[RDS Postgres] failed: Connection timed out

Question

Hi, I have multiples postgres database instance on RDS, but, for some reason, out of nowhere, some of them now have no public connection, without any active intervention.
What is weird is that I can still connect to some instances with psql. Both instances (the ones I can connect and the ones I can't) have the same:
 * Security Groups (with my ext ip as an inbound rule to postgres)
 * Publicly accessible: Yes
 * VPC
 * Subnets
 * Availability Zone

And their DNS looks ok, which I verified with:
```
$ nslookup ########.rds.amazonaws.com
Server:         ###.###.###.###
Address:        ###.###.###.######

Non-authoritative answer:
##########.rds.amazonaws.com     canonical name = ##########.amazonaws.com.
Name:   ##########.amazonaws.com
Address: ###.###.###.###
```
But, when checking with 
```
$ nc -zv ##########.rds.amazonaws.com ####
```
I get "*succeeded*" from one, and "*nc: connect to ###.###.###.### port #### (tcp) failed: Connection timed out*" from the other.

Accepted Answer

Hello.

Is there a route to the Internet gateway configured in the subnet where RDS is running?  
For example, are all the subnets set in the subnet group public subnets?  
https://repost.aws/knowledge-center/rds-connectivity-instance-subnet-vpc  
https://repost.aws/knowledge-center/rds-cannot-connect

By the way, public access of RDS is not recommended for security reasons, so I think it is better to connect using Session Manager's port forwarding function as introduced in the document below.  
https://aws.amazon.com/jp/blogs/mt/use-port-forwarding-in-aws-systems-manager-session-manager-to-connect-to-remote-hosts/

[RDS Postgres] failed: Connection timed out

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