Linux SSH and Secure File Transfer

SSH provides encrypted remote shells and file transfer. This article assumes basic navigation and permissions from Linux Users, Groups, and Permissions.

Install and verify the server

On Debian or Ubuntu:

sudo apt update
sudo apt install openssh-server
sudo systemctl enable --now ssh
sudo systemctl status ssh

Check the listening socket and firewall policy before testing from another host:

sudo ss -tlnp | grep ':22'
sudo ufw allow OpenSSH

Use a key

Generate a key on the client, not on the server:

ssh-keygen -t ed25519 -C 'admin@example.invalid'
ssh-copy-id user@server
ssh user@server

Protect the private key with a passphrase. Never copy it to a server or commit it to a repository.

Harden deliberately

After confirming key login in a second session, consider disabling password authentication and direct root login in /etc/ssh/sshd_config:

PasswordAuthentication no
PermitRootLogin no

Validate before reloading:

sudo sshd -t
sudo systemctl reload ssh

Do not disable password access until a tested key-based recovery path exists. A firewall, updates, rate limiting, and least-privileged accounts remain important.

Transfer files

scp ./backup.tar.gz user@server:/tmp/
scp user@server:/tmp/backup.tar.gz ./

For large or repeat transfers, rsync -av --progress -e ssh source/ user@server:/srv/backup/ can avoid retransmitting unchanged data.

Troubleshooting

ssh -vv user@server
sudo journalctl -u ssh --since '10 minutes ago'

Use hostnames or stable DNS records instead of relying on a container’s temporary IP address.