1. Lecture 10 Doman Name System CPE 401 / 601 Computer Network Systems slides are modified from Dave Hollinger and Kurose & Ross

2. Hostnames People: many identifiers:  SSN, name, passport # Internet hosts, routers:  IP address (32/128 bit) - used for addressing datagrams  “name”, e.g., ww.yahoo.com - used by humans  http://www.youtube.com/watch?v=9ilYrY9iBhk http://www.youtube.com/watch?v=9ilYrY9iBhk  IP Addresses are great for computers  IP address includes information used for routing.  IP addresses are tough for humans to remember.  IP addresses are impossible to guess.  ever guessed at the name of a WWW site? DNS 2

3. Domain Name System  The domain name system is usually used to translate a host name into an IP address.  Domain names comprise a hierarchy so that names are unique, yet easy to remember.  distributed database  implemented in hierarchy of many name servers  hosts, routers, communicate with name servers to resolve names (address/name translation) DNS 3

4. Host name structure  Each host name is made up of a sequence of labels separated by periods.  Each label can be up to 63 characters  The total name can be at most 255 characters.  Examples:  whitehouse.gov  barney.the.purple.dinosaur.com  cnl.cse.unr.edu DNS 4

5. Domain Name  The domain name for a host is the sequence of labels that lead from the host (leaf node in the naming tree) to the top of the worldwide naming tree.  A domain is a subtree of the worldwide naming tree. DNS 5 Root DNS Servers com DNS servers org DNS serversedu DNS servers unr.edu DNS servers dri.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers

6. DNS services  hostname to IP address translation  host aliasing  Canonical, alias names  mail server aliasing  load distribution  replicated Web servers: set of IP addresses for one canonical name Why not centralize DNS?  single point of failure  traffic volume  distant centralized database  maintenance DNS 6 doesn’t scale!

7. Distributed, Hierarchical Database Client wants IP for www.unr.edu; 1 st approx:  client queries a root server to find edu DNS server  client queries edu DNS server to get unr.edu DNS server  client queries unr.edu DNS server to get IP address for www.unr.edu DNS 7

8. DNS: Root name servers  contacted by local name server that can not resolve name  root name server:  contacts authoritative name server if name mapping not known  gets mapping  returns mapping to local name server DNS 8 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 36 other locations) i Autonomica, Stockholm (plus 28 other locations) k RIPE London (also 16 other locations) m WIDE Tokyo (also Seoul, Paris, SF) a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations)

9. Top-Level Domain Servers  responsible for com, org, net, edu, gov, …  Network Solutions maintains servers for com TLD  Educause for edu TLD  Countries each have a top level domain  2 letter domain name  uk, fr, ca, jp, …  New top level domains include.aero.biz.coop.info.name.pro DNS 9

10. Authoritative Servers  organization’s DNS servers,  providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).  The organization that owns a domain name is responsible for running a DNS server that can provide the mapping between hostnames within the domain to IP addresses.  So - some machine run by UNR is responsible for everything within the rpi.edu domain.  can be maintained by organization or service provider DNS 10

11. DNS Distributed Database  There is one primary server for a domain, and typically a number of secondary servers containing replicated databases. UNR 11 rpi.edu DNS DB rpi.edu DNS DB unr.edu DNS DB Authoritative unr.edu DNS DB Replicas unr.edu DNS server

12. Local Name 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  acts as proxy, forwards query into hierarchy DNS 12

13. DNS: caching and updating records  once (any) name server learns mapping, it caches mapping  cache entries timeout (disappear) after some time  TLD servers typically cached in local name servers Thus root name servers not often visited  DNS Cache poisoning attacks  http://www.youtube.com/watch?v=1d1tUefYn4U http://www.youtube.com/watch?v=1d1tUefYn4U  update/notify mechanisms under design by IETF  RFC 2136  http://www.ietf.org/html.charters/dnsind-charter.html DNS 13

14. 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 DNS 14

16. DNS Clients  A DNS client is called a resolver.  A call to gethostbyname() is handled by a resolver (typically part of the client).  Most Unix workstations have the file /etc/resolv.conf that contains the local domain and the addresses of DNS servers for that domain. DNS 16

17. /etc/resolv.conf search cse.unr.edu me.unr.edu engr.unr.edu ce.unr.edu it.unr.edu unr.edu nameserver 134.197.5.1 nameserver 134.197.6.1 DNS 17

18. nslookup  nslookup is an interactive resolver that allows the user to communicate directly with a DNS server.  nslookup is usually available on Unix workstations.  dig and host are also DNS clients DNS 18

19. DNS Servers  Servers handle requests for their domain directly.  Servers handle requests for other domains by contacting remote DNS server(s).  Servers cache external mappings. DNS 19

20. Server - Server Communication  If a server is asked to provide the mapping for a host outside it’s domain (and the mapping is not in the server cache):  The server finds a nameserver for the target domain.  The server asks the nameserver to provide the host name to IP translation.  To find the right nameserver, use DNS! DNS 20

21. DNS Data  DNS databases contain more than just hostname-to-address records:  Name server recordsNS  Hostname aliases CNAME  Mail ExchangersMX  Host InformationHINFO DNS 21

22. Server Operation  If a server has no clue about where to find the address for a hostname, ask the root server.  The root server will tell you what nameserver to contact.  A request may get forwarded a few times. DNS 22

23. DNS name resolution example  Host at cse.unr.edu wants IP address for rec.dri.edu DNS 23 requesting host cse.unr.edu rec.dri.edu root DNS server local DNS server ns1.unr.edu 1 2 3 4 5 6 authoritative DNS server dns.dri.edu 7 8 TLD DNS server iterated query: rcontacted server replies with name of server to contact r“I don’t know this name, but ask this server”

24. Recursion  A request can indicate that recursion is desired - this tells the server to find out the answer (possibly by contacting other servers).  If recursion is not requested - the response may be a list of other name servers to contact. DNS 24

25. DNS name resolution example DNS 25 requesting host cse.unr.edu rec.dri.edu root DNS server local DNS server ns1.unr.edu 1 2 4 5 6 authoritative DNS server dns.dri.edu 7 8 TLD DNS server 3 recursive query: rputs burden of name resolution on contacted name server rheavy load?

26. UDP & TCP  Both UDP and TCP are used:  TCP for transfers of entire database to secondary servers (replication)  UDP for lookups  If more than 512 bytes in response requestor resubmits request using TCP. DNS 26

27. DNS records DNS: distributed db storing resource records (RR) DNS 27 RR format: (name, value, type, ttl) rType=A  name is hostname  value is IP address rType=CNAME  name is alias name for some “canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com  value is canonical name rType=MX  value is name of mailserver associated with name rType=NS  name is domain (e.g. foo.com)  value is hostname of authoritative name server for this domain rType=AAA

28. DNS protocol, messages DNS protocol : query and reply messages, both with same message format DNS 28 ridentification: 16 bit # for query, reply to query uses same # rflags: QR: Query=0, Response=1 AA: Authoritative Answer TC: response truncated (> 512 bytes) RD: recursion desired RA: recursion available rcode: return code Response

29. DNS protocol, messages DNS 29 Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used

30. Name to Address Conversion  There is a library of functions that act as DNS client (resolver).  you don’t need to write DNS client code to use DNS!  With some OSs you need to explicitly link with the DNS resolver library: -lnsl ( nsl is “Name Server Library”) DNS 30 Suns (Solaris) need this!

31. DNS library functions gethostbyname gethostbyaddr gethostbyname2 DNS 31 IPV6!

32. gethostbyname  Return IP addresses of a host struct hostent *gethostbyname( const char *hostname);  struct hostent is defined in netdb.h:  All the IP addresses returned via the hostent are in network byte order! DNS 32

33. hostent picture h_name h_aliases h_addrtype h_length h_addr_list DNS 33 alias 1 alias 2 null Official Name IP address 1 IP address 2 null

34. Which Address?  On success, gethostbyname returns the address of a hostent that has been created.  has an array of ptrs to IP addresses  Usually use the first one: #define h_addr h_addr_list[0] DNS 34

35. gethostbyname and errors  On error gethostbyname return null.  Gethostbyname sets the global variable h_errno to indicate the exact error:  HOST_NOT_FOUND  TRY_AGAIN  NO_RECOVERY  NO_DATA  NO_ADDRESS DNS 35 All defined in netdb.h

36. Getting at the address: char **h_addr_list; h = gethostbyname("joe.com"); sockaddr.sin_addr.s_addr = *(h->h_addr_list[0]); This won't work!!!! h_addr_list[0] is a char* ! DNS 36

37. Using memcpy  You can copy the 4 bytes (IPv4) directly: h = gethostbyname("joe.com"); memcpy(&sockaddr.sin_addr, h->h_addr_list[0], sizeof(struct in_addr)); DNS 37

38. gethostbyaddr  Return hostname corresponding to an IP. struct hostent *gethostbyaddr( const char *addr size_t len, int family); DNS 38 AF_INET (could be AF_INET6) sizeof(struct in_addr)

39. Some other functions uname get hostname of local host getservbyname get port number for a named service getservbyaddr get name for service associated with a port number DNS 39