GNet Solutions » GNU/Linux Administration

• Table of contents
• Key Exchange
• Authentication
• Client Authentication
• Symmetric Ciphers
• Message Authentication Codes
• Generate Strong Client Keypairs
• Resources

This is guide for hardening an SSH server.

• WARNING: You will need a recent OpenSSH version. It should work with 6.5 but I have only tested 6.6; you can check the current version by running:
  

  ssh -V
  

Key Exchange¶

• Add a strong Key Exchange:
  

  echo 'KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256' >> /etc/ssh/sshd_config
  

• Open /etc/ssh/moduli if exists, and delete lines where the 5th column is less than 2000.
  

  awk '$5 > 2000' /etc/ssh/moduli > "${HOME}/moduli" 
  wc -l "${HOME}/moduli" 
  mv "${HOME}/moduli" /etc/ssh/moduli
  

  • If wc -l is empty, create it:
    

    ssh-keygen -G "${HOME}/moduli" -b 4096
    ssh-keygen -T /etc/ssh/moduli -f "${HOME}/moduli" 
    rm "${HOME}/moduli" 
    

Authentication¶

• Edit the sshd config file:
  

  vi /etc/ssh/sshd_config
  

  • And modify the following parameters to only use SSHv2 and ed25519 and rsa key algorithms:
    

    Protocol 2
    HostKey /etc/ssh/ssh_host_ed25519_key
    HostKey /etc/ssh/ssh_host_rsa_key
    

Client Authentication¶

• edit the sshd config file:
  

  vi /etc/ssh/sshd_config
  

  • And modify the following parameters to disable password based logins:
    

    PasswordAuthentication no
    ChallengeResponseAuthentication no
    PubkeyAuthentication yes
    

Symmetric Ciphers¶

NOTE: For some reason I could not get the aes256-gcm@openssh.com and aes128-gcm@openssh.com ciphers to work on OpenSSH 6.6.

• Add strong Symmetric Ciphers:
  

  echo 'Ciphers chacha20-poly1305@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr' >> /etc/ssh/sshd_config
  

Message Authentication Codes¶

• Add strong Message Authentication Codes:
  

  echo 'MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-ripemd160-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160,umac-128@openssh.com' >> /etc/ssh/sshd_config
  

Generate Strong Client Keypairs¶

• Generate strong client ssh keypairs:
  

  ssh-keygen -t ed25519 -o -a 512
  ssh-keygen -t rsa -b 4096 -o -a 512
  

Resources¶

• https://stribika.github.io/2015/01/04/secure-secure-shell.html