IT3010 Lecture 03: Network Servers and Architecture

DNS and Caching

• DNS responses are cached to provide quick responses for repeated translations.
• Cached data periodically times out, with a lifetime (TTL) controlled by the owner of the data.
• The TTL is passed with every record.

DNS Reliability

• DNS servers are replicated to ensure name service availability even if one or more replicas are down.
• Queries can be load balanced between replicas.
• UDP is used for queries, with exponential backoff when retrying the same server.
• The same identifier is used for all queries, ensuring that it doesn't matter which server responds.

Dynamic Host Configuration Protocol (DHCP)

• DHCP is a standardized networking protocol used for dynamically distributing network configuration parameters.
• It provides IP addresses for interfaces and services.

Properties of DNS Host Entries

• Different kinds of mappings are possible, including 1-1 mapping between domain name and IP address, multiple domain names to the same IP address, and single domain name to multiple IP addresses.
• Some valid domain names don't map to any IP address.

DNS Design: Hierarchy and Zone Definitions

• Each node in the hierarchy stores a list of names that end with the same suffix.
• A zone is a contiguous section of the name space, with an associated set of name servers.
• Zones are created by convincing owner nodes to create or delegate a subzone.

DNS Design: cont…

• Records within a zone are stored in multiple redundant name servers.
• Primary/master name servers are updated manually, while secondary/redundant servers are updated by zone transfer of the name space.
• Zone transfer is a bulk transfer of the "configuration" of a DNS server, using TCP to ensure reliability.

Client-Server Architecture

• Client-server architecture is the dominant model for developing network applications.
• A server is a process that offers a service, while a client is a process that requests the service.
• The server waits for requests from clients, and roles of the client and server processes are asymmetric.

Domain Name System (DNS)

• DNS is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network.
• It translates a humanly-meaningful, text-based identifier to a system-internal, often numeric identification or addressing component.

Name Server

• A name server is a computer hardware or software server that implements a network service for providing responses to queries against a directory service.
• It translates an often humanly-meaningful, text-based identifier to a system-internal, often numeric identification or addressing component.

DNS Message Format

• DNS messages consist of 12 bytes, including name, type fields for a query, and RRs in response to the query.
• Records for authoritative servers, additional "helpful info", and other information are also included.

DNS Header Fields

• Identification is used to match up requests and responses.
• Flags are used to mark the query or response, indicate authoritative or non-authoritative responses, and support recursive resolution.

DNS Record RR Format

• Resource records (RRs) are tuples containing class, name, value, type, and TTL.
• Classes include Internet (IN), Chaosnet (CH), etc., and each class defines a value associated with the type.
• For the "IN" class, types include A, CNAME, NS, and MX.