Windows built-in firewall does not block SSH connections those are going out & there is no need to create an exception for SSH connections from Windows to LINUX hosts. Still many things could mess up connections.
One of the major culprits being the DNS settings. We had a migration for Primary domain controller from 2008 R2 to Windows Server 2016. Prior the upgrade we brought a temporary VM using VirtualBox to insure that the migration could happen seamlessly. The migration to temporary server happened without issues and we rushed to change the DNS settings for all production servers with the new VM IP address, after removing the IP address for the old PDC from the list.
After the confirmations, we built a fresh VM on VMWare ESXi environment and promoted it as PDC & we ahead with decommissioning the VM built on VirtualBox and forgot that “one thing”. Changing DNS server details for our LINUX servers.
Initially we didn’t give much attention to the SSH connection slowness as these LINUX server(s) running Oracle Applications were known to be slow in responding. However, things didn’t look quite promising as we noticed delays like 40s+ to establish connection to the host while trying to upload files using FileZilla client. It took us almost 8 days to realize that the connection issues were not from the client end, rather from the servers.
Checking the network configurations revealed these servers were still configured with the IP address of the temporary DC at the first entry, that we had for a while. Resetting the DNS entries to existing DNS servers IP addresses resolved the problems instantly.
Side note: No, you cannot overcome this issue by using IP addresses to establish the SSH connection. Regardless, the Linux HOST will try to resolve the client’s IP using the given DNS entries. So the only solution was to correct the DNS entries!
If you ever run into a slow SSH, well, hope know where to look at first after reading this post.