Name Services and DNS

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1 In Academic Affiliation with Name Services and DNS Distributed Computer Systems Lecture 3 Module Leader: Mrs. Bobby Rishikesh 1

2 Contents What is a name service? SNS: A Simple Name Service DNS: Domain Name Srvice The X.500 Directory Seervice 2

3 What is a name service? Client processes use name services to keep track of entities such as: Address of named resources or objects. These objects may be: users file and directory names computers network domains and services, ports,... 3

4 A mechanism is needed to convert names to addresses TCP/IP systems convert names to IP addresses by using: A host file Domain Name System (DNS) NFS systems use Network Information Service (NIS) 4

5 The host file TCP/IP systems normally have a file called hosts that contains: a simple list of IP addresses and names that relate to them Each address can have several names: usually a formal name and a number of nicknames or aliases. 5

6 Purpose of naming Addresses are used to locate objects Names are easier to remember than numbers You would like to get to the address or other objects using a name DNS provides a mapping from names to resources of several types 6

7 Example of a hosts file Parts of the /etc/hosts file in bs47c # # IP Hosts Database # # Allocation of ranges: # # n - scismes, scismstations, DECstations # # n - Suns, Acorns # # n - Apples, FastPaths (*** Internet Access enabled ***) # # n - PCs and clones (*** Internet Access enabled ***) # # n - Comms. devices (bridges, servers, etc.) # "Large" multi-user/server Alphas/scismes etc bs47a.staffs.ac.uk bs47a bs47b.staffs.ac.uk bs47b bs47c.staffs.ac.uk bs47c bs41.staffs.ac.uk bs bsmail.staffs.ac.uk bsmail bs33m web.staffs.ac.uk web www bs33w news.staffs.ac.uk news bs33n # Reserved for School of Computing mail.soc.staffs.ac.uk bsroute121.staffs.ac.uk bsroute crroute121.staffs.ac.uk crroute121 # [no smtp] 7

8 Name Services A name service stores a database of bindings between textual names and attributes for objects. Major operations of a name service: Name Resolution-lookup an address, etc. Creating new bindings Deleting bindings Listing bound names 8

9 Composed naming domains Names often consist of several components which must be looked up in separate parts of the database: Name Type: Value: User login name Computer: network address 9

10 The namespace needs to be made hierarchical to be able to scale. The idea is to name objects based on location unit within that location object within unit Fully Qualified Domain Name (FQDN) labels separated by dots DNS provides a mapping from FQDNs to resources of several types Names are used as a key when fetching data in the DNS 10

11 The DNS maps names into data using Resource Records. Resource Record A More detail later Address Resource 11

12 Domain names can be mapped to a tree. New branches at the dots No restriction to the amount of branches. Domains are namespaces Everything below.com is in the com domain. Everything below ripe.net is in the ripe.net domain and in the net domain 12

13 Name service operations in the SNS Name resolution (Lookup) Binding (Bind) Unbinding(Unbind) 13

14 Domain Naming System (DNS) DNS is a distributed mapping system between host names and IP numbers Each host on the Internet with a registered name has one name associated with its IP number. 14

15 DNS functions Two main services are provided by dedicated name servers: Lookup services name resolution (mapping host names onto IP addresses) Provision of a database that defines these mappings 15

16 The DNS hierarchy DNS is implemented by a hierarchical system of name servers The root level is maintained by InterNIC InterNIC assigns domain names in all root level domains including: EDU COM MIL NET ORG... 16

17 Example of DNS name space hierarchy The host name soc.staffs.ac.uk is part of domain staffs.ac.uk (staffordshire University) staffs.ac.uk is part of academic community in uk: the section of DNS name space that is allocated to staffordshire university ac is the domain for academic communities in uk... 17

18 The DNS example... root... co uk ac staffs soc org com edu gov... shef bs47c... 18

19 How DNS query works When a DNS client needs to look up a name used in a program, it queries DNS servers to resolve the name. Each query message the client sends contains three pieces of information, specifying a question for the server to answer: A specified DNS domain name, stated as a fully qualified domain name (FQDN) A specified query type, which can either specify a resource record by type or a specialized type of query operation A specified class for the DNS domain name. For Windows DNS servers, this should always be specified as the Internet (IN) class. 19

20 DNS Name Resolution The nearest DNS name server maintains ONLY a small portion of the total DNS name space. DNS uses a name resolution protocol to resolve names by Using a special program called resolver that searches for a proper DNS name server. 20

21 Alternate query responses The previous discussion of DNS queries assumes that the process ends with a positive response returned to the client. However, queries can return other answers as well. These are the most common: An authoritative answer A positive answer A referral answer A negative answer An authoritative answer is a positive answer returned to the client and delivered with the authority bit set in the DNS message to indicate the answer was obtained from a server with direct authority for the queried name. 21

22 A positive response can consist of the queried RR or a list of RRs (also known as an RRset) that fits the queried DNS domain name and record type specified in the query message. A referral answer contains additional resource records not specified by name or type in the query. This type of answer is returned to the client if the recursion process is not supported. The records are meant to act as helpful reference answers that the client can use to continue the query using iteration. A negative response from the server can indicate that one of two possible results was encountered while the server attempted to process and recursively resolve the query fully and authoritatively: 22

23 Accessing a web page You type into your web browser and hit enter. What happens now? 23

24 Step 1: Your PC sends a resolution request to its configured DNS Server, typically at your ISP. Your PC ISP Recursive DNS server Tell me the Address of 24

25 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 ISP Recursive DNS server Root Servers I don t know the address but I know who s authoritative for the com domain ask them 25

26 Step 3: Your ISPs recursive name server then asks one of the com name servers as directed. Tell me the Address of ISP Recursive DNS server com DNS servers I don t know the address but I know who s authoritative for the google.com domain ask them 26

27 Step 4: Your ISPs recursive name server then asks one of the google.com name servers as directed. Tell me the Address of ISP Recursive DNS server google.com DNS server The Address of is

28 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. Your PC ISP Recursive DNS server The Address of is

29 Step 6: Your PC can then make the actual HTTP request to the web server. Send me the web page Your PC web server Here it is! 29

30 Summary WEB (HTTP) Request root DNS servers google.com.au Web Server 2 6 The actual web request Your PC 1 5 DNS ISP Recursive DNS server 4 3 com DNS servers DNS requests google.com DNS server 30

ECE 650 Systems Programming & Engineering. Spring 2018

ECE 650 Systems Programming & Engineering Spring 2018 Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) Tyler Bletsch Duke University Slides are adapted from Brian Rogers (Duke) Dynamic