Domain name service
- 1. Domain Name Service(DNS) • A person can be identified by several names: Full name, nick name etc. Similarly identifiers for hosts are: • Users prefer hostnames and routers or any machine refers IP address so a service is needed that converts hostnames / domains to values which is done by DNS. Hostnames IP Address 1. Variable length(facebook, google) 1. Fixed length(4 parts) 2. mnemonic- Easy to remember 2. Difficult to remember the integers. 3. Carry no information to help route packets towards them. 3. Routing information embedded within them.
- 2. Definitions • A name in a distributed system is a string of bits or characters that is refer to an entity, ex Hosts, printers, disks, files, Processes, users, mailboxes, news groups, Web pages etc • Three types of names: – Address: the name of an access point to an entity Identifier: An identifier refers to at most one entity . An identifier always refers to the same entity Human-friendly name: • E.g. unix file name, DNS names • Domain name- Label that defines a realm of administrative autonomy.ex facebook domain comprises of host belonging to facebook. • Names are always organized in a name space – A name space is an organization mechanism for a group of names Zone: Contiguous section of name space. Ex. Complete tree, a single node or subtree. Domain Name Resolvers: Also called DNS resolvers, are the computers which are used by ISPs to respond to a user request to resolve a domain name. "Resolving a domain name" refers to the translation of a domain name into an IP Address.
- 4. Overall work of DNS WEB BROWSER User enters “www.facebook.com”Web browser calls the service DNS IP Address TCP connection to the IP Address
- 5. Services of DNS 1. Conversion of host name to IP address. 2. Host aliasing: “alias- alternate names” . - Many names may map finally to same address. - Ex. www.facebook.com: www.facebok.com, www.facbook.com - DNS prevents the user from getting trapped by false website names created by frauds. - Lets consider , there is a single machine running two services: Web server, e- mail server. For users, Web Server, server name: www.abc.com, E-mail server name: www.xyz.com When the user types www.xyz.com, it will connect to www.server.com. Ex. Facebook.com (alias name)- for users, star.c10r.facebook.com(canonical name)- original server name If the single machine is shifted, then the users will be connected to new machine. - Helps run multiple services from same server.(in case there is a small Organization who cannot maintain their Own server, they create a alias for their server but user gets connected to the server from where the organization got its service)
- 6. 3. Mail server aliasing : help specify mail server of a given domain Ex. Mail address is abc@cbit.ac.in (as request) DNS finds out who is the host for e-mail server DNS sends the host name as xyz.cbit.ac.in and then the mail is sent. 4. Load distribution: Help distributed load across replicated servers(every server is not working singly but group of servers to prevent failures). DNS rotates the server IP addresses among the replicated servers and give back the result the IP address of the server which can take the load.
- 7. Implementation of DNS 1. Centralized architecture • Host asks for IP address to a central authority and gets back the reply. • Problem: 1.Single point of failure 2.Has to cope with high traffic volume. 3.Location: Where will be the DNS server placed? If Its far from user then more time will be required to get the reply. 4.Huge database maintenance 5.Overall its not scalable.
- 8. 2. Hierarchical & distributed Implementation Local DNS server- provides DNS service to hosts within an organization. Root DNS Server com There are 13 root servers Controlled by US com org edu in uk Top level domain amazon facebook co ac gov cbit Authoritative DNS servers
- 9. Ways to resolve domain names client Local DNS Root .com TLD server Facebook authoritative server 1 2 3 4 5 6 7 8 IP address Of facebook.com? 1. Iterative way
- 10. • Iterative way:- 1. Client requests for IP address of facebook.com to its local DNS. 2. Local DNS sends the request to the root . 3. Root returns the IP address of the DNS server which handles the .com domain 4. Same request to .com TLD server. 5. .com returns the IP address of facebook authoritative server address. 6. Same request to facebook.com server 7,8. The actual IP address is returned to client.
- 11. 2. Recursive solution client facebook .com Root Local DNS 1 8 2 7 3 6 4 5 Disadvantage: Root server is bottleneck, it it breaks then other servers cannot be contacted.