In this project, we measured the stability of DNS servers based on the most popular 500 domains. In the first part of the project, DNS server replica counts and maximum DNS server separation are found for each domain. In the second part, these domains are queried for a one-month period in order to find their uptime percentages

Boyaci, Omer

Schulzrinne, Henning G.

Columbia University Academic Commons

Measurements of DNS Stability              Omer Boyaci Henning Schulzrinne        Department of Computer ScienceColumbia University, New York, NY 10027               {hgs@cs,ob2108}columbia.eduJanuary 20041. IntroductionThe Domain Name System (DNS) maps domain names to IP addresses and vice versa. The Domain Name System consist of two pieces, DNS servers and resolvers. Resolvers are client applications which deliver the IP address(es) of a domain name upon request of a user application or operating system. DNS servers receives queries from resolvers and they return the corresponding IP address(es) of the domain name back to clients. The DNS is one of the core protocols of the Internet.In this project, we measured the stability of DNS servers based on the most popular 500 domains. In the first part of the project, DNS server replica counts and maximum DNS server separation are found for each domain. In the second part, these domains are queried for a one-month period in order to find their uptime percentages.2. Measurement SetupWe have developed a number of Java applications for this project. The names and functionalities of main ones are given below. AlexaParser: Alexa.com is an amazon.com company which provides the most visited 500 domains. Their rankings are based on the information which they collect from the millions of Alexa toolbar users. This Java application parses the HTML files of alexa.com and produce a list of the top 500 domain names.AuthoritativeDNSServerFinder: This application finds the authoritative DNS servers of a given domain.MaximumDNSSeperationFinder: This application traceroutes each DNS server of a domain and finds the maximum separation, which is explained in 3.2, between them. It uses the traceroute utility for path discovery. UptimeProber: This application periodically tries to resolve a domain name via its authoritative DNS servers.The measurements were done on Unix systems of the Columbia University Department of Computer Science in December 2004.3.  Results1. DNS Server ReplicasIt can be seen from Table 1 that 3/5 of the domains only have two authoritative DNS servers and most of the others have three or four. Table 1. Authoritative DNS Server CountFigure 1 shows the percentage for each DNS replication count. Sixty percent of the domains have two DNS servers, another 30 percent have three or four. The complete list of domain names and their DNS server count can be seen in the Appendix.Figure 1. DNS Server Counts2. Maximum DNS Server SeparationThis metric is a good indicator for DNS stability. If the DNS servers of a domain is not separated enough from each other then the users can not access to these servers in case of a link or router failure. The following algorithm is used to calculate the DNS Count Domain Number1 42 2983 824 685 226 or more 20maximum DNS server separation:Let us assume that there are 4 DNS servers ns1, ns2, ns3, ns4 for a given domain. First we perform a traceroute to each DNS server and then calculate the separation between each pair [there are 6 pairs (ns1,ns2)(ns1,ns3)(ns1,ns4)(ns2,ns3)(ns2,ns4)(ns3,ns4)]. The difference for each pair is calculated by counting the different routers on their ways. For example consider the comparison of ns1 and ns2.  Lets assume that their traceroutes are:ns1 > router1 router2 router6 router9 router7ns2 > router1 router2 router3 router4 router8Then their separation difference is 3 because their route differs in last three hops. We define the “maximum DNS server separation” for a given domain as the maximum of these six differences. The maximum DNS server separation for the top 500 domains are given below. Figure 2. Maximum DNS server separationMore than 75% of the domains have a maximum DNS server separation of at least 4. The complete list of domains and their maximum DNS server separation are given in the Appendix.1918171615141312111098765432100 5 10 15 20 25 30 35 40 45 50253527294848283231242340263913140723Maximum DNS Server SeparationMaximum DNS Server SeparationNumber of Domains (out of 500)3. Uptime PercentagesWe measured the uptime percentages by probing each domain 300,000 times over thirty days. We used a customized prober which behaves like a resolver. For each domain it sequentially queries the DNS servers one by one until it receives a response. Each DNS server was queried at most three times with a timeout values of 2, 4 and 8 seconds. If none of the DNS servers responded, we incremented the missed queries counter of this domain. Table 2 shows the DNS query misses of domains. Almost 80% of the domains missed less than 50 packets during the experiment. Table 2. Number of misses4. Analysis of the resultsTable 4 shows the combined results from our two experiments. The values for columns “DNS Replica Count” and “Maximum DNS Separation” are averaged values. It can be seen that when we go from perfect to below 99.99% the DNS replica count and maximum DNS server separation decrease. This is an indication that DNS replica count and maximum DNS server separation play an important role achieving stable Domain Name Service. Table 3. Analysis of the resultsPerfect 175 35%1-5 misses 75 15%6-10 misses 40 8%11-20 misses 51 10%21-50 misses 52 10%51-100 misses 25 5%101-500 misses 40 8%More than 500 misses 42 8%Perfect 170 3.6 9.6Above 99.99% 180 2.3 6.5Below 99.99% 120 2.1 4.9Uptime PercentagesNumber of DomainsAverage DNS Replica CountMaximum DNS SeparationAppendixIn the table below domains with low uptime percentages are listed. The table is sorted by uptime percentages in an increasing order. The first entry is www.adobe.com because it has the lowest uptime percentage, %94. The rank column shows the place of the domain among top 500 domains. DNS replica count is given in column 3 and maximum DNS server separation is given in column 4. The total number of DNS queries sent to a domain is given in the last column. The number of queries answered by the DNS is given in column titled “Response”, where as the number of queries missed by the authoritative DNS servers of a domain is given in column titled “Missed”. The uptime percentage is calculated by dividing responses by the total number of queries.References[1] Alexa, http://www.alexa.com[2] P. Mockapetris, “Domain Names--Implementation and Specification,” Request for Comments 1987, Internet Engineering Task Force, Nov. 1987. No Rank Domain DNS Replicas Uptime Missed Response Query1 128 3 10 93.96 20195 314267 3344622 305 2 0 97.18 9460 325827 3352873 53 2 2 97.80 7324 325014 3323384 327 3 1 99.34 2196 330134 3323305 28 2 8 99.35 2163 328932 3310956 308 2 8 99.48 1712 330626 3323387 375 2 1 99.49 1697 328862 3305598 169 2 1 99.52 1615 333671 3352869 401 2 0 99.52 1592 328967 33055910 290 2 2 99.56 1471 329334 33080511 319 2 2 99.57 1448 333841 33528912 78 4 2 99.68 1074 330448 33152213 148 2 0 99.68 1044 329515 33055914 459 2 0 99.72 918 329640 33055815 249 2 2 99.75 832 333627 33445916 389 2 9 99.76 805 331934 33273917 48 2 3 99.76 805 333654 33445918 60 2 7 99.77 762 330333 331095Max DNS Sepwww.adobe.comwww.blackplanet.comwww.maplestory.comwww.fc2web.comwww.msn.co.ilwww.5460.netwww.1stblaze.comwww.domainsponsor.comwww.haohz.comwww.revenue.netwww.incredimail.comwww.sportsline.comwww.17173.comwww.totalvelocity.comwww.cjb.netwww.online.sh.cnwww.cnool.netwww.nypost.com

Measurements of DNS Stability

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Measurements of DNS Stability

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