AWS Web Services says port 25 is open, but it is still inaccessible

Question

Hello,

I have filled the request to open port 25 on my LightSail instance and it has been done (at least they told me so).
I decided to test it by running a simple webserver on that port **CURL**ing it from the outside. That didn't work. But it works if I run it on port 24 for example.

OS is CentOS 9, SElinux is disabled, firewall-cmd is not installed, and reverse DNS has been set up properly.

EDIT: added more info

Answer

As you know, 25 port you want to access is usually for Simple Mail Transfer Protocol (SMTP) email routing traffic and AWS throttles traffic over port 25 from Amazon Lightsail instances by default [1].

To enable SMTP on your instance, you must also configure reverse DNS for your instance.
For more information, see 'Configuring reverse DNS for an email server on your Amazon Lightsail instance [2]'.

And you shared that you have filled the request to open port 25 on your LightSail instance and it has been done according to Amazon Lightsail.

According to AWS's official article [3], It might take up to 48 hours to process your request.

If your request is approved, then you receive an email to notify you that the port 25 restriction is removed.

If you don't receive an update within 48 hours, then reply to the initial email that you received.

Therefore, I would like to recommend you to reply to the initial email that you sill can't access to your instances via 25 port.

And If you need to utilize 25 port immediately, I want to recommend you to utilize other ports or VPC peering feature [4] temporarily.

Since firewall rules affect only traffic that flows in through the public IP address of an instance,
It does not affect traffic that flows in through the private IP address of an instance or resources in a peered virtual private cloud (VPC) in the same AWS Region.

I hope above I shared helps, please tell me If you have further questions.

Thank you.

References:

[1] Instance firewalls in Lightsail

https://lightsail.aws.amazon.com/ls/docs/en_us/articles/understanding-firewall-and-port-mappings-in-amazon-lightsail

[2] Configuring reverse DNS for an email server on your Amazon Lightsail instance

https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-configuring-reverse-dns

[3] How do I remove the restriction on port 25 from my Amazon EC2 instance or Lambda function?

https://repost.aws/knowledge-center/ec2-port-25-throttle

[4] Set up Amazon VPC peering to work with AWS resources outside of Amazon Lightsail

https://lightsail.aws.amazon.com/ls/docs/en_us/articles/lightsail-how-to-set-up-vpc-peering-with-aws-resources

AWS Web Services says port 25 is open, but it is still inaccessible

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