FreeBSD

Note

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

These instructions have been tested against the following versions:

• 13.5-RELEASE
• 14.3-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 sntrup761x25519-sha512,sntrup761x25519-sha512@openssh.com,mlkem768x25519-sha256,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512\n\nCiphers aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\nMACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com\n\nHostKeyAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nCASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nHostbasedAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nPubkeyAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\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 aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\n\n  KexAlgorithms sntrup761x25519-sha512,sntrup761x25519-sha512@openssh.com,mlkem768x25519-sha256,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n\n  MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\n  HostKeyAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\n  CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\n  HostbasedAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-256\n\n  PubkeyAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-256\n" >> ~/.ssh/config