SSH-Hardening
Based on https://stribika.github.io/2015/01/04/secure-secure-shell.html (with a stripped down list of Ciphers and MACs)
Server configurationBearbeiten
Remove all HostKey directives in /etc/ssh/sshd_config, then append at the bottom:
# SSH hardening, see https://wiki.fsinf.at/wiki/SSH-Hardening # Disable SSHv1 Protocol 2 # Only allow Public Key Authentication PubkeyAuthentication yes PasswordAuthentication no ChallengeResponseAuthentication no # Only allow users in the "users" group (no system users!). # NOTE: See /etc/adduser.conf (EXTRA_GROUPS and ADD_EXTRA_GROUPS) to # automatically add new non-system users to a group. AllowGroups users # Don't forget to remove HostKey directives above HostKey /etc/ssh/ssh_host_rsa_key HostKey /etc/ssh/ssh_host_ed25519_key KexAlgorithms curve25519-sha256@libssh.org,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-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
... and execute:
cd /etc/ssh/ rm ssh_host_* ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key -N "" ssh-keygen -t ed25519 -f ssh_host_ed25519_key -N ""
restart serverBearbeiten
When restarting, current session is not affected, make sure you keep it open, in case you've done something wrong:
systemctl restart sshd
Client configurationBearbeiten
On top of your ~/.ssh/config, add:
Host * KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1 Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
Note that we add diffie-hellman-group-exchange-sha256 in comparison to the server config - this is for comparability with Ubuntu 12.04 (remove it if you don't care).
If possible, you can also regenerate your own SSH keys:
ssh-keygen -t ed25519 -o -a 100 ssh-keygen -t rsa -b 4096 -o -a 100
WARNING: This overwrites your old keys if you're not careful. If you don't add a new key to your servers before removing the old ones, you're locked out.