DNS: Domain Name System

Transcription

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

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: how to map between IP address and name, and vice versa? Domain Name System: v distributed database implemented in hierarchy of many name servers v application-layer protocol: hosts, name servers communicate to resolve names à addresses note: core Internet function, implemented as applicationlayer protocol complexity at network s edge Application Layer 2-2

3 DNS: services, structure DNS services v Resolution hostname à IP address v Aliasing canonical, alias names mail server aliasing v Load balancing with replicated web servers: many addresses map to one name why not centralize DNS? v single point of failure v traffic volume v distant centralized database v maintenance doesn t scale! Application Layer 2-3

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 /16 have domain name suffix umass.edu All hosts on network /24 are in the School of Computer Science at UMass Amherst.

6 DNS Name hierarchy DNS hierarchy can be represented by a tree Root and top-level domains are administered by an Internet central name registration authority (ICANN) org edu. (root) gov com Top-level Domains Below top-level domain, administration of name space is delegated to organizations Each organization can delegate further uci.edu math.toronto.edu toronto.edu ece.toronto.edu Managed by UofT Managed by ECE Dept. neon.ece.toronto.edu

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). edu virginia.edu Example: DNS domains are., edu, virginia.edu, cs.virginia.edu cs.virginia.edu neon.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 com edu gov int mil net org Commercial organizations Educational institutions Government institutions International organizations U.S. military institutions Networking organizations Non-profit organizations

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

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) Zone. (root).edu.uci.edu math.virginia.edu.virginia.edu cs.virginia.edu A server can divide part of its zone and delegate it to other servers Zone and domain Domain

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 DNS resolution: distributed, hierarchical Top-level domain servers Root DNS Servers com DNS servers org DNS servers edu DNS servers yahoo.com DNS servers amazon.com DNS servers Authoritative name servers pbs.org DNS servers poly.edu umass.edu DNS servers DNS servers client wants IP for 1 st approx: v client queries root server to find.com TLD DNS server v client queries.com TLD DNS server for amazon.com auth server v client queries amazon.com DNS auth server to get IP address for Application Layer 2-14

15 DNS: root name servers v contacted when no info about top-level or auth server v root name server can: return top-level or auth name server address or contact auth server and return final resolved address e. NASA Mt View, CA f. Internet Software C. Palo Alto, CA (and 48 other sites) c. Cogent, Herndon, VA (5 other sites) d. U Maryland College Park, MD h. ARL Aberdeen, MD j. Verisign, Dulles VA (69 other sites ) 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) k. RIPE London (17 other sites) i. Netnod, Stockholm (37 other sites) m. WIDE Tokyo (5 other sites) 13 root name servers worldwide Application Layer 2-15

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 2-16

17 Local DNS name server v does not strictly belong to hierarchy v deployed by ISP (residential, company, university) also called default name server v 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 2-17

18 DNS name resolution example root DNS server v host at cis.poly.edu wants IP address for gaia.cs.umass.edu TLD DNS server iterated query: v contacted server replies with name of server to contact v I don t know this name, but ask this server local DNS server dns.poly.edu 1 8 requesting host cis.poly.edu 7 6 authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu Application Layer 2-18

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

20 DNS: caching, updating records v 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 v 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 v update/notify mechanisms proposed IETF standard RFC 2136 Application Layer 2-20

21 DNS records DNS: distributed db storing resource records (RR) RR format: (name, value, type, ttl) type=a name is hostname value is IP address type=ns name is domain (e.g., foo.com) value is hostname of authoritative name server for this domain type=cname name is alias name for some canonical (the real) name is really servereast.backup2.ibm.com value is canonical name type=mx value is name of mailserver associated with name Application Layer 2-21

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

23 DNS protocol, messages 2 bytes 2 bytes identification # questions flags # answer RRs name, type fields for a query RRs in response to query records for authoritative servers additional helpful info that may be used # authority RRs # additional RRs questions (variable # of questions) answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs) Application Layer 2-23

24 Inserting records into DNS v example: new startup Network Utopia v 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, , A) v create authoritative server type A record for type MX record for networkutopia.com Application Layer 2-24

25 Resource Records Resource records are stored in configuration files (zone files) at name servers. Example resource records for a zone: db.mylab.com $TTL mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial ; refresh 7200 ; retry ; expire ; ttl ) ; mylab.com. IN NS PC4.mylab.com. ; localhost A PC4.mylab.com. A PC3.mylab.com. A PC2.mylab.com. A PC1.mylab.com. A

26 Resource Records db.mylab.com $TTL mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial ; refresh 7200 ; retry ; expire ; ttl ) ; mylab.com. IN NS PC4.mylab.com. ; localhost A PC4.mylab.com. A PC3.mylab.com. A PC2.mylab.com. A PC1.mylab.com. A 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 * [email protected] is the 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) ConfiguraEon: Router2(config)#ip nat inside source static PC4% route add net netmask gw Which ping works and why? PC3% ping c PC3% ping c Router1% ping c Router1% ping c PC4% ping c PC4% ping c NAT Table on Router2 Router2#show ip nat translations Pro Inside global Inside local Outside local Outside global

29 Exercise 1(B) ConfiguraEon: Router2(config)#ip nat inside source static PC4% route add net netmask gw Which ping works and why? PC3% ping c PC3% ping c Router1% ping c Router1% ping c PC4% ping c PC4% ping c NAT Table on Router2 Router2#show ip nat translations Pro Inside global Inside local Outside local Outside global

30 Exercise 1(B) ConfiguraEon: Router2(config)#ip nat inside source static Router2(config)#ip nat inside source static Router2(config)#ip nat inside source static Which ping works and why? PC3% ping c PC3% ping c Router1% ping c Router1% ping c PC4% ping c PC4% ping c NAT Table on Router2 Router2#show ip nat translations Pro Inside global Inside local Outside local Outside global

31 Exercise 1(B) ConfiguraEon: Router2(config)#ip nat inside source static Router2(config)#ip nat inside source static Router2(config)#ip nat inside source static Which ping works and why? PC3% ping c PC3% ping c Router1% ping c Router1% ping c PC4% ping c PC4% ping c NAT Table on Router2 Router2#show ip nat translations Pro Inside global Inside local Outside local Outside global

32 Exercise 1(B) Show IP source/desenaeon addresses before/aoer Router2 PC3% ping c Before Router2: Src: ( ), Dst: ( ) AOer Router2: Src: ( ), Dst: ( ) NAT Table on Router2 Router2#show ip nat translations Pro Inside global Inside local Outside local Outside global

33 Exercise 1(C)- NAT/PAT/Masquerade telnet commands; which one successful? PC1% telnet (Router1) PC1% telnet (PC4) Router1# telnet (PC1) Router1# (PC4) PC4: telnet (PC3) 33

34 Exercise 1(C)- NAT/PAT/Masquerade telnet commands; which one successful? PC1% telnet (Router1) PC1% telnet (PC4) Router1# telnet (PC1) Router1# (PC4) PC4: telnet (PC3) 34

35 Exercise 1(C)- NAT & telnet PC1% telnet (PC4) Before translaeon (PC2) Internet Protocol Source: Destination: Transmission Control Protocol Source port: Destination port: telnet (23) Sequence number: AOer translaeon (PC2) Internet Protocol Source: Destination: Transmission Control Protocol Source port: Destination port: telnet (23) Sequence number:

36 Exercise 1(C)- PAT & ICMP (ping) Ping (ICMP) does not use port number IdenEficaEon is used to help with NAT PC1% ping (PC4) Internet Protocol, Src Addr: , Dst Addr: Identification: 0x0000 Protocol: ICMP (0x01) Source: Destination: Internet Protocol, Src Addr: , Dst Addr: Identification: 0x0000 Protocol: ICMP (0x01) Source: Destination:

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

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