# Domain Name System

IP addresses are not human readable, and they point to single machines when applications have many.

# Domain name System

Distributed database of mapping between hostnames and IP addresses. Application layer protocol that runs on top of UDP.

Resource Records (name, value, type, ttl)

• Type A: name - hostname, value - ip
• Type NS: name - domain, value - hostname of authoritative name server for this domain

# Root Servers

In recursive lookups, 13 root servers (IP addresses stable) that are replicated.

Any Cast: advertise a single IP prefix with multiple distinct locations. Hosts routed to closest one.

# Message Format

• identification: reply will match query
• flags: tell if query or reply
• questions: query of hostname lookup
• answers: responses to query
• authority: if responder does not know answer, nameserver authoritative for part of the hostname
• additional: to help prevent further lookups (hints for common lookups)