Why the connection is timing out with Elastic IP but not with the public DNS

Question

I've setup a Redis Server on AWS. The Redis server has its own security group: sg-redis. I also have a webserver under another security group (sg-webserver) which is suppose to be able to connect to the redis server. For this security group I've opened:

• TCP 6379 for sg-webserver
• TCP 22 for sg-webserver

My redis server has an Elastic IP associated with the EC2 instance.

From the webserver, if I telnet my redis server using the Elastic IP I get a connection timeout:

[root@web-server:~]# telnet 54.11.22.333 6379
Trying 54.11.22.333...
telnet: connect to address 54.11.22.333: Connection timed out

Now I do the same using the Public DNS of the Redis server:

[root@web-server:~]# telnet ec2-11-22-33-444.compute-1.amazonaws.com 6379
Trying 10.22.33.444...
Connected to ec2-11-22-33-444.compute-1.amazonaws.com.
Escape character is '^]'.

I can see that the webserver is translating the public DNS to the AWS internal IP (not the Elastic IP) so I guess this is why it's working.

Now my question is why this is not working with the Elastic IP ? Do I have to use the Public DNS internally?

Cheers, Maxime

Answer 1

Yes, you should use public DNS internally. For that reason alone, it will resolve to the internal or external ip depending where you use it.