L4 Domain Name System (DNS)
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1 L4 Domain Name System (DNS) by T.S.R.K. Prasad EA C451 Internetworking Technologies 19/01/2013
2 References / Acknowledgements Sec 2.5: DNS, [Kurose] Sec 9.3.1: Name Service (DNS), [Peterson] Sec 12.3: DNS, [Farrel] References
3 Optional Readings [rfc920] - Domain Requirements [rfc1101] - DNS Encoding of Network Names and Other Types [rfc1034] - Domain Names Concepts and Facilities [rfc1035] - Domain Names Implementation and Specification [rfc1546] - Host Anycasting Service [rfc4367] - What's in a Name: False Assumptions about DNS Names Optional Readings
4 Optional Readings [rfc4592] - Wildcards in dns [rfc5782] - DNS Blacklists and Whitelists [Weaver] [Walfish-SFR] Nicholas Weaver, Christian Kreibich, and Vern Paxson, Redirecting DNS for Ads and Profit Michael Walfish, Hari Balakrishnan, and Scott Shenker, Untangling the Web from DNS Optional Readings
5 Presentation Overview Problem Areas Queries and Responses Resource Records Name Space DNS Basics Lecture Outline
6 Presentation Overview Problem Areas Queries and Responses Resource Records Name Space DNS Basics Lecture Outline
7 Host Names vs. IP addresses Host names Mnemonic name appreciated by humans Variable length, full alphabet of characters Provide little (if any) information about location Examples: and bbc.co.uk IP addresses Numerical address appreciated by routers Fixed length, binary number Hierarchical, related to host location Examples: and DNS Basics Host Names vs IP Addresses
8 Separating Names and Addresses Names are easier to remember vs Addresses can change underneath Move to E.g., renumbering when changing providers Name could map to multiple IP addresses to multiple (8) replicas of the Web site Map to different addresses* in different places Address of a nearby copy of the Web site E.g., to reduce latency, or return different content Multiple names for the same address E.g., aliases like and cnn.com DNS Basics Separating Names and Addresses
9 Scalable (Name Address) Mappings Originally: per-host file Flat namespace /etc/hosts (what is this on your computer today?) SRI kept master copy Downloaded regularly Single server doesn t scale Traffic implosion (lookups & updates) Single point of failure Amazing politics Need a distributed, hierarchical collection of servers DNS Basics Names and Addresses Mappings
10 Properties of Present DNS Hierarchical namespace Hierarchical name space divided into zones Hierarchy of DNS servers Root (hardwired into other servers) Top-level domain (TLD) servers Authoritative DNS servers Zones distributed over collection of DNS servers Performing the translations Local DNS servers Resolver software DNS Basics Properties
11 Presentation Overview Problem Areas Queries and Responses Resource Records Name Space DNS Basics Lecture Outline
12 DNS Name Space Domain Zone Name Space
13 Root Servers Name Space Root Servers
14 Root Servers in India I & J Name Space Root Servers in India
15 gtld aero biz com coop edu gov info int mil museum name net org pro Description Airlines and aerospace companies Businesses or firms Commercial organizations Cooperative business organizations Educational institutions Government institutions Information service providers International organizations Military groups Museums Personal names / Individuals Network support centers Nonprofit organizations Profesional individual organizations Generic Top Level Domains (gtlds) Name Space gtlds
16 TLD and Authoritative DNS Servers Top-level domain (TLD) servers Generic domains (e.g., com, org, edu) Country domains (e.g., uk, fr, cn, jp) Special domains (e.g., arpa) Typically managed professionally Network Solutions maintains servers for com Educause maintains servers for edu Authoritative DNS servers Provide public records for hosts at an organization Private records may differ, though not part of original design s intent For the organization s servers (e.g., Web and mail) Can be maintained locally or by a service provider Name Space Servers
17 Local DNS Server does not strictly belong to hierarchy each ISP (residential ISP, company, university) has one also called default name server when host makes DNS query, query is sent to its local DNS server has local cache of recent name-to-address translation pairs (but may be out of date!) acts as proxy, forwards query into hierarchy Name Space Servers
18 Presentation Overview Problem Areas Queries and Responses Resource Records Name Space DNS Basics Lecture Outline
19 Resource Records (RR) Format <Name, Type, Value(Rlength+Rdata), Class, TTL> Common Types: A NS MX PTR (many others) A6, AAAA, SOA, CNAME, TXT, SPF, SRV, HINFO, DNAME, GPOS, LOC, NSAP, KEY, Resource Records
20 Root Server Zone File ( in 11/1987). IN SOA SRI-NIC.ARPA. HOSTMASTER.SRI-NIC.ARPA. ( ;serial 1800 ;refresh every 30 min 300 ;retry every 5 min ;expire after a week 86400) ;minimum of a day NS A.ISI.EDU. NS C.ISI.EDU. NS SRI-NIC.ARPA. MIL NS SRI-NIC.ARPA NS A.ISI.EDU. Resource Records Root Server Zone File
21 Root Server Zone File ( in 11/1987) EDU NS SRI-NIC.ARPA NS C.ISI.EDU. SRI-NIC.ARPA. A A MX 0 SRI-NIC.ARPA. HINFO DEC-2060 TOPS IN-ADDR.ARPA. PTR IN-ADDR.ARPA. PTR IN-ADDR.ARPA. PTR IN-ADDR.ARPA. PTR IN-ADDR.ARPA. PTR SRI-NIC.ARPA. ACC.ARPA. SRI-NIC.ARPA. C.ISI.EDU. A.ISI.EDU. ACC.ARPA. A HINFO PDP-11/70 UNIX MX 10 ACC.ARPA. A.ISI.EDU A C.ISI.EDU A USC-ISIC.ARPA. CNAME C.ISI.EDU. Resource Records Root Server Zone File
22 From ARPA. Zone File arpa IN NS a.root-servers.net. arpa IN NS m.root-servers.net. in-addr.arpa IN NS a.in-addr-servers.arpa. in-addr.arpa IN NS f.in-addr-servers.arpa. a.in-addr-servers.arpa IN A a.in-addr-servers.arpa IN AAAA 2001:500:13:0:0:0:0:73 f.in-addr-servers.arpa IN A f.in-addr-servers.arpa IN AAAA 2001:67c:e0:0:0:0:0:1 ARPA - Address and Routing Parameter Area Domain Resource Records From ARPA. Zone File
23 Presentation Overview Problem Areas Queries and Responses Resource Records Name Space DNS Basics Lecture Outline
24 DNS Message Format <DNS Message>::= <common header> <question> [<question>...] [<answer>...] [<authority>...] [<additional info>...] Queries and Responses DNS Message Format
25 DNS Header Format RCode Meaning 0 Success / Message Request 1 Format Error 2 Server Failure 3 Name Error 4 Not Implemented 5 Refused Queries and Responses DNS Header Format
26 DNS Header Message Bits Bit Q A T D R Meaning Query Authoritative Answer Truncated Response Recursion Desired Recursion Available Queries and Responses DNS Header Format
27 DNS Record Formats Question Record Response Record Queries and Responses DNS Record Formats
28 Iterative Query Address for bits-pilani.ac.in? 2 Don t know; Ask ISP DNS Server Queries and Responses Iterative Query Authoritative Name Server Serves both.in. and.ac.in. domains
29 Iterative Query Address for bits-pilani.ac.in? 4 Don t know; Ask the Root Server Queries and Responses Iterative Query Authoritative Name Server Serves both.in. and.ac.in. domains
30 Iterative Query 5 5 Address for bits-pilani.ac.in? 6 Don t know; Ask TLD Server Queries and Responses Iterative Query 6 Authoritative Name Server Serves both.in. and.ac.in. domains
31 Iterative Query 7 7 Address for bits-pilani.ac.in? 8 Don t know; Ask the authoritative name server Queries and Responses Iterative Query 8 Authoritative Name Server Serves both.in. and.ac.in. domains
32 Iterative Query 9 9 Address for bits-pilani.ac.in? 10 Address is Queries and Responses Iterative Query 10 Authoritative Name Server Serves both.in. and.ac.in. domains
33 Recursive Query Serves both.in. and.ac.in. domains Queries and Responses Recursive Query
34 Types of DNS Queries Forward DNS Query Domain name to other resources Uses dedicated forward DNS infrastructure Reverse DNS Query IP address to domain name Uses dedicated reverse DNS infrastructure Inverse DNS Query Resources to domain name Need to know the authoritative name server Queries and Responses Types of DNS Queries
35 Sample Query Header OPCODE=SQUERY Question QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=A Answer <empty> Authority <empty> Additional <empty> Queries and Responses Sample Query
36 Response to Sample Query Header OPCODE=SQUERY, RESPONSE, AA Question QNAME=SRI-NIC.ARPA., QCLASS=IN, QTYPE=A Answer SRI-NIC.ARPA IN A IN A Authority <empty> Additional <empty> Queries and Responses Sample Response
37 Response for Another Query Answer for question: QNAME=BRL.MIL, QTYPE=A Header OPCODE=SQUERY, RESPONSE Question QNAME=BRL.MIL, QCLASS=IN, QTYPE=A Answer <empty> Authority MIL IN NS SRI-NIC.ARPA NS A.ISI.EDU Additional A.ISI.EDU. A SRI-NIC.ARPA. A A Queries and Responses Sample Response
38 Reverse DNS Query for (flickr.com) The ordered list of name servers used for the reverse query - 1. NS: b.root-servers.net [ ] (Marina Del Rey, California, United States) 2. NS: c.in-addr-servers.arpa [ ] (Mauritius) 3. NS: y.arin.net [ ] (Sterling, Virginia, United States) 4. NS: ns3.yahoo.com [ ] (Mumbai, Maharashtra, India) Answer: in-addr.arpa undefined PTR Queries and Responses Reverse DNS Query
39 DNS Caching Performing all these queries takes time And all this before actual communication takes place E.g., 1-second latency before starting Web download Caching can greatly reduce overhead The top-level servers very rarely change Popular sites (e.g., visited often Local DNS server often has the information cached How DNS caching works DNS servers cache responses to queries Responses include a time to live (TTL) field Server deletes cached entry after TTL expires Queries and Responses DNS Caching
40 Presentation Overview Problem Areas Queries and Responses Resource Records Name Space DNS Basics Lecture Outline
41 DNSRedirect Using Wildcards Problem Areas Wild Cards
42 Result of DNS Redirect Problem Areas Wild Cards
43 DDoS Attacks on Root Servers (1) Attacker directs zombies to begin attack (2) All zombies send DNS query for record foo in domain bar.<tld> to open recursive servers and set source IP= (3) Open resolvers ask bar.<tld> for record foo (5) Open resolvers send DNS response to target name server (4) bar.<tld> responds with record foo (4000 byte DNS TXT RR) Problem Areas DDoS Attacks
44 Other Problems Domain Squatting (cyber squatting) DNS Cache poisoning DNS ID Spoofing Mobility Dynamic DNS Updates Web DNS Updates Security Problem Areas Other Problems
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