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SSH-Hardening, 2015-01-10, based on (with a stripped down list of Ciphers and MACs)


On Debian Testing (= Jessie) and Ubuntu 14.04 (= Trusty) you don't have to do anything)

On Debian Stable (= Wheezy):

echo "deb wheezy-backports main" >> /etc/apt/sources.list
apt-get update
apt-get -t wheezy-backports install openssh-server

On Ubuntu 12.04 (= Precise): To lazy and/or incompetent for backports.

Server configuration

Remove all HostKey directives in /etc/ssh/sshd_config, then append at the bottom:

# SSH hardening, see

# Don't forget to remove HostKey directives above
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

... 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 ""

When restarting, current session is not affected, make sure you keep it open, in case you've done something wrong:

/etc/init.d/ssh restart

Client configuration

On top of your ~/.ssh/config, add:

Host *

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.