IP Address Assigned to EC2 Instance

Question

Two questions:
1) When an EC2 instance is started, what actually provides the IP addresses that are assigned to it?
2) How can I see the public IP address assigned to an EC2 instance, from within (i.e. after connecting) the EC2 instance? When an EC2 is started, I can see the public IP address that is assigned to it, simply by looking in the Management Console.  I can also see the Private IP assigned to that same EC2 instance, once I connect to it, and issue the "ip a s" command - I'm running a RHEL OS.  Is there a command that I can run/execute within Linux that will display the public IP address associated with that EC2 instance?

Answer

1. When an EC2 instance is started, what actually provides the IP addresses that are assigned to it?

According to the document, a public IP address is assigned to your instance from Amazon's pool of public IPv4 addresses.

> A public IP address is assigned to your instance from Amazon's pool of public IPv4 addresses, and is not associated with your AWS account. When a public IP address is disassociated from your instance, it is released back into the public IPv4 address pool, and you cannot reuse it.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html#concepts-public-addresses

2. How can I see the public IP address assigned to an EC2 instance, from within (i.e. after connecting) the EC2 instance?

Instance metadata can be used to retrieve public IP addresses.
```
curl http://169.254.169.254/latest/meta-data/public-ipv4
```
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instancedata-data-retrieval.html

Answer

I got the private ip address with the help of this post in my Ubuntu instance.

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc mq state UP group default qlen 1000
    link/ether 02:93:07:be:da:d9 brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.76/24 brd 10.0.0.255 scope global dynamic eth0
       valid_lft 2915sec preferred_lft 2915sec
    inet6 fe80::93:7ff:febe:dad9/64 scope link 
       valid_lft forever preferred_lft forever

It is same with the ip address in the following command.

TOKEN=`curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600"` && curl -H "X-aws-ec2-metadata-token: $TOKEN" -v http://169.254.169.254/latest/meta-data/local-ipv4
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    56  100    56    0     0  28000      0 --:--:-- --:--:-- --:--:-- 28000
*   Trying 169.254.169.254:80...
* TCP_NODELAY set
* Connected to 169.254.169.254 (169.254.169.254) port 80 (#0)
> GET /latest/meta-data/local-ipv4 HTTP/1.1
> Host: 169.254.169.254
> User-Agent: curl/7.68.0
> Accept: */*
> X-aws-ec2-metadata-token: AQAAAEKVNCUakqrrHX0SIzCtzRcmpmaNNl2ypoguN1KR6NpvRlCnoQ==
> 
* Mark bundle as not supporting multiuse
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Accept-Ranges: bytes
< Content-Length: 9
< Content-Type: text/plain
< Date: Tue, 29 Nov 2022 11:37:08 GMT
< Last-Modified: Wed, 23 Nov 2022 18:36:36 GMT
< X-Aws-Ec2-Metadata-Token-Ttl-Seconds: 21600
< Connection: close
< Server: EC2ws
< 
* Closing connection 0
10.0.0.76

But I can not get the public ip address with the curl command.

curl http://169.254.169.254/latest/meta-data/public-ipv4

Looks like there is no public-ipv4 under meta-data directory.

$ TOKEN=`curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600"` && curl -H "X-aws-ec2-metadata-token: $TOKEN" -v http://169.254.169.254/latest/meta-data
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    56  100    56    0     0  28000      0 --:--:-- --:--:-- --:--:-- 56000
*   Trying 169.254.169.254:80...
* TCP_NODELAY set
* Connected to 169.254.169.254 (169.254.169.254) port 80 (#0)
> GET /latest/meta-data HTTP/1.1
> Host: 169.254.169.254
> User-Agent: curl/7.68.0
> Accept: */*
> X-aws-ec2-metadata-token: AQAAAEKVNCUB3KPoodSB0SFUkkn1aSah2MRUxAVQQx_3SeVBllVoFA==
> 
* Mark bundle as not supporting multiuse
* HTTP 1.0, assume close after body
< HTTP/1.0 200 OK
< Accept-Ranges: bytes
< Content-Length: 324
< Content-Type: text/plain
< Date: Tue, 29 Nov 2022 11:29:43 GMT
< Last-Modified: Wed, 23 Nov 2022 18:36:36 GMT
< X-Aws-Ec2-Metadata-Token-Ttl-Seconds: 21600
< Connection: close
< Server: EC2ws
< 
ami-id
ami-launch-index
ami-manifest-path
auth-identity-credentials/
block-device-mapping/
events/
hostname
iam/
identity-credentials/
instance-action
instance-id
instance-life-cycle
instance-type
local-hostname
local-ipv4
mac
metrics/
network/
placement/
profile
public-keys/
reservation-id
security-groups
services/

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IP Address Assigned to EC2 Instance

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