2.4 The Domain Name System (DNS)

Understanding the Domain Name System (DNS)

Overview of DNS

• The Domain Name System (DNS) translates host names like gaia.cs.umass.edu into IP addresses such as 128.119.40.186, functioning as a critical part of the internet.

• DNS operates at the application layer, utilizing TCP and UDP services, which emphasizes its role as an application-level service.

Structure and Functioning of DNS

• The session will cover the structure, functioning, query resolution process, DNS records, and message format of the DNS protocol.

• The DNS serves to provide translation between names and IP addresses; it is akin to personal identifiers like social security numbers or employee IDs.

Characteristics of DNS

• The DNS is a distributed database containing records that facilitate name translation among host names and IP addresses.

• It consists of a hierarchy of servers across the internet that communicate to deliver name translation services while maintaining simplicity in network core design.

Functions Provided by DNS

• Besides translating IP addresses to host names, DNS also offers aliasing functions for complex internal hostnames and service resolution for domain-associated mail servers.

• Load balancing is another function where multiple IP addresses can serve requests for a web server, allowing efficient distribution of traffic.

Centralized vs Decentralized Approach

• A centralized approach would create single points of failure and high traffic concentration; thus, decentralization enhances performance and resilience against failures.

• Given the scale with trillions of daily queries (e.g., Akamai handles over a trillion), decentralized systems are essential for managing computational demands effectively.

Summary Insights on DNS Design

• The design philosophy behind DNS emphasizes high scalability and performance while handling massive volumes of requests efficiently.

• Organizationally decentralized with hundreds of thousands responsible for their records within this distributed database presents significant challenges but ensures robustness.

Hierarchical Structure of DNS

Understanding Hierarchy in DNS

• The hierarchical structure includes root servers at the top level followed by Top-Level Domain (TLD) servers responsible for domains like .com, .edu, etc.

• Authoritative name servers hold ultimate responsibility for resolving names within their specific domains (e.g., umass.edu).

Query Resolution Process

Understanding the DNS Hierarchy

The Role of Root Servers

• Root servers serve as a last resort for name resolution, acting as a contact point to obtain translations rather than providing them directly.

• They are crucial for internet functionality, likened to the central nervous system, with security being a significant concern.

• There are 13 logical root servers globally, replicated into nearly 1,000 physical servers, with over 200 located in the U.S.

Top-Level Domains (TLD)

• TLD servers resolve addresses ending in .com, .edu, .net, and .org; they are managed by internet registries.

• Authoritative name servers hold the ultimate authority over an organization's names and provide definitive answers regarding those names.

Local DNS Servers

• Every internet host has an associated local DNS server that resolves names by contacting it first.

• If the local server has cached information about a name translation, it responds immediately; otherwise, it initiates a resolution process.

DNS Name Resolution Process

Example of DNS Querying

• A request from engineering.nyu.edu to resolve gaia.cs.umass.edu illustrates how queries traverse through various DNS levels.

• The local NYU DNS server queries the root server which provides TLD server IP addresses based on the .edu suffix.

Iterative vs. Recursive Queries

• The example demonstrates iterative querying where each query is sent sequentially until resolution occurs; four messages were exchanged in total.

• In contrast to iterative queries, recursive queries place the burden on higher-level servers to resolve requests completely before responding back down the chain.

Caching Mechanisms in DNS

Importance of Caching

• Caching allows previously resolved mappings to be stored temporarily for quicker future access and reduces overall query traffic on DNS infrastructure.

Time-to-Live (TTL)

• Cached entries have a TTL after which they expire; this helps manage outdated information but can lead to temporary inaccuracies if records change before expiration.

Understanding DNS Protocol Messages

Overview of DNS Records

• DNS database records consist of a four-tuple: name, value, type, and TTL (time to live).

• Common types of DNS records include:

• A Record: Maps a hostname to its IP address for name-to-address translation.

• NS Record: Indicates the authoritative name server for a domain (e.g., umass.edu).

• CNAME Record: Used for aliasing one domain name to another.

• MX Record: Specifies the mail server associated with a domain.

DNS Protocol Message Formats

• Both query and reply messages share the same format; they are part of a query-response protocol.

• The ID field is a 16-bit number set by the querier, which matches in responses to indicate correspondence.

• The flag field indicates if the message is a query or reply and whether recursion is requested or if the reply is authoritative.

Query and Response Structure

• In queries, questions like resolving hostnames are included; replies contain resource records (e.g., A record with hostname and IP).

Setting Up an Internet Presence

• To establish an online presence (e.g., networkutopia.com), registration with a DNS registrar is necessary.

• After obtaining IP addresses, provide your authoritative name server's details to the registrar. This includes:

• NS record for your name server's name.

• A record for its IP address in the global DNS database.

Importance of DNS Security

• Protecting DNS from denial-of-service attacks primarily involves firewalls.

• Authentication services are crucial to ensure that only authorized sources can enter records into the database.