Embedded Files
Objectives
Objectives
In this page we look into Domain Name System (DNS) and brief information on the following related to DNS:
Domain Name System (DNS)
Domain Name System (DNS)
DNS is a global service that converts or resolves host names to IP addresses. DNS is an application-layer protocol that allows hosts to query the hierarchical distributed database. The DNS protocol runs over UDP and uses port 53.
It is a mechanism for
querying and updating the DNS database
replicating the information across DNS database servers
DNS Name Space
DNS Name Space
Upside-down tree structure. A web host is regarded by hostname and domain, separated by dots, e.g.www.uts.edu.au
Root Domain (null): represented by “” or .
Top Level Domains (TLD) (First Level)
Generic: .org, . gov, .com, .edu , and now .biz, .info, .tv, etc
Country code: e.g., .au, .uk , .us, .md, nz , etc
ICANN selects organizations to manage TLD
Second Level domains (and further)
2nd level domain (edu.au & ns.edu. au)
3rd level domain (uts.edu.au & ns.uts.edu.au)
4th level domain (it.uts.edu.au & ns.it.uts.edu.au)
DNS Slaves and aching
DNS Slaves and aching
The Distributed database includes master & slave servers - slaves update off master (zone xfer). Distributed database performs queries too (hierarchical)
Roughly defined terminology:
Primary (Master) DNS: Required, authoritative server
Secondary (Slave) DNS:
Optional, to improve performance
periodically downloads a zone file from a master DNS server
slave can then answer authoritative queries about that zone
Caching - only DNS
caching to improve performance
Normally used by ISP
Time To Live (TTL) before it has to download the data again
How Does DNS Works?
How Does DNS Works?
Clients access libraries called resolver to perform DNS queries. DNS query can be iterative or recursive.
Iterative: Server points client to a different DNS server
Recursive: Server responds with destination IP address or an error message
Figure 1 above shows:
The host cis.poly.edu first sends a DNS query message to its local DNS server, dns.poly.edu. The query message contains the hostname to be translated, namely, gaia.cs.umass.edu.
The local DNS server forwards the query message to a root DNS server.
The root DNS server takes note of the edu suffix and returns to the local DNS server a list of IP addresses for TLD servers responsible for edu.
The local DNS server then resends the query message to one of these TLD servers.
The TLD server takes note of the umass.edu suffix and responds with the IP address of the authoritative DNS server for the University of Massachusetts, namely, dns.umass.edu.
The local DNS server resends the query message directly to dns.umass.edu
The authoritative DNS server responds to the local DNS server with the IP address of gaia.cs.umass.edu.
Finally, the local DNS server returns back to the host with the IP address of gaia.cs.umass.edu
Note that in this example, in order to obtain the mapping for one hostname, eight DNS messages were sent: four query messages and four reply messages!
Figure 2 shows a DNS query chain for which all of the queries are recursive.
DNS Binding
DNS Binding
BIND is one implementation of a DNS server
DNS Client
DNS Client
The DNS client sends a query containing the hostname to a DNS server. For instance, a DNS client to determine the IP address for the hostname www.cisco.com would send a query to its local DNS server first.
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