How installing nftables broke ssh-copy-id (opens in new tab)

While testing a new feature for WarehousePG in our Docker containers, I ran into an unexpected problem: I wanted to block certain network traffic between the Docker hosts, and therefore installed nftables. Suddenly very early in the startup process the ssh-copy-id ssh key exchange broke. The error message: 1 2 3 4 5 6 7 8 9 10 11 12 coordinator | Adding ssh keys for user gpadmin to host standby coordinator | Attempt 1/5: Adding keys for user gpadmin to host standby .