# Listen on separate port for backup purposes.
Port 2222

# Use the SSH protocol version 2 (which is safer).
Protocol 2

# Define dedicated host keys for backup SSH server.
HostKey /etc/ssh-backup/ssh_host_rsa_key
HostKey /etc/ssh-backup/ssh_host_ecdsa_key
HostKey /etc/ssh-backup/ssh_host_ed25519_key

# Use privilege separation for increased security.
UsePrivilegeSeparation yes

# Configure logging.
SyslogFacility AUTH
LogLevel INFO

# Users logging-in have 10 seconds to login upon established connection.
LoginGraceTime 10

# Don't allow root accounts logins.
PermitRootLogin no

# Enforce strict checking of home directory mode. However, this is not used for
# the chroots (chroots must check mode).
StrictModes yes

# Allow public key authentication.
PubkeyAuthentication yes

# Don't read the user's ~/.rhosts and ~/.shosts files for eventual
# RhostsRSAAuthentication or HostbasedAuthentication.
IgnoreRhosts yes

# Disable host-based authentication.
HostbasedAuthentication no

# Do not allow logins with empty passwords.
PermitEmptyPasswords no

# Don't allow challenge-response authentication.
ChallengeResponseAuthentication no

# Disable password-based authentication.
PasswordAuthentication no

# Disable X11 forwarding.
X11Forwarding no

# Do not print motd to avoid eventual issues for clients.
PrintMotd no

# Do not print the date and time of the last user login.
PrintLastLog no

# Use TPC keepalives for detecting dead connections.
TCPKeepAlive yes

# Use the internal SFTP so we can also easily utilise chroot.
Subsystem sftp internal-sftp

# Use PAM. But thanks to PasswordAuthentication being set to "no", PAM will be
# used just for session stuff.
UsePAM yes

# Specify a dedicated PID file for the backup SSH.
PidFile /run/sshd-backup.pid

# Users logging-in are forced to use the SFTP server.
ForceCommand internal-sftp

# Chroot logged-in users to their home directories.
ChrootDirectory %h

# Do not allow any TCP forwarding.
AllowTCPForwarding no

# Only allow the members of this group to log-in into this instance of OpenSSH
# server.
AllowGroups backup