2.
Network addresses are numbers.
Addresses are topologically oriented
Used for routing purposes
Moving a host may require change of address
Are not easy to remember
Names can be used for users and for applications
Easy for humans
Can be used as a low level service discovery mechanism.
Changing the server machine requires just changing the name-> IP binding
Names can have a logical structure.
REQUIREMENT OF DNS
3.
In the beginning, there was the hosts.txt
-A file containing the names and addresses of all hosts in the network
-Problems: maintainability, size
-Still used as a backup (local network host information)
DNS
– Tree-structured
– Delegation
– Separated from network structure and topology
– uses UDP, port number 53 for queries, TCP for zone transfers.
HISTORY
4.
Terms:
-namespace = set of possible names, flat or hierarchical.
-naming system maintains a collection of bindings of names to values.
-given a name, a resolution mechanism returns the corresponding value.
-a name server is an implementation of the resolution mechanism.
-DNS (Domain Name System) = name service in Internet.
-Zone is an administrative unit, domain is a subtree.
DNS System
5.
First level hierarchy
-- domains for each country + edu., com., gov., mil., org., net., int.
– New domains: aero., biz., coop., info., museum., name., pro.
– DNS first level managed by Internet Corporation for Assigned Names
& Numbers (ICANN), also manages address allocations.
Hierarchy is partitioned into subtrees, zones
-- zone corresponds to administrative boundaries in DNS (and, often also of
DNS-servers).
DNS Domain Hierarchy
6.
7.
1. Global Distribution
Data is maintained locally, but retrievable globally
No single computer has all DNS data
DNS lookups can be performed by any device
Remote DNS data is locally catchable to improve performance.
2. Loose Coherency
Changes to the master copy of the database are replicated according to
timing set by the zone administrator.
Cached data expires according to timeout set by zone administrator
DNS features
8.
3. Scalability
No limit to the size of the database
No limit to the number of queries
-- 24,000 queries per second handled easily
Queries distributed among masters, slaves, and caches
4. Reliability
Data is replicated
--Data from master is copied to multiple slaves
Clients can query
Clients will typically query local caches
DNS protocols can use either UDP or TCP
Contd…
9.
5. Dynamicity
Database can be updated dynamically
Modification of the master database triggers replication
Only master can be dynamically updated
Contd….
10.
Name servers store information about the name space in units
called “zones”.
Usually, more than one name server are authoritative for the same
zone
Also, a single name server may be authoritative for many zones
Types of Name Server
1.Authoritative – maintains the data
Master – where the data is edited
Slave – where data is replicated to
2. Caching – stores data obtained from an authoritative server.
Name Servers and Zones
