Proper, Secure SSH Configuration

This article was written under the influence of the much-respected expert Anton Tsvet.

This article is intended for those who already understand that simply turning on a server and running a program on it is not a difficult task. However, making that program secure and resilient against external attacks in production is a challenge of an entirely different level. Here we’ll talk about SSH security and how to protect your server from potential threats.

Proper SSH configuration
First of all, /etc/ssh/sshd_config and /etc/ssh/ssh_config are different files.
The file /etc/ssh/sshd_config is responsible for configuring the ssh server so that one can connect TO that server.

The file /etc/ssh/ssh_config (or ~/.ssh/config) on your personal computer, on the other hand, governs which parameters YOU want to use when connecting to a server.

1. You can use ansible to verify a correct configuration from here.
2. Use ED25519 SSH keys.
3. Disable password access.
4. Limit the number of login attempts**:** Use the MaxAuthTries parameter.
5. Disable root access.
   1. Create an sshusers group.
   2. Create separate users from the keys in /root/.ssh/authorized_keys.
   3. Add the created users to the sshusers group.
   4. In the config /etc/ssh/sshd_config add AllowGroup sshusers.
6. The resulting config file:

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

# SSH server listening address and port
#Port 22
#ListenAddress 0.0.0.0
#ListenAddress ::

# only listen to IPv4
#AddressFamily inet

# only listen to IPv6
#AddressFamily inet6

########## 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 ssh-agent forwarding to prevent lateral movement
AllowAgentForwarding no

# prevent TCP ports from being forwarded over SSH tunnels
# **please be aware that disabling TCP forwarding does not prevent port forwarding**
# any user with an interactive login shell can spin up his/her own instance of sshd
AllowTcpForwarding no

# prevent StreamLocal (Unix-domain socket) forwarding
AllowStreamLocalForwarding no

# disables all forwarding features
# 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 instad 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 ##########

# permit only the specified users to login
# AllowUsers sshtunnel

# permit only users within the specified groups to login
AllowGroups sshusers

# uncomment the following options to permit only pubkey authentication
# be aware that this will disable password authentication
#   - AuthenticationMethods: permitted authentication methods
#   - PasswordAuthentication: set to no to disable password authentication
#   - UsePAM: set to no to disable all PAM authentication, also disables PasswordAuthentication when set to no
#AuthenticationMethods publickey
PasswordAuthentication no

# PAM authentication enabled to make password authentication available
# remove this if password authentication is not needed
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 authenticaion retries to prevent brute force attacks
MaxAuthTries 3

# disallow connecting using empty passwords
PermitEmptyPasswords no

# prevent root from being logged in via SSH
PermitRootLogin no

# enable pubkey authentication
PubkeyAuthentication yes

########## 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,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521
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
# see this link for more details: https://github.com/k4yt3x/sshd_config#deactivating-short-diffie-hellman-moduli
#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
#KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,sntrup761x25519-sha512@openssh.com

########## 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

# 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

# allow scanners to login via password
Match User scanners
  PasswordAuthentication yes

/etc/ssh/sshd_config

Sources:

CIS Debian Linux 12 Benchmark

CIS Ubuntu Linux 24.04 LTS Benchmark

Academy

SSH | Russian-language Ubuntu documentation