1. 16, May, 2010 Baseer Ahmad Baheer
DNS Domain Name Systerm/Service/Server The Internet's Directory Service th Lecture
16, May, 2010
Baseer Ahmad Baheer

2. Human beings can be identified in many ways:
Name
SSN
Driver’s license numbers

3. Internet hosts can be identified by:
Hostname
Appreciated by humans.
Host’s location mohe.gov.af
Difficult to process by routers.
IP Address
Hierarchical structure.
Routers use this IP address to route datagram towards its destination.

4. Services Provided by DNS
DNS is:
A distributed database implemented in a hierarchy of name servers
An application-layer protocol that allows hosts and name servers to communicate in order to provide the translation service. (Over UDP with 53 port number)

5. Translating hostnames to their underlying IP addresses. Host aliasing:
Canonical hostname.
Hostname: relay1.west-coast.enterprise.com
Two alises name: and enterprise.com
Mail server aliasing
Load Distribution

6. How DNS Works?

8. Why not centerlize DNS? Single point of failure Traffic volume
Distant centralized database
Maintenance

9. No server has all name-to-IP address mappings
Local name servers:
Each ISP, company has local (default) name server
Host DNS query first goes to local name server
Root name servers
Authoritative name server:
For a host: stores that host’s IP address, name
Can perform name/address translation for that host’s name

12. DNS Records
The name servers that together implement the DNS distributed database, store Resource Records (RR) for the hostname to IP address mappings.

13. A resource record is a four-tuple that contains the following fields:

14. The meaning of Name and Value depend on Type:
If Type=A, then Name is a hostname and Value is the IP address for the hostname. Thus, a Type A record provides the standard hostname to IP address mapping. As an example, (relay1.bar.foo.com, , A) is a Type A record.

15. If Type=NS, then Name is a domain (such as foo
If Type=NS, then Name is a domain (such as foo.com) and Value is the hostname of a server that knows how to obtain the IP addresses for hosts in the domain. This record is used to route DNS queries further along in the query chain. As an example, (foo.com, dns.foo.com, NS) is a Type NS record.

16. If Type=CNAME, then Value is a canonical hostname for the alias hostname Name. This record can provide querying hosts the canonical name for a hostname. As an example, (foo.com, relay1.bar.foo.com, CNAME) is a CNAME record.

17. If Type=MX, then Value is a hostname of a mail server that has an alias hostname Name. As an example, (foo.com. mail.bar.foo.com, MX) is an MX record. MX records allow the hostnames of mail servers to have simple aliases.

19. References