VPC DNS

In this, we will learn about VPC DNS.

• DNS resolves names used on Internet to their corresponding IP addresses.
• DNS hostname is a name that uniquely and absolutely names a computer. Morever, this is composed of a host name and a domain name.
• DNS servers resolve DNS hostnames to their corresponding IP addresses.
• Public IPv4 addresses enable communication over Internet
• private IPv4 addresses enable communication within network of instance (EC2-Classic or  VPC).
• two kinds of subnets in AWS VPC
• public subnet – attached to an Internet Gateway. adds a routing table entry to subnet’s routing table sending all Internet traffic to an Internet Gateway.
• private subnet – if traffic from subnet destined for Internet is sent to either a NAT instance, or a AWS managed NAT device
• EC2 instance running in either subnet can choose to have a public IP address or not.
• Once you give instance a public IP address, it becomes “reachable” from Internet (or ingress) and of course can reach “Internet” resources from instance (or egress).
• During instance launch into default VPC, instance is given public and private DNS hostnames for public IPv4 and private IPv4 addresses for instance.
• If launched in non-default VPC, private and public DNS hostname given as per DNS attributes for VPC
• AWS provided private (internal) DNS hostname resolves to private IPv4 address of instance
• Private DNS hostname for communication between instances in same network can also be used but can’t resolve DNS hostname outside network that instance is in.
• Public DNS hostname is resolved to public IPv4 address of instance outside network of instance, and to private IPv4 address of instance from within network of instance.
• No DNS hostnames for IPv6 addresses.

DNS Support VPC

VPC has attributes for instances launched, on

• whether they  receive public DNS hostnames corresponding to their public IP addresses
• whether DNS resolution through Amazon DNS server is supported for VPC.

With both attributes as true,

• Instances with a public IP address receive corresponding public DNS hostnames.
• Amazon-provided DNS server can resolve Amazon-provided private DNS hostnames.

If either or both of attributes is set to false

• Instances with a public IP address do not receive corresponding public DNS hostnames.
• Amazon-provided DNS server cannot resolve Amazon-provided private DNS hostnames.
• Instances receive custom private DNS hostnames if there is a custom domain name in DHCP options set. If you are not using Amazon-provided DNS server, custom domain name servers must resolve hostname as appropriate.

• By default, both attributes are set to true in a default VPC or a VPC created by VPC wizard.
• By default, only enableDnsSupport attribute is set to true in a VPC created any other way.
• If you use custom DNS domain names defined in a private hosted zone in Route 53, or use private DNS with interface VPC endpoints (AWS PrivateLink), you must set enableDnsHostnames and enableDnsSupport attributes to true.

DNS Limits

• EC2 instance number of packets sent to Amazon-provided DNS server, is maximum of 1024 packets per second per network interface.
• This limit cannot be increased.
• number of DNS queries per second supported by Amazon-provided DNS server varies by
  • type of query
  • size of response
  • protocol in use.

Simple AD flow of DNS queries originating from within the VPC.