SSH2 subsystems are a useful convenience feature to predefine remote commands for SSH clients to invoke easily. Subsystems provide a layer of abstraction for defining and invoking the remote commands. A subsystem need not be a separate program; it can invoke a function built into the SSH server itself. SFTP is the most common SSH subsystem that you are going to encounter. For example on Linux distributions, the default /etc/ssh/sshd_config file defines one subsystem, This is the configuration line on Fedora 20: # override default of no subsystems Subsystem sftp /usr/libexec/openssh/sftp-server Do not remove or comment out the above line.