# ultra-secure OpenSSH server configuration

# only allow version 2 of SSH protocol
Protocol 2

# listen on port 22 by default
Port 22

# listen on these interfaces and protocols
AddressFamily any
ListenAddress 0.0.0.0

# limit authentication thresholds
LoginGraceTime 30s
MaxAuthTries 6

# limit maximum instanctes to prevent DoS
MaxStartups 5

# ensure proper logging
SyslogFacility AUTH
LogLevel INFO

# enforce permission checks before a login is accepted
# (prevents damage because of hacked systems with world-writeable
# home directories or similar)
StrictModes yes

# only allow safe crypto algorithms (may break some _very_ outdated clients)
# see also: https://stribika.github.io/2015/01/04/secure-secure-shell.html
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com

# enable data compression after successful login only
Compression delayed

# only allow cryptographically safe SSH host keys (adjust paths if needed)
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_rsa_key

# only allow login via public key by default
PubkeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication no
PermitEmptyPasswords no

# permit root login as there is no other user in IPFire 2.x
PermitRootLogin yes

# ignore user ~/.rhost* files
IgnoreRhosts yes

# ignore user known hosts file
IgnoreUserKnownHosts yes

# ignore user environments
PermitUserEnvironment no

# do not allow any kind of forwarding (provides only low security)
# some of them might need to be re-enabled if SSH server is a jump platform
X11Forwarding no
AllowTcpForwarding no
AllowAgentForwarding no
PermitTunnel no
GatewayPorts no
PermitOpen none

# detect broken sessions by sending keep-alive messages to
# clients (both via TCP and SSH)
TCPKeepAlive yes
ClientAliveInterval 10

# close unresponsive SSH sessions which fail to answer keep-alive
ClientAliveCountMax 6

# add support for SFTP
Subsystem	sftp	/usr/lib/openssh/sftp-server

# EOF