Host Address and Canonical Name (CNAME) Records

---

Host Address Records

• an address (A) record is used to resolve a host name to an IPv4 address
• an AAAA record resolves a host name to an IPv6 address

Info

• DNS uses UDP port 53 by default
  • UDP has max packet size of 512 bytes
• IPv6 is larger, so AAAA records exceed the limit
  • results in UDP packets being fragmented
    • can result in being blocked by firewalls
  • ensure DNS servers and firewalls can accepts these transmissions

CNAME Records

A canonical name (CNAME) record is used to configure an alias for an existing address record (A or AAAA).

• aka alias
• e.g., IP address of a web server with host record lamp could also be resolved by the alias www
• often used to make DNS administration easier
  • e.g., an alias can be redirected to a completely different host temporarily during system maintenance
• multiple different-named resource records can refer to the same IP address
  • and vice versa in load balancing

Info

• can configure multiple address records to point different host names to the same IP address
  • using CNAME is better practice
• possible to configure multiple A or AAAA records with the same host name but different IP addresses
  • usually done as a basic load balancing technique called round robin DNS

Info

• a name server can be configured to allow automatic creation, updating, and deletion of host records using Dynamic DNS (DDNS)
  • allows a client or DHCP server to configure records
    • rather than requiring the DNS server admin to create and update manually
• in Windows, running ipconfig /registerdns causes a client to attempt to use DDNS