1, ANSWER: 0, AUTHORITY: 7, ADDITIONAL: 0
;; QUERY SECTION:
;; anyhost.nt.com, type = A, class = IN
;; AUTHORITY SECTION:
IN SOA a.root-servers.net. nstld.verisign-grs.com.
SOA = Start of Authority Final Dot = Authoritative
A.ROOT-SERVERS.NET. <- SPECIFY DNS NEXT TIME
;; Total query time: 4 msec
;; FROM: bigzilla.ece.gatech.edu to SERVER: default
;; WHEN: Fri Feb 20 08:00:
;; MSG SIZE sent: 28 rcvd: 156 16

17. >dig @C.ROOT-SERVERS.NET nt.com. +norecurse ; SPECIFY NAME SERVER
; (1 server found)
;; res options: init defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26071
;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 15
;; QUERY SECTION:
;; nt.com, type = A, class = IN
;; AUTHORITY SECTION:
com D IN NS C.GTLD-SERVERS.NET. {TOP-LEVEL NS for .com}
com D IN NS L.GTLD-SERVERS.NET.
com D IN NS A.GTLD-SERVERS.NET. …
;; ADDITIONAL SECTION:
A.GTLD-SERVERS.NET D IN A
A.GTLD-SERVERS.NET D IN AAAA :503:a83e::2:30
B.GTLD-SERVERS.NET D IN A
B.GTLD-SERVERS.NET D IN AAAA :503:231d::2:30
•••
;; Total query time: 23 msec
;; FROM: bigzilla.ece.gatech.edu to SERVER: C.ROOT-SERVERS.NET
;; WHEN: Fri Feb 20 08:06:
;; MSG SIZE sent: 24 rcvd: 512 17

18. >dig @192.5.6.30 nt.com. +norecurse -q=any
; (1 server found)
;; res options: init defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24266
;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 5, ADDITIONAL: 5
;; QUERY SECTION:
;; nt.com, type = A, class = IN
;; AUTHORITY SECTION: {Authoritative Name Servers for domain nt.com}
nt.com D IN NS ns-guy2.nortelnetworks.com.
nt.com D IN NS ns-har2.nortelnetworks.com.
•••
;; ADDITIONAL SECTION:
ns-guy2.nortelnetworks.com. 2D IN A
ns-har2.nortelnetworks.com. 2D IN A
;; Total query time: 58 msec
;; FROM: bigzilla.ece.gatech.edu to SERVER:
;; WHEN: Fri Feb 20 08:07:
;; MSG SIZE sent: 24 rcvd: 229
Next: anyhost.nt.com. +norecurse -q=any 18

19. >whois nt.com
Whois Server Version 2.0
Domain names in the .com and .net domains can now be registered
with many different competing registrars. Go to
for detailed information.
Domain Name: NT.COM
Registrar: GODADDY.COM, INC.
Whois Server: whois.godaddy.com
Referral URL:
Name Server: NS-GUY2.NORTELNETWORKS.COM
Name Server: NS-HAR2.NORTELNETWORKS.COM
Status: clientDeleteProhibited
Status: clientRenewProhibited
Status: clientTransferProhibited
Status: clientUpdateProhibited
Updated Date: 20-jan-2009
Creation Date: 28-sep-1990
Expiration Date: 27-sep-2010
>>> Last update of whois database: Fri, 20 Feb :33:26 UTC <<< 19

20. DNS Cache Poisoning root DNS server 3 3
TLD DNS server - IP = 4
1 Spoofed Request for no.bigbank.com
source IP = 5
local DNS server
dns.poly.edu
2 Spoofed Response
- source IP =
Auth. DNS for bigbank.com = 8
Future requests for all
URLs in bigbank.com
go to =
authoritative
DNS server
dns.bigbank.com
requesting host
cis.poly.edu
IP = 20

21. wireshark Display of DNS Response
ID is random nonce
used to authenticate
Response to Query 21

22. DNS protocol, spoofed messages
< bits > *
Name, type fields
for a query
RRs in response
to query
records for
authoritative servers
additional “helpful”
info that may be used
* 16-bit ID is used to match responses to requests. To spoof,
use 1 request and 65,536 responses, or
use 256 requests and 256 responses (Birthday attack).
06a DNS.ppt 22

23. DNS Cache Poisoning - Anticipated Attack
Sends a Request
for a URL,
and N fake Replies
with random IDs
Lookup
Time is is is
Correct guess of
<- ID Nonce
Probable no. of hits
= N / 65,354 is is is is is is
UDP Connection closed >
cached
->
(not noticed) is is is
Local DNS NS-CNN.COM Hacker 23

24. DNS Cache Poisoning – Bellovin Birthday Attack
<- Sending 260
requests for same domain, cnn.com,
and N Replies
with fake Auth. N.S. IP address.
with random IDs
Lookup
Time * is is
<- Correct guess
of one ID.
Probable no. of hits
260*N/(2^16)
=1 if N =252
Prob(hits>0)=0.63
Total packets = 512 is is is is is is
Local DNS ->
caches = is is
* Local DNS sends 260 queries with different IDs.
DOS Attack
Local DNS NS-CNN.COM Hacker
DNS Hack #3: Change DNS IP configured in local cache. 24

25. Fast-Flux DNS (Botnet Distributed Phishing)
Botmaster registers his DNS server, with the “ru” TLD (Top Level DNS) as the Authority for “bg4589.ru”
Botnet hosts sent out to lure victims to a Phishing Web site (IP )
Problem: as soon as BNY Network Security person sees one of the s, they do a DNS lookup (get ), a “whois”, and shut the host down.
Solution: vary the IP address returned to one of a thousand botnet Web servers.
Problem: BNY NetSec repeatedly does DNS lookups to get a complete list of botnet hosts.
Solution: After several lookups from the same IP, have many other bots do a Distributed Denial of Service attack (DDoS) for several days against BNY NetSec.
DNS Hack #4: Use a Fast Flux DNS to prevent total shutdown. 25

26. authoritative DNS server
Fast Flux DNS URL in Phish -> One of Many bots
root DNS server
Host at poly.edu wants IP address for
Host sends a "recursion-requested" query request to dns.poly.edu.
[Host is doing a non-recursive search]
Local DNS server does a "recursive" search. This requires contacting several other DNS servers before the final answer is given to host. 2 3
TLD DNS server 4 5
Fast Flux - many IP’s of bot Phishing sites.
local DNS server
dns.poly.edu 7 6 1 8
Note: the dot after "com" below is necessary to avoid getting the same cached answer from dns.poly.edu.
requesting host
joe.poly.edu
authoritative DNS server
dns.urhcked.com
$ nslookup
answer
$ nslookup
answer
06a DNS.ppt 26
Adapted from “Computer Networking: A Top Down Approach Featuring the Internet”, by Jim Kurose & Keith Ross

27. New Protocols supplement DNS and DHCP on LAN
* In addition to network configuration (DHCP) and Name Resolution (DNS), it’s nice to have a list of servers available on your LAN. For example, be able to select from a list of printers.
Link-Local: Rather than initially using the Network IP Address (host bits 0), randomly select one of 65,534 addresses in x.x/16 (you don’t have to know the network address).
Apple’s mDNS (UDP port 5353) is an open specification that lets a host announce a chosen Name (e.g., “Lab Printer”) in a multicast message to ). Hosts running mDNS will all listen to this multicast address and add “Lab Printer” and its IP address and MAC address to the list of servers available. DNS-SD advertises Server Names and services.
Microsoft’s NetBios Name Service (NBNS) is similar, but uses the Network Broadcast Address (host bits all 1). UPnP protocol Simple Service Delivery (SSDP) advertises services.
The IETF is developing a similar Internet standard - Service Location Protocol (SLP).
en.wikipedia.org/wiki/MDNS 2/26/10 27

28. DNSSEC – DNS Security Extensions
DNSSEC works by digitally signing records for DNS lookup using public-key cryptography. The correct DNSKEY record is authenticated via a chain of trust, starting with a set of verified public keys for the DNS root zone which is the trusted third party.
New DNS record types were created or adapted to use with DNSSEC: RRSIG, DNSKEY, DS, NSEC, NSEC3, NSEC3PARAM.
When DNSSEC is used, each answer to a DNS lookup will contain an RRSIG DNS record, in addition to the record type that was requested. The RRSIG record is a digital signature of the answer DNS resource record set. The digital signature can be verified by locating the correct public key found in a DNSKEY record.
Stub resolvers (host software) are "minimal DNS resolvers that use recursive query mode to offload most of the work to a recursive name server.”
A validating stub resolver can also potentially perform its own signature validation by setting the Checking Disabled (CD) bit in its query messages, for end-to-end DNS security for domains implementing DNSSEC.
en.wikipedia.org/wiki/DNSSEC 2/26/13 28

29. Reverse DNS Lookups
Reverse DNS Lookups (e.g., nslookup )
> nslookup
in-addr.arpa name = seiya.ece.gatech.edu.
The “arpa” top-level domain is now only used for reverse lookups. ARPA has been redefined as standing for “Address and Routing Parameter Area.”
For looking up a IPv4 address, the dotted-decimal text string is reversed, and the domain name “in-addr.arpa” is added. Like normal name lookups, the recursion proceeds from right to left. If the block of addresses /16 are assigned to the Autonomous System (AS) gatech.edu, then Georgia Tech (OIT) is responsible for maintaining a database for x.y (addresses to ).
The domain for IPv6 addresses is ip6.arpa.
en.wikipedia.org/wiki/DNSSEC 2/26/13 29

30. dig www.ece.gatech.edu [ MX records not shown]
; <<>> DiG 9.6-ESV-R4-P3 <<>>
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 56886
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 5
;; QUESTION SECTION:
; IN A
;; ANSWER SECTION:
IN A
;; AUTHORITY SECTION:
ece.gatech.edu IN NS dns1.gatech.edu.
ece.gatech.edu IN NS dns3.gatech.edu.
;; ADDITIONAL SECTION:
dns1.gatech.edu IN A
dns1.gatech.edu IN AAAA 2610:148:1f00:f400::3
dns3.gatech.edu IN A
;; Query time: 37 msec
;; SERVER: #53( ) {note: mail handlers (MTAs) not shown.}
;; WHEN: Mon Feb 25 12:50:
;; MSG SIZE rcvd: 213 30

31. nslookup -q=ANY gatech.edu {note: use domain name only, for MX info}
Server:
Address: #53
gatech.edu
origin = brahma5.dns.gatech.edu
mail addr = hostmaster.gatech.edu
serial =
refresh = 10800
retry = 3600
expire =
minimum = 60
Name: gatech.edu
Address:
gatech.edu nameserver = dns1.gatech.edu.
gatech.edu nameserver = heath.dpo.uab.edu {Backup DNS at U. Alabama}
gatech.edu nameserver = dns3.gatech.edu.
gatech.edu nameserver = dns2.gatech.edu.
gatech.edu mail exchanger = 10 mx1.gatech.edu.
gatech.edu mail exchanger = 10 mx2.gatech.edu.
gatech.edu text = "MS=ms "
gatech.edu text = "MS=ms " 31

32. Five DNS Hacks
DNS Hack #0 – modify /etc/resolv.conf or Windows’ Registry, to change the IP of the Local DNS Server.
DNS Hack #1 – add a line to /etc/hosts or Windows’ Registry.
DNS Hack #2 – In URL link, hide the actual domain: e.g.,
DNS Hack #3 – Fast-Flux DNS: gives different IP every time.
DNS Hack #4 – Poison the Local DNS Server’s cache
(using a “Birthday” Attack) 32