OpenSSH
OpenSSH Server
OpenSSH has an adequate security policy by default on newer systems, but there are additional measures you can take.
Ubuntu 22.04+
Create a file to store your secure settings
nano /etc/ssh/sshd_config.d/hardening.conf
Add the following lines
KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,gss-curve25519-sha256-,diffie-hellman-group16-sha512,gss-group16-sha512-,diffie-hellman-group18-sha512,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-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com
HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com
And restart OpenSSH
systemctl restart sshdRHEL and derivatives 8+ (RockyLinux, AlmaLinux, CloudLinux)
Create a file to store your secure settings
nano /etc/crypto-policies/back-ends/opensshserver.config
Add the following line
CRYPTO_POLICY='-oCiphers=chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr -oMACs=hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com -oGSSAPIKexAlgorithms=gss-curve25519-sha256- -oKexAlgorithms=curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256 -oHostKeyAlgorithms=ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512 -oPubkeyAcceptedKeyTypes=ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512'And restart OpenSSH
nano systemctl restart sshd
OpenSSH Client
Edit your SSH configuration file at
nano ~/.ssh/config
and add
Host *
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,gss-group16-sha512-,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256
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
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
MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com
CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256
GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-
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