CS5229 09/10 Lecture 11: DNS
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DNS
6 November 2009 CS5229 Semester 1 2009/10
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2 CS5229 Semester 1 2009/10
root
.com .org .sg
nus pbs google yahoo ntu
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3 CS5229 Semester 1 2009/10
root server
.sg
.nus local DNS
dig +trace www.nus.sg
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uses UDP
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Resource Record"(name, TTL, class, type, value)"
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ns1.nus.edu.sg. 6562 IN A 137.132.123.4
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TTL-based Caching
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Resource Type"A NS PTR MX CNAME …"
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ns1.nus.edu.sg. 6562 IN A 137.132.123.4
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DNS-based "Load Balancing
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DNS-based "Server Selection
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how fast?"how efficient? "
is caching useful?"how big should TTL be?
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“DNS Performance and Effectiveness of Caching”"
J Jung et. al."IEEE TON 2002
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Internet
Logger
MIT Subnets
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3 traces"1 week each
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DNS query/respose"TCP SYN/FIN/RST
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basic analysis
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60.4 24.6
6.8 6.4
DNS Query Types for mit-jan00
A PTR MX ANY Others
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496,802"lookups followed by TCP
connections"
3,619,173"TCP connections
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80-87%"DNS cache hit ratio"
for A lookup
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not very high since it includes Web browser’s"
caching too
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how fast is DNS lookup?
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lookup latency ≈ "time between first query"
and last response
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kaist
mit
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74.62
24.07
1.16 0.15
mit-jan00
0
1
2
> 2
Number of referals per lookup
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0
1
2
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NS Cache"miss: first query to gTLD or root servers"
hit: otherwise
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miss
hit
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2,486,104"iterative lookups"
6,039,582"query packets
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significant number of retransmissions
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~24%"of lookups are unanswered
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persistent retransmission"referral loops
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zero referral
answered
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no need to retransmit beyond 2-3 retransmissions!
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loops
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each loop generated on average"
10"query packets
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~60%"of all queries are generated by
unanswered lookup
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popularity of domain name follows "
Zipf"distribution
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Frequency of the x-th most popular item is
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popularity of web pages"population of countries"occurances of English words" :" :
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long tail :"
46%"of domain names "are accessed once
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20%"DNS responses are "
from root/gTLD servers
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without caching NS records:"
5X"loads on root/gTLD servers
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kaist
mit
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popular sites have shorter TTL"
TLL reduces over time
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sharing cache does not help
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large TTL does not help
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“King: Estimating Latency between Arbitrary End
Hosts”"Gummadi et. al."
IMC 2002
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how to find alternate path "with lower latency?
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need to find latency between pair of Internet hosts
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assume name server "is close to end host
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authorative NS
authorative NS
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how to find a name server close to an end host?"
look at domain name, IP prefix
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how likely does a name server allow recursive query?"
> 72% of name server tested
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how accurate is the estimated latency?
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how close, really, is an end host to its authorative NS?
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many diverged paths consist of routers that are physically
adjacent
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75-80% of the disjoint paths have latency less than 10ms"
in the Napster trace
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Summary:"Measurement study of DNS"
Use DNS to do measurement