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DNS: Domain Name System

DNS: Domain Name System. CMPSCI 491G: Computer Networking Lab V. Arun. Slides adapted from Liebeherr & Zarki , Kurose & Ross, Kermani. Domain Name System: distributed database implemented in hierarchy of many name servers

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DNS: Domain Name System

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  1. DNS: Domain Name System CMPSCI 491G: Computer Networking Lab V. Arun Slides adapted from Liebeherr & Zarki, Kurose & Ross, Kermani

  2. Domain Name System: distributed database implemented in hierarchy of many name servers application-layer protocol: hosts, name servers communicate to resolvenames  addresses note: core Internet function, implemented as application-layer protocol complexity at network’s “edge” people: many identifiers: SSN, name, passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., www.yahoo.com - used by humans Q: how to map between IP address and name, and vice versa ? DNS: domain name system Application Layer

  3. why not centralize DNS? single point of failure traffic volume distant centralized database maintenance DNS services Resolution hostname  IP address Aliasing canonical, alias names mail server aliasing Load balancing with replicated web servers: many addresses map to one name DNS: services, structure doesn’t scale! Application Layer

  4. Before there was DNS …. …. there was the HOSTS.TXT file • Before DNS (until 1985), name resolution was done by FTP’ing a single file (hosts.txt) from a central server. • Names in hosts.txt are not structured. • hosts.txt still works on most operating systems. It can be used to define local names.

  5. Design principle of DNS • DNS naming system based on a hierarchical and logical tree structure called domain namespace. • An organization obtains authority for parts of the name space, and can add additional layers of the hierarchy • Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system • In practice, allocation of the domain names generally follows the allocation of IP address, e.g., • All hosts with network prefix 128.143/16 have domain name suffix virginia.edu • All hosts on network 128.143.136/24 are in the Computer Science Department of the University of Virginia

  6. DNS hierarchy can be represented by a tree Root and top-level domains are administered by an Internet central name registration authority (ICANN) Below top-level domain, administration of name space is delegated to organizations Each organization can delegate further Managed by UofT Managed by ECE Dept. DNS Name hierarchy

  7. Domain name system • Each node in the DNS tree represents a DNS name • Each branch below a node is a DNS domain. • DNS domain can contain hosts or other domains (subdomains) • Example: DNS domains are ., edu, virginia.edu, cs.virginia.edu

  8. Top-level domains • Three types of top-level domains: • Organizational: 3-character code indicates the function of the organization • Used primarily within the US • Examples: gov, mil, edu, org, com, net • Geographical: 2-character country or region code • Examples: us, va, jp, de • Expanded top-level domains (gTLDs) • Essentially arbitrary TLDs • Reverse domains: A special domain (in-addr.arpa) used for IP address-to-name mapping

  9. Organizational top-level domains

  10. Hierarchy of name servers • The resolution of the hierarchical name space is done by a hierarchy of name servers • Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone. • Zone is a part of the subtree • DNS server answers queries about hosts in its zone

  11. Authority and delegation • Authority for the root domain is with the Internet Corporation for Assigned Numbers and Names (ICANN) • ICANN delegates to accredited registrars (for gTLDs) and countries for country code top level domains (ccTLDs) • Authority can be delegated further • Chain of delegation can be obtained by reading domain name from right to left. • Unit of delegation is a “zone”.

  12. DNS domain and zones • Each zone is anchored at a specific domain node, but zones are not domains. • A DNS domain is a branch of the namespace • A zone is a portion of the DNS namespace generally stored in a file (could consist of multiple nodes) • A server can divide part of its zone and delegate it to other servers

  13. Primary and secondary name servers • For each zone, there must be a primary name server and a secondary name server • The primary server (master server) maintains a zone file which has information about the zone. Updates are made to the primary server • The secondary server copies data stored at the primary server. Adding a host: • When a new host is added (“gold.cs.virginia.edu”) to a zone, the administrator adds the IP information on the host (IP address and name) to a configuration file on the primary server

  14. Root DNS Servers org DNS servers edu DNS servers com DNS servers poly.edu DNS servers umass.edu DNS servers pbs.org DNS servers yahoo.com DNS servers amazon.com DNS servers DNS resolution: distributed, hierarchical Top-level domain servers … … Authoritative name servers client wants IP for www.amazon.com; 1st approx: • client queries root server to find .com TLD DNS server • client queries .com TLD DNS server for amazon.com auth server • client queries amazon.com DNS auth server to get IP address for www.amazon.com Application Layer

  15. contacted when no info about top-level or auth server root name server can: return top-level or auth name server address or contact auth server and return final resolved address DNS: root name servers c. Cogent, Herndon, VA (5 other sites) d. U Maryland College Park, MD h. ARL Aberdeen, MD j. Verisign, Dulles VA (69 other sites ) k. RIPE London (17 other sites) i. Netnod, Stockholm (37 other sites) m. WIDE Tokyo (5 other sites) e. NASA Mt View, CA f. Internet Software C. Palo Alto, CA (and 48 other sites) 13 root name “servers” worldwide a. Verisign, Los Angeles CA (5 other sites) b. USC-ISI Marina del Rey, CA l. ICANN Los Angeles, CA (41 other sites) g. US DoD Columbus, OH (5 other sites) Application Layer

  16. TLD, authoritative servers top-level domain (TLD) servers: • responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp • Network Solutions maintains servers for .com TLD • Educause for .edu TLD authoritative DNS servers: • organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts • can be maintained by organization or service provider Application Layer

  17. Local DNS name server • does not strictly belong to hierarchy • deployed by ISP (residential, company, university) • also called “default name server” • acts as proxy between host and DNS hierarchy • has local cache of recent name-to-address translation pairs (but may be out of date!) Application Layer

  18. host at cis.poly.edu wants IP address for gaia.cs.umass.edu local DNS server dns.poly.edu DNS name resolution example root DNS server 2 3 TLD DNS server 4 5 iterated query: • contacted server replies with name of server to contact • “I don’t know this name, but ask this server” 6 7 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu Application Layer

  19. local DNS server dns.poly.edu DNS name resolution example root DNS server 3 2 7 recursive query: • puts burden of name resolution on contacted name server • heavy load at upper levels of hierarchy? 6 TLD DNS server 4 5 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu Application Layer

  20. any name server can cache learned mappings cache entries timeout (disappear) after some time (TTL) TLD servers typically cached in local name servers, so root name servers not often visited cached entries may be out-of-date (best effort name-to-address translation!) if name host changes IP address, may not be known Internet-wide until all TTLs expire update/notify mechanisms proposed IETF standard RFC 2136 DNS: caching, updating records Application Layer

  21. DNS: distributed db storing resource records (RR) type=NS name is domain (e.g., foo.com) value is hostname of authoritative name server for this domain DNS records RR format:(name, value, type, ttl) type=A • nameis hostname • valueis IP address type=CNAME • name is alias name for some “canonical” (the real) name • www.ibm.comis really servereast.backup2.ibm.com • valueis canonical name type=MX • valueis name of mailserver associated withname Application Layer

  22. queryand reply messages, both with same message format 2 bytes 2 bytes identification flags # questions # answer RRs # additional RRs # authority RRs questions (variable # of questions) answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs) DNS protocol, messages msg header • identification: 16 bit # for query, reply to query uses same # • flags: • query or reply • recursion desired • recursion available • reply is authoritative Application Layer

  23. 2 bytes 2 bytes identification flags # questions # answer RRs # additional RRs # authority RRs questions (variable # of questions) answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs) 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 Application Layer

  24. Inserting records into DNS • example: new startup “Network Utopia” • register name networkuptopia.com at DNS registrar (e.g., Network Solutions) • provide names, IP addresses of authoritative name server (primary and secondary) • registrar inserts two RRs into .com TLD server:(networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, 212.212.212.1, A) • create authoritative server type A record for www.networkuptopia.com; type MX record for networkutopia.com Application Layer

  25. Resource Records • The database records of the distributed data base are called resource records (RR) • Resource records are stored in configuration files (zone files) at name servers. • Resource records for a zone:

  26. Resource Records Max. age of cached data in seconds * Start of authority (SOA) record. Means: “This name server is authoritative for the zone Mylab.com” * PC4.mylab.com is the name server * hostmaster@mylab.com is the email address of the person in charge Name server (NS) record. One entry for each authoritative name server Address (A) records. One entry for each hostaddress

  27. Lab 7 (DHCP/NAT) review

  28. Exercise 1(B)-3 Configuration: Router2(config)#ip nat inside source static 10.0.1.2 200.0.0.2 .. .. PC4% route add –net 200.0.0.0 netmask 255.255.255.0 gw 128.195.7.32 Which ping works and why? PC3% ping –c3 10.0.1.3 PC3% ping –c3 128.143.136.1 Router3% ping –c3 10.0.1.2 Router3% ping –c3 128.143.136.1 PC4% ping –c3 10.0.1.2 PC4% ping –c3 200.0.0.2 NAT Table on Router2 Router2#show ipnat translations Pro Inside global Inside local Outside local Outside global --- 200.0.0.2 10.0.1.2 --- ---

  29. Exercise 1(B)-3 Configuration: Router2(config)#ip nat inside source static 10.0.1.2 200.0.0.2 .. .. PC4% route add –net 200.0.0.0 netmask 255.255.255.0 gw 128.195.7.32 Which ping works and why? PC3% ping –c3 10.0.1.3 PC3% ping –c3 128.143.136.1 Router3% ping –c3 10.0.1.2 Router3% ping –c3 128.143.136.1 PC4% ping –c3 10.0.1.2 PC4% ping –c3 200.0.0.2 NAT Table on Router2 Router2#show ipnat translations Pro Inside global Inside local Outside local Outside global --- 200.0.0.2 10.0.1.2 --- ---

  30. Exercise 1(B)-4 Configuration: Router2(config)#ipnat inside source static 10.0.1.2 200.0.0.2 Router2(config)#ipnat inside source static 10.0.1.1 200.0.0.1 Router2(config)#ipnat inside source static 10.0.1.3 200.0.0.3 Which ping works and why? PC3% ping –c3 10.0.1.3 PC3% ping –c3 128.143.136.1 Router3% ping –c3 10.0.1.2 Router3% ping –c3 128.143.136.1 PC4% ping –c3 10.0.1.2 PC4% ping –c3 200.0.0.2 NAT Table on Router2 Router2#show ipnat translations Pro Inside global Inside local Outside local Outside global --- 200.0.0.1 10.0.1.1 --- --- --- 200.0.0.2 10.0.1.2 --- --- --- 200.0.0.3 10.0.1.3 --- ---

  31. Exercise 1(B)-4 Configuration: Router2(config)#ipnat inside source static 10.0.1.2 200.0.0.2 Router2(config)#ipnat inside source static 10.0.1.1 200.0.0.1 Router2(config)#ipnat inside source static 10.0.1.3 200.0.0.3 Which ping works and why? PC3% ping –c3 10.0.1.3 PC3% ping –c3 128.143.136.1 Router3% ping –c3 10.0.1.2 Router3% ping –c3 128.143.136.1 PC4% ping –c3 10.0.1.2 PC4% ping –c3 200.0.0.2 NAT Table on Router2 Router2#show ipnat translations Pro Inside global Inside local Outside local Outside global --- 200.0.0.1 10.0.1.1 --- --- --- 200.0.0.2 10.0.1.2 --- --- --- 200.0.0.3 10.0.1.3 --- ---

  32. Exercise 1(B) Show IP source/destination addresses before/after Router2 PC3% ping –c3 128.143.136.1 Before Router2: Src: 10.0.1.2 (10.0.1.2), Dst: 128.143.136.1 (128.143.136.1) After Router2: Src: 200.0.0.2 (200.0.0.2), Dst: 128.143.136.1 (128.143.136.1) NAT Table on Router2 Router2#show ipnat translations Pro Inside global Inside local Outside local Outside global --- 200.0.0.1 10.0.1.1 --- --- --- 200.0.0.2 10.0.1.2 --- --- --- 200.0.0.3 10.0.1.3 --- ---

  33. Exercise 1(C)- NAT/PAT/Masquerade telnet commands; which one successful? PC1% telnet 10.0.1.3 (Router1) PC1% telnet 128.143.136.1 (PC4) Router1# telnet 10.0.1.2 (PC1) Router1# 128.143.136.1 (PC4) PC4: telnet 10.0.1.2 (Router2)

  34. Exercise 1(C)- NAT/PAT/Masquerade telnet commands; which one successful? PC1% telnet 10.0.1.3 (Router2) PC1% telnet 128.143.136.1 (PC4) Router1# telnet 10.0.1.2 (PC1) Router1# 128.143.136.1 (PC4) PC4: telnet 10.0.1.2 (Router2)

  35. Exercise 1(C)- NAT & telnet PC1% telnet 128.143.136.1 (PC4) Before translation (PC2) Internet Protocol Source: 10.0.1.2 Destination: 128.143.136.1 Transmission Control Protocol Source port: 32774 Destination port: telnet (23) Sequence number: 1857633137 After translation (PC2) Internet Protocol Source: 128.143.136.22 Destination: 128.143.136.1 Transmission Control Protocol Source port: 32774 Destination port: telnet (23) Sequence number: 1857633137

  36. Exercise 1(C)- PAT & ICMP (ping) • Ping (ICMP) does not use port number • “Identification” is used to help with NAT PC1% ping 128.143.136.1 (PC4) Internet Protocol, SrcAddr: 10.0.1.2, DstAddr: 128.143.136.1 Identification: 0x0000 Protocol: ICMP (0x01) Source: 10.0.1.2 Destination: 128.143.136.1 Internet Protocol, SrcAddr: 128.143.136.22, DstAddr: 128.143.136.1 Identification: 0x0000 Protocol: ICMP (0x01) Source: 128.143.136.22 Destination: 128.143.136.1

  37. Exercise 1(D)- NAT & FTP • FTP uses 2 ports • Control connection, port 21 • Data connection port 20 • No problem with NAT & control connection. • For data connection, the server initiates a connection from its port 20 to a (random) port on client • Causes problem with NAT • Only client can initiate connection • PASSIVE mode solves this problem

  38. Exercise 1(D)- NAT & FTP PC3% ftp 128.143.136.22 (PC2)

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