FreeBSD

Note

Taken from: https://github.com/bsdlabs/ssh-hardening

These instructions have been tested against the following versions:

• 12.4-RELEASE
• 12.4-STABLE
• 13.2-RELEASE
• 13.2-STABLE
• 14.0-RELEASE
• 14.0-STABLE
• 15.0-CURRENT

Server

1. Remove existing key-pairs, disable DSA & ECDSA

   rm -f /etc/ssh/ssh_host_*
   sysrc sshd_dsa_enable="no"
   sysrc sshd_ecdsa_enable="no"
   sysrc sshd_ed25519_enable="yes"
   sysrc sshd_rsa_enable="yes"

2. Regenerate RSA and Ed25519 keys

   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 ""

3. Remove Diffie-Hellman moduli smaller than 3071

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

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

   printf "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms [email protected],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-sha2-512,[email protected],rsa-sha2-256,[email protected]\n" >> /etc/ssh/sshd_config

5. Restart sshd

   service sshd restart

Client

1. Run the following in a terminal to harden the OpenSSH client for the local user

   mkdir -p -m 0700 ~/.ssh; printf "\nHost *\n  Ciphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr\n  KexAlgorithms [email protected],curve25519-sha256,[email protected],diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n  MACs [email protected],[email protected],[email protected]\n  HostKeyAlgorithms ssh-ed25519,[email protected],[email protected],[email protected],rsa-sha2-256,[email protected],rsa-sha2-512,[email protected]\n" >> ~/.ssh/config