I set up an RDS instance running SQL Server, configured to be "publicly accessible". The security group allows inbound connections on 1433 for the server itself and for a second (different) specific security group.
I have two servers running in the second security group, on the same subnet. One runs Windows Server 2016 and the other runs Ubuntu. From the Ubuntu box, I have no problem connecting to the database server (telnet, nmap, nc, and sqsh are all happy). The Windows server can't connect -- it reports timeouts.
In installed nmap on the Windows server, and it reports the port is "filtered" with the reason "no-response".
I disabled Windows Firewall, but the same thing happens.
Can anybody suggest how I can proceed to figure out what's going wrong, here?