• Used to look up records associated with domains
• Most common function is name to address lookup
• An A record (IPv4) or AAAA record (IPv6)
• But may also return other record types
• Usually runs recursively
• Requires hosts to know a DNS resolver to use (DHCP)
• Hosts register themselves in DNS
• Use dynamic DNS (often used for home ADSL networks where the IP address alocated by the ISP changes over time)

• Recursively:
• First ask a root DNS server
• A list of these is configured in the local resolver
• Then recurse through delegated DNS hierarchy

• DNS can use TCP or UDP on port 53
• By default it uses UDP
• Much more efficient for small/fast lookups
• UDP messages originally limited to 512 bytes
• Extended by EDNS0: "Extension mechanisms for DNS"
• Allows larger responses
• IPv6 records
• DNSSEC records (signaled responses)

• Avoid hitting external DNS servers heavily
• DNS records have a Time-To-Live value
• Typically 1 hour to 24 hours
• Tradeoff between DNS load and update responsiveness

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