Advanced Networking. Domain Name System

Transcription

1 Advanced Networking Domain Name System

2 Purpose of DNS servers Human being has many identifications: 1) Our name can be used for identification Problem: Two differenet people may have same name. 2) Mobile numbers can be used for identification Hello! I am , meet my wife ! Problem: What will happen if SIM card is lost? 3) Driving Licence can be used for identification Again you have to remember some numbers for each people. Population of Bangladesh is 15 Crore!

3 Purpose of DNS servers Same thing for Internet hosts! Human readable name, easy to rember 1) 2) They also have unique IP addresses: 1) ) Problem: Human beings prefer english names, Internet routers and other hosts on Internet prefer IP addresses (because those are of fixed length!)

4 Purpose of DNS servers Solution: Use both human readable names and IP addresses with a mechanism for translation between them. Translate host name to IP address

5 Early Translation Mechanisms Provide a host.txt file on every host with two columns (IP address, Host name) Sample host.txt file router.foo.com router dns.foo.com dns mail2.foo.com www ftp mail2 dns mail.foo.com mail pc1.foo.com pc pc2.foo.com pc2 sysman

6 Problem with early solutions! Early solution is OK with a network consisting of few hundreds of hosts! Not suitable for today s Internet, which is A network of networks with about twenty million hosts 10 % growth per year

7 How Internet looks like? (Captured on January 2009)

8 How Internet looks like?

9 What is the solution then? Distribute the translation database all over the Internet

10 Name space Needs an unambiguous English name for every hosts Name Name Space Space Flat Flat Name Name Space Space Hierarchical Name Name Space Space Does not have any structure Any host may take any name Problem: How to avoid duplicity?

11 Hierarchical name space Name is made of several parts. First part: The nature of organization i.e. whether education, profitable/non-profitable etc Next part can be name of the organization Next part can be a department name within the organization And so on. Example: Universities Educational Institute Amazon (book seller) Profit-making organization Within a university: Several departments Computer Science, Electrical engineering, communication engineering etc...

12 Hierarchical name space Benefits of Hierarchical name space: Authority to assign and control the name space can be decentralized i.e. A particular entity can control all educational institute another body can control all profit-making organizations etc Easy to provide unambiguous names: For example, Both CS and EEE department can have a computer named challenger within UIU, the name of two computers may be: challenger.cs.uiu.edu challenger.eee.uiu.edu

13 Domain Name Space To have a hierarchical name space, a domain name space was designed An inverted-tree structure with the root at top Each node in the tree has a label root

14 Fully qualified domain names (FQDN) ATC: Advanced technology Center FHDA: Foothill De Anza Community college

15 Domain A domain is is a sub sub tree tree of of a larger larger tree tree identified by by a domain name name

16 Domain illustrated root domain TLDs & cctlds 2lds 3lds 4lds

17 DNS vs File System

18 Naming a Domain Naming a Directory Start Here Start Here yahoo.com.au. A. is used as separator C:\windows\system32\d rivers\ A \ is used as separator

19 Domain Illustrated Domain com.au The ausregistry.com.au sub-domain of com.au

20 Distribution of Domain name space Now the host.txt file will look like (with FQDN) as follows: router.foo.com router dns.foo.com dns mail2.foo.com www ftp mail2 dns mail.foo.com mail pc1.foo.com pc pc2.foo.com pc2 sysman Where to to store this information?

21 Centralized solution Store it in a single machine in the Internet with wellknown IP address Problem A single point of failure (if it fails, so the Internet!) Traffic volume A single server needs to handle all requests (near-impossible solution) Distant centralized database A single server can not be close to all hosts Difficulty of Maintenance Huge database, frequent update

22 Decentralized solution

23 Three category of servers DNS DNS Servers Servers Root Root DNS DNS servers servers Top Top level level DNS DNS servers servers (TLD) (TLD) Authoritative DNS DNS servers servers

24 Root servers Those are the starting point for name resolution 13 root servers geographically spread over the world Those have previously set well-known IP addresses Contain the IP address of TLD servers

25 Location of Root servers

26 Root servers IP address Full info at ftp://rs.internic.net/domain/named.cache A.ROOT- SERVERS.NET A B.ROOT- SERVERS.NET A C.ROOT- SERVERS.NET A D.ROOT- SERVERS.NET A E.ROOT- SERVERS.NET A F.ROOT- SERVERS.NET A G.ROOT- SERVERS.NET A H.ROOT- SERVERS.NET A I.ROOT- SERVERS.NET A

27 Top-level Domain servers (TLD) Responsible for com, org, net, edu, etc, and all top- level country domains uk, fr, ca, jp. Network solutions maintains servers for com TLD Educause for edu TLD Contain IP address of the lower domain s DNS server s IP addresses

28 Authoritative DNS servers Organization s DNS servers, Providing authoritative hostname to IP mappings for organization s servers (e.g., Web and mail) and other hosts. Can be maintained by organization or service provider

29 What happens when you type Step 1: Your PC sends a resolution request to its configured DNS Server, typically at your ISP. Tell me the Address of

30 Step 2: Your ISPs recursive name server starts by asking one of the root servers predefined in its hints file. Tell me the Address of I don t know the address but I know who s authoritative for the com domain ask them

31 Step 3: Your ISPs recursive name server then asks one of the com name servers as directed. Tell me the Address of I don t know the address but I know who s authoritative for the google.com domain ask them

32 Step 5: ISP DNS server then send the answer back to your PC. The DNS server will remember the answer for a period of time. The Address of is

33 Step 6: Your PC can then make the actual HTTP request to the web server. Send me the web page Here it is!

34 Summary The actual web request DNS

35 Let s dirty our hand Type the followings to get IP address of To know the IP address of a root server enter: dig -t ANY. Answer: To know the IP address of a TLD server for com domain enter: dig -t com Answer: To know the IP address of an Authoritative DNS server for google.com domain type: dig -t google.com Answer: To know the IP address of host type: dig -t Answer: On Internet Explorer type

36 Two ways of sending DNS query DNS DNS Query Query Sending Approaches Iterative queries queries Recursive queries queries

37 Iterative queries

38 Recursive queries

39 The A type record The Address record One or more normally defines a host Contains an IPv4 Address (the address computers use to uniquely identify each other on the internet) Eg. The record: A In the ausregistry.com.au domain, defines the host uniquely identifiable as to be reachable at the IPv4 Address

40 DNS records DNS: distributed db storing resource records (RR) Resource records are analogous to files RR format: (name, value, type, ttl) Four Four types types of of Resource Records A A type type NS NS type type CNAME type type MX MX type type

41 The NS type record An NS record defines the authoritative Name servers for the domain. The Name Server records also define the name servers of children domains Eg. The record: google.com IN NS ns1.google.com. google.com IN NS ns2.google.com. google.com IN NS ns4.google.com. google.com IN NS ns3.google.com.

42 The CNAME type record A CNAME defines an alias The alias will then be resolved, if another CNAME is encountered then the process continues until an A record is found Normally used for load balancing purpose, or two or more server pointing to same host Example: IN CNAME IN A IN A IN A IN A IN A IN A

43 The MX type record An MX record defines the mail servers for a particular domain Mail exchange records hold the name of hosts, and their priorities, able to deliver mail for the domain. buet.ac.bd IN MX 5 dakghar.buet.ac.bd. buet.ac.bd IN MX 10 dakpeon.buet.ac.bd.

44 Case Study Domain: foo.com IP address: mail.foo.com dns.foo.com router.foo.com Mail server DNS server Router ============ Internet ============================================ LAN User # System Manager WWW/FTP server; backup DNS/mail - pc1.foo.com pc2.foo.com mail2.foo.com Alias: dns2.foo.com sysman.foo.com Aliases: ftp.foo.com,

45 Resource Records on DNS of foo.com domain ; Name Server (NS) records. foo.com. IN NS dns.foo.com. foo.com. IN NS dns2.foo.com. ; Mail Exchange (MX) records. foo.com. IN MX 20 mail.foo.com. foo.com. IN MX 40 mail2.foo.com. ; Address (A) records. router.foo.com. IN A dns.foo.com. IN A mail2.foo.com. IN A dns2.foo.com. IN A mail.foo.com. IN A pc1.foo.com. IN A pc2.foo.com. IN A ; Aliases in Canonical Name (CNAME) records... ftp.foo.com. IN CNAME mail2.foo.com. IN CNAME mail2.foo.com. sysman.foo.com. IN CNAME pc2.foo.com.

46 DNS: caching 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

47 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header identification: 16 bit # for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative

48 DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional helpful info that may be used

49 Adding a new domain Example: just created startup Bangla Network Register name BanglaNetwork.bd at a registrar (e.g., contact BTTB, they are in charge of bd domain) Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary) Registrar inserts two RRs into the bd TLD server: (banglanetwork.bd, dns1.banglanetwork.bd, NS) (dns1.banglanetwork.bd, , A) Put in authoritative server Type A record for www. banglanetwork.bd and Type MX record for banglanetwork.bd How do people get the IP address of your Web site?

50 Sending an DNS is not just used in HTTP protocol (web pages) DNS is involved in almost every protocol in use on the internet Next example is how DNS facilitates the transfer of electronic mail.

51 Step 1: Your PC sends the to its configured outbound mail server. A DNS request similar to the previous example is required to find the address of the mail server. Please send this message to someone@gmail.com

52 Step 2: Your mail server follows the same intensive process to find the authoritative servers for gmail.com. Tell me the name servers for gmail.com Here are the name servers for gmail.com

53 Step 3: Ask the gmail.com name server for the list of Mail exchangers (MX) for that domain. Tell me the MX s for gmail.com The MXs are mx10.gmail.com and mx20.gmail.com

54 Step 4: Select a Mail server and deliver the mail. Here is some mail for the gmail.com domain Mail accepted for delivery

55 Thank you!