IST362. Domain Name Service.

Domain Name Service.

transport layer service used: UDP or TCP

port number: 53

Services provided by DNS

Host aliasing. One server can have a canonical name and a set of aliases. For example, www.yahoo.com is an alias for the server with canonical name www.yahoo.akadns.net.
Mail server aliasing. To be able to send an e-mail to someone@marshall.edu your SMTP server needs to know the IP address of SMTP server marshall.edu.
Load balancing. Busy sites like www.yahoo.com or www.cnn.com are replicated via multiple servers, with each server running on a different end system and each having a different IP address. For replicated servers, a set of IP addresses is associated with one canonical name. Thus, when DNS server gets a request for resolving this canonical name or one of its aliases for an IP address it sends back the entire set of IPs, but rotates the ordering of the addresses within each reply.

Centralized database vs. Distributed database.

Disadvantages of the centralized approach:

A single point of failure.
Traffic volume.
Distant centralized database.
Maintenance and authorization problems.

Distributed DB:

Local name servers. Each ISP has a local DNS server. When a host issues a DNS request it is first sent to the local name server which is typically manually configured on each host.
Root name servers. When a local name server cannot immediately resolve the request, it behaves as a DNS client and queries one of the root servers (there are about a dozen of them). The root server either knows the answer for the query or it knows the address of an authoritative name server that has mapping for the host name in the request.
Authoritative name servers. Every host is registered with an authoritative name server. (Typically, the authoritative server is a name server in the host's local ISP. Actually, each host is required to have at least two such servers in case of failure.)

Example: host dementiev.marshall.edu wants to connect to a web server athabasca.library.emory.edu.

First it sends a request to a Marshall name server
If Marshall name server doesn't know the IP of athabasca.library.emory.edu , it sends a query to a root server responsible for .edu addresses
The root server most probably doesn't know the answer either, but it knows the IP of the name server of the emory.edu network. So, it sends a query to that server.
Assume emory.edu name server doesn't remember the needed address, but it knows the IP of the name server of the library.emory.edu network and sends the query there
Since host athabasca belongs to this network this name server is the authoritative name server for the host. That is, library.emory.edu knows the IP. It sends it back to emory.edu name server
name server of the emory.edu network response to the root server
the root server sends the answer to the Marshall local name server
Marshall local name server sends it to the asking host.

DNS records

The name servers that together implement the DNS database store resource records for the hostname-to-IP mappings. Each DNS reply carries one ore more resource records. A resource record is a four-tuple that contains the following fields:

(Name, Value, Type, TTL)

where TTL (time to live) determines the time the record should be stored in a cache. It's an integer number containing the number of seconds.

The meaning of Value and Name depend on Type:

Type Name Value Example

A hostname IP address for the hostname (ist.marshall.edu, 206.212.61.157, A, 3600)

NS domain hostname of an authoritative name server that knows how to obtain the IP s for hosts of the domain (marshall.edu, mudnsa.marshall.edu, NS, 36000)

CNAME alias canonical hostname (www.yahoo.com, www.yahoo.akadns.net, CNAME, 600)

MX mail alias canonical name of the mail server (hotmail.com, mx4.hotmail.com, MX, 86400)

Other types

Type	Name	Value	Example
A	hostname	IP address for the hostname	`(ist.marshall.edu, 206.212.61.157, A, 3600)`
NS	domain	hostname of an authoritative name server that knows how to obtain the IP s for hosts of the domain	`(marshall.edu, mudnsa.marshall.edu, NS, 36000)`
CNAME	alias	canonical hostname	`(www.yahoo.com, www.yahoo.akadns.net, CNAME, 600)`
MX	mail alias	canonical name of the mail server	`(hotmail.com, mx4.hotmail.com, MX, 86400)`

HINFO -- the host CPU and operating system type

MINFO -- the mailbox or mail list information.

PTR -- the host name if the query is an Internet address; otherwise, the pointer to other information

SOA -- the domain's "start-of-authority" information.

TXT, UINFO, WKS, ANY, AXFR, MB, MD, MF, NULL are described in the RFC-1035 documentation.

NSLookUp

Very useful tool. Available via a web interface even via two inerfaces. Find more information about it on the manual page or in the Microsoft knowledge base.

Official documentation:

Domain Names - Concepts and Facilities

Domain Names - Implementation and Specification