How to enable passwordless SSH login to a Synology NAS

Several things need to be done to allow passwordless SSHing into a Synology NAS box. This guide was written against DSM 6.2.1-23824 Update 1.

1. Enable support for SSH on the Synology target machine
   1. Click the Control Panel Icon
   2. Choose Terminal & SNMP
   3. Check Enable SSH service
2. Allow the user, on the Synology target machine, to SSH
   1. Edit /etc/passwd
   2. Change the command/shell of the user that will SSH into the box from /sbin/nologin to /bin/sh
   • Read this to understand the format of the /etc/passwd file a bit better.
3. Add the authorized key of the client to the authorized_keys file of target
   1. on the client run ssh-keygen
   2. copy the .pub file (likely something like /home/user/.ssh/id_ecdsa.pub) to the target machine with a command like

      scp /home/user/.ssh/id_ecdsa.pub target_user@target_server:/var/services/homes/target_user/.ssh/id_ecdsa.pub

   3. either rename id_ecdsa.pub to authorized_keys or copy the contents into your authorized_keys file.
4. SSH once manually from the client machine (as the SSHing client user) to the target machine to add the target machine to the client's known_hosts file.