Resolving password authentication failures when transferring files from CentOS 7 to Synology NAS requires verifying SSH service status, confirming correct user credentials, and ensuring key-based or password-based authentication is properly configured. This guide details the diagnostic steps for both systems to eliminate common connectivity barriers.
Verifying SSH Service Availability
The most frequent cause of connection refusal or immediate disconnection is that the SSH daemon is not running on the target machine. On the Synology NAS, you must enable the SSH service through the graphical interface before any command-line troubleshooting can succeed. Navigate to Control Panel, select Terminal and SNMP, and check the box labeled Enable SSH service. Ensure the port is set to the default22 unless you have intentionally changed it in your transfer commands. If you are using a non-standard port, you must specify it explicitly during the transfer process.
On the CentOS 7 source machine, verify that the SSH client tools are installed by running rpm -q openssh-clients. If the package is missing, install it using yum install openssh-clients. Additionally, confirm that the firewall on CentOS allows outbound connections on the target port. You can test basic network reachability from the CentOS terminal using nc -zv synology_ip_address 22. A successful response indicates that the network path is clear and the SSH port is listening.
Authentication Configuration and Credentials
Password problems often stem from mismatched credentials or restrictive authentication policies. When connecting from CentOS to Synology, use the standardscp or sftp commands with the correct username. The default administrative user on Synology is typically admin, but for security reasons, many administrators disable this account. Use a dedicated user account that has been granted sudo privileges if file operations require root access.
If you are using password authentication, ensure that the PasswordAuthentication directive is enabled in the SSH configuration. On Synology, this is managed via the DSM interface rather than direct file editing. However, if you are using key-based authentication, which is more secure and avoids interactive password prompts, verify that the public key from the CentOS machine is appended to the ~/.ssh/authorized_keys file on the Synology NAS. Incorrect permissions on the .ssh directory or the authorized_keys file will cause the server to reject the key silently. The directory should be 700 and the file 600.
For those seeking alternative methods to avoid complex SSH configurations entirely, consider exploring Graphically Browse and Transfer Files to SSH Server Using Various Linux Clients Without SSHFS. These tools can sometimes handle authentication quirks more gracefully than raw command-line utilities.
Troubleshooting Connection Refusals and Timeouts
When a transfer fails, the error message provides critical clues. A "Connection refused" error usually means the SSH service is down or blocked by a firewall. A "Permission denied (publickey,password)" error indicates that the credentials are incorrect or the authentication method is disabled. To gain deeper insight into these failures, examine the logs on the Synology side. While traditional Linux systems use/var/log/auth.log, Synology DSM manages logs differently. Check the system log in Log Center within the DSM Control Panel. Look for entries related to sshd that indicate why the connection was terminated.
On the CentOS side, run the transfer command with verbose output by adding the -v flag to scp or sftp. For example, scp -v local_file.txt user@synology_ip:/remote/path. This output will show each step of the negotiation process, including key exchange and authentication attempts. It helps identify whether the client is trying the right method and where exactly the handshake fails.
Another common issue is host key verification. If the SSH host keys on the Synology NAS have changed due to a reinstallation or major update, the CentOS client will refuse the connection for security reasons. You must remove the old entry from the ~/.ssh/known_hosts file on the CentOS machine and accept the new key upon reconnection.
Ensuring File Permissions and Paths
Even after successful authentication, transfers may appear to fail if the destination directory does not exist or lacks write permissions. Always create the target directory on the Synology NAS before initiating the transfer. Usemkdir -p /path/to/destination via an SFTP session or SSH shell. Verify that the user account used for the transfer has write access to this directory. If you are transferring files owned by another user, you may need to adjust ownership using chown after the transfer completes.
For large files or multiple files, consider using parallel transfer tools to improve speed and reliability. As discussed in Transfering Files to Remote Hosts Using Parallel SSH with GnuParallel, splitting the workload can prevent timeouts on unstable networks. This approach is particularly useful when moving large datasets between Linux servers and NAS devices.
Using Alternative Transfer Protocols
If SSH continues to present difficulties, evaluate whether other protocols are suitable for your environment. SMB/CIFS is natively supported by Synology and can be accessed directly from CentOS usingsmbclient or mounted via cifs-utils. This method bypasses SSH entirely and uses standard Windows-style shares. Configure the share in DSM under Shared Folder settings and enable SMB support. Then, mount the share on CentOS using a command like mount -t cifs //synology_ip/share_name /mnt/local_mount -o username=user,password=pass.
WebDAV is another option provided by Synology's Web Station package. It allows file transfers over HTTP/HTTPS, which can traverse firewalls more easily than SSH. However, it may introduce overhead for very large files. Choose the protocol that best aligns with your security requirements and network infrastructure.
Frequently Asked Questions
Why does my SCP command hang after entering the password?
It may be hanging because the remote directory does not exist or lacks write permissions for the user. Verify the path exists on the Synology NAS and check the folder permissions in the DSM Shared Folder settings.Can I use Synology as an SSH server without enabling it in DSM?
No, the SSH daemon must be explicitly enabled in the Control Panel under Terminal and SNMP. By default, it is disabled for security reasons to prevent unauthorized access.How do I reset the admin password on Synology if I cannot connect?
You can reset the admin password using the Synology Assistant tool on your local network or by physically accessing the NAS to press the reset button for four seconds. This restores the password to the default state printed on the device label.What is the difference between scp and sftp for file transfers?
scp is a simple command-line utility for copying files, while sftp provides an interactive file transfer protocol similar to FTP but encrypted. sftp allows browsing directories and resuming interrupted transfers, making it more flexible for complex operations.