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1.    If you access Amazon EC2 using the command line tools or an API, you'll need your access key ID and secret access key.

2.    Instances can fail or terminate for reasons outside of your control. If an instance fails and you launch a replacement instance, the replacement has a different public IP address than the original. 

3.    However, if your application needs a static IP address, you can use an Elastic IP address.

4.    You can use security groups to control who can access your instances. These are analogous to an inbound network firewall that enables you to specify the protocols, ports, and source IP ranges that are allowed to reach your instances.

5.    You can create multiple security groups and assign different rules to each group. You can then assign each instance to one or more security groups, and we use the rules to determine which traffic is allowed to reach the instance. You can configure a security group so that only specific IP addresses or specific security groups have access to the instance.

6.    Amazon EC2 stores the public key only, and you store the private key (ssh keys, to connect your EC2 instance).

7.    Anyone who possesses your private key can decrypt your login information, so it's important that you store your private keys in a secure place.

8.    You can have up to five thousand key pairs per region.

9.    When you launch an instance, you should specify the name of the key pair you plan to use to connect to the instance. If you don't specify the name of an existing key pair when you launch an instance, you won't be able to connect to the instance

10. Amazon EC2 doesn't keep a copy of your private key; therefore, if you lose a private key, there is no way to recover it. 

11. If you lose the private key for an instance store-backed instance, you can't access the instance; you should terminate the instance and launch another instance using a new key pair. 

12. If you lose the private key for an EBS-backed Linux instance, you can regain access to your instance.

13. If you have several users that require access to a single instance, you can add user accounts to your instance. (By sharing private keys among users)

14. You can create a key pair for each user, and add the public key information from each key pair to the .ssh/authorized_keys file for each user on your EC2 instance. 

15. You can then distribute the private key files to your users. That way, you do not have to distribute the same private key file that's used for the root account to multiple users.

16. Amazon EC2 does not accept DSA keys. Make sure your key generator is set up to create RSA keys. Supported lengths: 1024, 2048, and 4096.

17. The public key that you specified when you launched an instance is also available to you through its instance metadata. To view the public key that you specified when launching the instance, use the following command from your instance:

18. GET http://169.254.169.254/latest/meta-data/public-keys/0/openssh-key

19. if you change the key pair that you use to connect to the instance, we don't update the instance metadata to show the new public key; you'll continue to see the public key for the key pair you specified when you launched the instance in the instance metadata.

20. When you delete a key pair, you are only deleting Amazon EC2's copy of the public key. Deleting a key pair doesn't affect the private key on your computer or the public key on any instances already launched using that key pair. You can't launch a new instance using a deleted key pair, but you can continue to connect to any instances that you launched using a deleted key pair, as long as you still have the private key (.pem) file.

21. If you're using an Auto Scaling group (for example, in an Elastic Beanstalk environment), ensure that the key pair you're deleting is not specified in your launch configuration. Auto Scaling launches a replacement instance if it detects an unhealthy instance; however, the instance launch fails if the key pair cannot be found.

22. If you create a Linux AMI from an instance, and then use the AMI to launch a new instance in a different region or account, the new instance includes the public key from the original instance. This enables you to connect to the new instance using the same private key file as your original instance. You can remove this public key from your instance by removing its entry from the .ssh/authorized_keys file using a text editor of your choice. 

23. If you lose the private key for an EBS-backed instance, you can regain access to your instance. You must stop the instance, detach its root volume and attach it to another instance as a data volume, modify the authorized_keys file, move the volume back to the original instance, and restart the instance.

24. This procedure isn't supported for instance store-backed instances. If the root device is an instance store volume, you must have the private key in order to connect to the instance.