Support #591
Updated by Daniel Curtis about 9 years ago
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:
<pre>
ssh -V
</pre>
h2. Key Exchange
* Add a strong Key Exchange:
<pre>
echo 'KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256' >> /etc/ssh/sshd_config
</pre>
* Open /etc/ssh/moduli if exists, and delete lines where the 5th column is less than 2000.
<pre>
awk '$5 > 2000' /etc/ssh/moduli > "${HOME}/moduli"
wc -l "${HOME}/moduli"
mv "${HOME}/moduli" /etc/ssh/moduli
</pre>
#* If @wc -l@ is empty, create it:
<pre>
ssh-keygen -G "${HOME}/moduli" -b 4096
ssh-keygen -T /etc/ssh/moduli -f "${HOME}/moduli"
rm "${HOME}/moduli"
</pre>
h2. Authentication
* Edit the sshd config file:
<pre>
vi /etc/ssh/sshd_config
</pre>
#* And modify the following parameters to only use SSHv2 and ed25519 and rsa key algorithms:
<pre>
Protocol 2
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
</pre>
h2. Client Authentication
* edit the sshd config file:
<pre>
vi /etc/ssh/sshd_config
</pre>
#* And modify the following parameters to disable password based logins:
<pre>
PasswordAuthentication no
ChallengeResponseAuthentication no
PubkeyAuthentication yes
</pre>
h2. 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:
<pre>
echo 'Ciphers chacha20-poly1305@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr' chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr' >> /etc/ssh/sshd_config
</pre>
h2. Message Authentication Codes
* Add strong Message Authentication Codes:
<pre>
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
</pre>
h2. Generate Strong Client Keypairs
* Generate strong client ssh keypairs:
<pre>
ssh-keygen -t ed25519 -o -a 512
ssh-keygen -t rsa -b 4096 -o -a 512
</pre>
h2. Resources
* https://stribika.github.io/2015/01/04/secure-secure-shell.html