Use SSH key-based authentication to bypass password prompts when transferring files from CentOS 7 to Synology. Generate a key pair on the Linux client, append the public key to the Synology user's authorized_keys file, and use scp or rsync for seamless, secure data movement without interactive password entry.
Understanding the Authentication Barrier
When attempting to transfer files between two Linux systems, such as a CentOS 7 server and a Synology NAS running DSM, you often encounter repeated password prompts. This happens because the Secure Shell (SSH) protocol requires authentication before allowing any file operations. While typing passwords manually works, it is inefficient for scripts, automated backups, or frequent large file transfers. The core issue is usually that the remote SSH service does not recognize the local machine's identity, forcing it to fall back to password verification for security reasons. To resolve this, you must establish trust between the two machines. This is achieved by exchanging cryptographic keys rather than relying on shared secrets like passwords. Once configured correctly, the CentOS machine can prove its identity to the Synology device automatically. This method is not only faster but also more secure, as it eliminates the risk of password interception and allows for stronger encryption standards.Verifying SSH Connectivity First
Before configuring keys, ensure basic connectivity exists. From your CentOS 7 terminal, try to connect to the Synology IP address using the standard SSH command. Replaceusername with your actual login name on the Synology and synology_ip with the target device's address.
ssh username@synology_ipGenerating SSH Keys on CentOS 7
The first step in overcoming password problems is generating a unique key pair on your source machine. Open a terminal on your CentOS 7 system and run the following command. This creates a private key that stays on your computer and a public key that will be shared with the Synology device.ssh-keygen -t rsa -b 4096.ssh directory. You may also choose to set a passphrase for an extra layer of security, though for automated scripts, leaving it empty is common. The command generates two files: id_rsa (private) and id_rsa.pub (public). Never share the private key with anyone or upload it to the remote server.
Copying the Public Key to Synology
There are two primary ways to get the public key onto the Synology NAS. The easiest method uses thessh-copy-id utility if it is installed on your CentOS system. This tool automates the process of appending your public key to the remote user's authorized keys list.
ssh-copy-id username@synology_ipssh username@synology_ip. It should log you in immediately without asking for a password. If ssh-copy-id is unavailable, you can manually copy the content of ~/.ssh/id_rsa.pub and paste it into the ~/.ssh/authorized_keys file on the Synology device via a text editor.
Transferring Files Securely
With key-based authentication established, you can now transfer files efficiently. Thescp command is the standard tool for copying files over SSH. To send a file from CentOS to Synology, use the following syntax.
scp /path/to/local/file.txt username@synology_ip:/path/to/remote/directory/rsync, which offers better performance and resume capabilities. It checks for differences between source and destination, transferring only what has changed.
rsync -avz /path/to/local/directory/ username@synology_ip:/path/to/remote/directory/Troubleshooting Permission Issues
If you still face authentication errors despite having the key, check permissions on the Synology side. The.ssh directory must have permissions set to 700, and the authorized_keys file must be 600. Incorrect permissions cause SSH to ignore the key for security reasons. Use chmod 700 ~/.ssh and chmod 600 ~/.ssh/authorized_keys on the Synology device to fix this. Additionally, ensure that the SSH daemon on Synology is configured to allow public key authentication in its configuration file, typically located at /etc/ssh/sshd_config. Look for the line PubkeyAuthentication yes and restart the SSH service if changes were made.
For advanced automation needs involving multiple hosts, parallel processing can speed up transfers significantly. Learn more about efficient bulk transfers in Transfering Files to Remote Hosts Using Parallel SSH with GnuParallel.
Frequently Asked Questions
Why does SSH still ask for a password after adding the key?
This usually indicates incorrect file permissions on the remote server. Ensure the .ssh folder is 700 and authorized_keys is 600. Also, verify that the key type matches what the server expects, such as RSA or Ed25519.
Can I use this method for root access on Synology?
Synology DSM typically disables direct root login via SSH for security. You should authenticate as a regular user with sudo privileges instead. Configure sudoers to allow passwordless commands if needed for automation.
What if my CentOS server is behind a NAT firewall?
NAT does not affect outbound SSH connections initiated from CentOS. As long as port 22 is reachable from your CentOS network to the Synology IP, key exchange will work normally regardless of internal routing.
How do I remove a compromised key?
Edit the authorized_keys file on the Synology device and delete the line corresponding to the compromised public key. Then, generate a new key pair on the CentOS machine and repeat the distribution process.
Is SCP safe for sensitive data?
Yes, SCP encrypts all data and credentials during transmission using the SSH protocol. It prevents eavesdropping and tampering, making it suitable for transferring confidential files between trusted Linux systems.