Proxmox VE 7.3 6

Same instructions as Ubuntu 22.04 LTS Server,
except, remove sntrup761x25519-sha512@openssh.com from KexAlgorithms in step 4, as support for it was dropped in pve.

That is :

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

0. Make a backup of your ssh folder as a precaution

cp -r /etc/ssh /etc/_ssh.back

1. 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 ""

2. Remove small Diffie-Hellman moduli

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

3. 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

4. Restrict supported key exchange, cipher, and MAC algorithms

cat <<EOF > /etc/ssh/sshd_config.d/ssh-audit_hardening.conf
# Restrict key exchange, cipher, and MAC algorithms, as per community-provided
# hardening guide at: https://github.com/jtesta/ssh-audit/wiki/Proxmox-VE-7.3-6/
KexAlgorithms diffie-hellman-group14-sha256,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
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com
HostKeyAlgorithms 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
EOF

5. Restart OpenSSH server

service ssh restart

6. Redistribute new hosts key fingerprints on cluster nodes

pvecm updatecerts