~> Affects Vault 1.4 and later

Anytime Vault uses the [instance metadata service][aws-ec2-mds] on an EC2
instance, such as for getting credentials from the instance profile, there may
be a delay with the introduction of [v2 of the instance metadata service
(IMDSv2)][aws-ec2-imdsv2]. The AWS SDK used by Vault first attempts to connect
to IMDSv2, and if that times out, it falls back to v1. In Vault 1.4, this
timeout can take up to 2 minutes. In Vault 1.5.5 and later, it can take up to 2
seconds with this fix: [#10133](https://github.com/hashicorp/vault/pull/10133).

The timeout occurs in situations where there is a proxy between Vault and
IMDSv2, and the instance hop limit is set to less than the number of "hops"
between Vault and IMDSv2. For example, if Vault is running in docker on an EC2
instance with the instance hop limit set to 1, the AWS SDK client will attempt
to connect to IMDSv2, timeout, and fall back to IMDSv1 because of the extra
network hop between docker and IMDS.

To avoid the timeout behavior, the hop limit may be [adjusted on the underlying
EC2 instances][aws-ec2-imdsv2-config]. With the docker example, setting the hop
limit to 2 will allow the AWS SDK in Vault to connect to IMDSv2 without delay.

[aws-ec2-mds]: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html
[aws-ec2-imdsv2]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html
[aws-ec2-imdsv2-config]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html#configuring-instance-metadata-options
