# Modified from K4YT3X Hardened OpenSSH Configuration

AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
AcceptEnv XMODIFIERS

# disallow automatically setting environment variables through ~/.ssh/environment
PermitUserEnvironment no
# disallow ssh-agent forwarding to prevent lateral movement
AllowAgentForwarding no

AllowTcpForwarding yes
AllowStreamLocalForwarding yes
# overrides all other forwarding switches
DisableForwarding yes

# disallow remote hosts from connecting to forwarded ports
# i.e. forwarded ports are forced to bind to 127.0.0.1 instead of 0.0.0.0
GatewayPorts no

# prevent tun device forwarding
PermitTunnel no

# suppress MOTD
PrintMotd no

# disable X11 forwarding since it is not necessary
X11Forwarding no

########## Authentication ##########

AuthenticationMethods publickey
PasswordAuthentication no
UsePAM no

# challenge-response authentication backend it not configured by default
# therefore, it is set to "no" by default to avoid the use of an unconfigured backend
ChallengeResponseAuthentication no

# set maximum authentication retries to prevent brute force attacks
MaxAuthTries 3

# disallow connecting using empty passwords
PermitEmptyPasswords no
PermitRootLogin no
PubkeyAuthentication yes

########## Cryptography ##########

# explicitly define cryptography algorithms to avoid the use of weak algorithms
# AES-CTR and Chacha20-Poly1305 modes have been removed to mitigate the Terrapin attack
#   https://terrapin-attack.com/
Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com
HostKeyAlgorithms rsa-sha2-512,rsa-sha2-256,ssh-ed25519
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com

# only use host keys with secure HostKeyAlgorithms
# omit the ECDSA key since it is only used in conjunction with the NIST P-curves
HostKey /etc/ssh/ssh_host_ed25519_key
#HostKey /etc/ssh/ssh_host_rsa_key

# short moduli should be deactivated before enabling the use of diffie-hellman-group-exchange-sha256
#   see this link for more details: https://github.com/k4yt3x/sshd_config#deactivating-short-diffie-hellman-moduli
# AES-CTR and Chacha20-Poly1305 modes have been removed to mitigate the Terrapin attack
#   https://terrapin-attack.com/
# ecdh-sha2-nistp* algorithms have been removed due to concerns around NIST P-curves' design
#   https://github.com/jtesta/ssh-audit/issues/213#issuecomment-1774204745
KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group16-sha512

########## Connection Preferences ##########

# Debian-based distributions only
# hide the Debian banner to prevent information disclosure
# (e.g., `SSH-2.0-OpenSSH_8.4p1 Debian-5+deb11u3`)
#DebianBanner no

# number of client alive messages sent without client responding
ClientAliveCountMax 2

# send a keepalive message to the client when the session has been idle for 300 seconds
# this prevents/detects connection timeouts
ClientAliveInterval 300

# compression before encryption might cause security issues
Compression no

# prevent SSH trust relationships from allowing lateral movements
IgnoreRhosts yes

# log verbosely for additional information
#LogLevel VERBOSE

# allow a maximum of two multiplexed sessions over a single TCP connection
MaxSessions 2

# enforce SSH server to only use SSH protocol version 2
# SSHv1 contains security issues and should be avoided at all costs
# SSHv1 is disabled by default after OpenSSH 7.0, but this option is
#   specified anyways to ensure this configuration file's compatibility
#   with older versions of OpenSSH server
Protocol 2

# override default of no subsystems
# path to the sftp-server binary depends on your distribution
#Subsystem sftp /usr/lib/openssh/sftp-server
#Subsystem sftp /usr/libexec/openssh/sftp-server
Subsystem sftp internal-sftp

# let ClientAliveInterval handle keepalive
TCPKeepAlive no

# disable reverse DNS lookups
UseDNS no