Amazon EC2 instance hostname types (original) (raw)

This section describes the Amazon EC2 instance guest OS hostname types available when you launch instances into your VPC subnets.

The hostname distinguishes the EC2 instances on your network. You may use the hostname of an instance if, for example, you want to run scripts to communicate with some or all of the instances on your network.

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Types of EC2 hostnames

There are two hostname types for the guest OS hostname when EC2 instances are launched in a VPC:

The EC2 instance guest OS hostname type depends on the subnet settings:

IP name

When you launch an EC2 instance with the Hostname type ofIP name, the guest OS hostname is configured to use the private IPv4 address.

Resource name

When you launch EC2 instances in IPv6-only subnets, the Hostname type of Resource name is selected by default. When you launch an instance in IPv4-only or dual-stack (IPv4+IPv6) subnets, Resource name is an option that you can select. After you launch an instance, you can manage the hostname configuration. For more information, see Change resource based naming options for Amazon EC2.

When you launch an EC2 instance with a Hostname type of Resource name, the guest OS hostname is configured to use the EC2 instance ID.

The difference between IP name and Resource name

DNS queries for both IP names and resource names coexist to ensure backward compatibility and to allow you to migrate from IP based-naming for hostnames to resource-based naming. For private DNS hostnames based on IP names, you cannot configure whether a DNS A record query for the instance is responded to or not. DNS A record queries are always responded to irrespective of the guest OS hostname settings. In contrast, for private DNS hostnames based on resource name, you can configure whether DNS A and/or DNS AAAA queries for the instance are responded to or not. You configure the response behavior when you launch an instance or modify a subnet. For more information, see Change resource based naming options for Amazon EC2.

Where to find resource names and IP names

You can see the hostname types, resource name and IP name, in the Amazon EC2 console.

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When creating an EC2 instance

When you create an EC2 instance, depending on which type of subnet you select, Hostname type of Resource name might be available or it might be selected and not be modifiable. This section explains the scenarios in which you see the hostname types resource name and IP name.

Scenario 1

You create an EC2 instance in the wizard (see Launch an EC2 instance using the launch instance wizard in the console) and, when you configure the details, you choose a subnet that you configured to be IPv6-only.

In this case, the Hostname type of Resource name is selected automatically and is not modifiable.DNS Hostname options of Enable IP name IPv4 (A record) DNS requests and Enable resource-based IPv4 (A record) DNS requests are deselected automatically and are not modifiable. Enable resource-based IPv6 (AAAA record) DNS requests is selected by default but is modifiable. If selected, DNS requests to the resource name will resolve to the IPv6 address (AAAA record) of this EC2 instance.

Scenario 2

You create an EC2 instance in the wizard (see Launch an EC2 instance using the launch instance wizard in the console) and, when you configure the details, you choose a subnet configured with an IPv4 CIDR block or both an IPv4 and IPv6 CIDR block ("dual stack").

In this case, Enable IP name IPv4 (A record) DNS requests is selected automatically and can't be changed. This means that requests to the IP name will resolve to the IPv4 address (A record) of this EC2 instance.

The options default to the configurations of the subnet, but you can modify the options for this instance depending on the subnet settings:

When viewing the details of an existing EC2 instance

You can see the hostname values for an existing EC2 instance in theDetails tab for the EC2 instance:

In addition, if you connect to your EC2 instance directly over SSH and enter thehostname command, you'll see the hostname in either the IP name or resource name format.

Choosing between resource names and IP names

When you launch an EC2 instance (see Launch an EC2 instance using the launch instance wizard in the console), if you choose a Hostname type of Resource name, the EC2 instance launches with a hostname in the resource name format. In such cases, the DNS record for this EC2 instance can also point to the resource name. This gives you the flexibility to choose whether that hostname resolves to the IPv4 address, the IPv6 address, or both the IPv4 and IPv6 address of the instance. If you plan to use IPv6 in the future or if you are using dual-stack subnets today, it’s best to use a Hostname type of Resource name so that you change DNS resolution for the hostnames of your instances without making any changes to the DNS records themselves. The resource name allows you to add and remove IPv4 and IPv6 DNS resolution on an EC2 instance.

If instead you choose a Hostname type of IP name, and use it as the DNS hostname, it can only resolve to the IPv4 address of the instance. It will not resolve to the IPv6 address of the instance even if the instance has both an IPv4 address and an IPv6 address associated with it.