Theo's sshd configuration

sshd_config

# Theo's sshd configuration
# Based on work by k4yt3x -- https://k4t.io/sshd
# See also https://infosec.mozilla.org/guidelines/openssh
# 
# Created 2020-10-05
# Updated 2023-09-24
# 
# Licensed under the GNU GPL v3
# https://www.gnu.org/licenses/gpl-3.0.txt


########## Binding ##########

# use an unusual port to avoid bruteforcing
# this may or may not be necessary, check auth.log
Port 6182

########## Features ##########

# accept locale-related environment variables
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 forwarding
# note that any user with shell access can run their own forwarders
AllowTcpForwarding no
AllowStreamLocalForwarding no
PermitTunnel no
X11Forwarding no

# uncomment to block ssh-agent forwarding
# enabled by default to simplify deployment
#AllowAgentForwarding no

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

# uncomment to permit only the specified users/groups
AllowGroups remote

# only allow pubkey authentication
AuthenticationMethods publickey
PubkeyAuthentication yes
PasswordAuthentication no
KbdInteractiveAuthentication no
UsePAM no

# limit retries to prevent bruteforcing
MaxAuthTries 3

# disallow empty passwords
PermitEmptyPasswords no

# disallow root ssh
PermitRootLogin no

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

# explicitly define cryptography algorithms to avoid the use of weak algorithms
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
HostKeyAlgorithms rsa-sha2-512,rsa-sha2-256,ssh-ed25519
MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com

# short moduli should be deactivated before enabling the use of diffie-hellman-group-exchange-sha256
# do this with:
#     awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.tmp && mv /etc/ssh/moduli.tmp /etc/ssh/moduli
# see also https://infosec.mozilla.org/guidelines/openssh
#KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256
KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256

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

# 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 addtional information
#LogLevel VERBOSE

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

# only use SSH protocol version 2
Protocol 2

# 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