SSH Hardening Guides

[xluffy]

Re-generate the RSA and ED25519 keys

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 ""

Remove small Diffie-Hellman moduli

awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduli

Enable the RSA and ED25519 keys

Enable the RSA and ED25519 HostKey directives in the /etc/ssh/sshd_config file:

sed -i 's/^\#HostKey \/etc\/ssh\/ssh_host_\(rsa\|ed25519\)_key$/HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_config

Restrict supported key exchange, cipher, and MAC algorithms

echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,[email protected],diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr\nMACs [email protected],[email protected],[email protected]\nHostKeyAlgorithms ssh-ed25519,[email protected],[email protected],[email protected],rsa-sha2-256,rsa-sha2-512,[email protected],[email protected]" > /etc/ssh/sshd_config.d/ssh-audit_hardening.conf

Restart OpenSSH server

service ssh restart

https://www.ssh-audit.com/hardening_guides.html#ubuntu_20_04_lts