Why my instance suddenly cannot be accessed via SSH?

Question

In the past, I was able to connect to my instance using SSH, and even used VS Code remote-ssh to connect without any issues. However, today I suddenly cannot connect anymore. When I try to connect using Terminal, I get a message saying the connection is refused. I checked the security rules for my instance, and my IP address is already added to the inbound rules, so it shouldn't be blocked by the firewall. What should I do?

Answer

Has your IP address changed, etc.?  
Check your IP address to be sure.  
It is recommended to check network ACLs and security groups afterwards.  
https://repost.aws/knowledge-center/ec2-linux-resolve-ssh-connection-errors

If this does not solve the problem, you can also try to connect using Session Manager.  
https://docs.aws.amazon.com/ja_jp/systems-manager/latest/userguide/session-manager.html

Answer

The Session Manager can be used in the following ways.  
https://docs.aws.amazon.com/ja_jp/systems-manager/latest/userguide/session-manager.html  
After logging in with Session Manager, check the status of sshd with the following command.

```
sudo systemctl status sshd
```
If it is stopped, restart sshd with the following command.  
``` 
sudo systemctl restart sshd
```

Answer

Hi, both status checks of my instance show normal, and there is no indication of abnormalities. However, the website still cannot be accessed, which is somewhat strange. It seems that AWS customer service requires payment to assist with this issue.

Answer

How can I check the firewall settings?
I have checked my security group, and the settings seem normal:
sgr-050eaa7e60aa4aa89 IPv6 HTTPS TCP 443 ::/0 –

–
sgr-02fb68a313da3f4bc IPv4 SSH TCP 22 0.0.0.0/0 –

–
sgr-02634c3a46d3011ce IPv4 HTTPS TCP 443 0.0.0.0/0 –

–
sgr-0d58092cd29d7b455 IPv4 Custom TCP TCP 34210 0.0.0.0/0 runcloud

–
sgr-0aa5912a1d4962712 IPv6 HTTP TCP 80 ::/0 –

–
sgr-0751e7b1cb5ff144e IPv4 HTTP TCP 80

The worst thing is that after I restarted the instance, even the website inside cannot be accessed anymore, and it seems that AWS charges a fee to help fix the problem.

Answer

Doesn't sound like a problem with security groups, etc.  
Are there any abnormalities in the EC2 status?  
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/monitoring-system-instance-status-check.html  
The following documents are common troubleshooting steps to take when you cannot connect to an instance.  
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-console.html  
Follow the instructions in the "Instance console output" section and check the logs.  
If sshd is stopped for some reason, the instance must be restarted.

Why my instance suddenly cannot be accessed via SSH?

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