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?

  • Can you post the contents of the firewall rules for each of the security groups? – duper51 Aug 31 '17 at 1:19

I've worked out the problem. It was wrong to set the RDS instance to "publicly accessible" -- I assumed that meant "listen on the network", but should have realized that couldn't be right.

When the RDS instances is "publicly accessible", it causes Windows to resolve the DNS name to the external (public) IP address, which confused the routing. By disabling the public accessibility, the DNS entry gets resolved to the private IP, and all is well.

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