====== SSH Hardening Guides ======

Рекомендации по настройке максимально безопасных настроек для SSH (и клиента и сервера): https://www.ssh-audit.com/hardening_guides.html

Там же можно запустить аудит: https://www.ssh-audit.com и посмотреть насколько плохо обстоят дела.

===== Ubuntu 22.04 LTS Server =====

Note: all commands below are to be executed as the root user.

Re-generate the RSA and ED25519 keys
<code>
rm /etc/ssh/ssh_host_*
ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
</code>

Remove small Diffie-Hellman moduli
<code>
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduli
</code>

Enable the RSA and ED25519 HostKey directives in the /etc/ssh/sshd_config file:
<code>
sed -i 's/^\#HostKey \/etc\/ssh\/ssh_host_\(rsa\|ed25519\)_key$/HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_config
</code>

Restrict supported key exchange, cipher, and MAC algorithms
<code>
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,gss-curve25519-sha256-,diffie-hellman-group16-sha512,gss-group16-sha512-,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\nHostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com" > /etc/ssh/sshd_config.d/ssh-audit_hardening.conf
</code>

Restart OpenSSH server
<code>
service ssh restart
</code>

Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.

{{tag>ssh hardening audit}}