Default sshd configs tend to focus more on compatibility instead security. Therefore hardening should be one of the first things after setup a new system.
I'm using the OpenSSH Daemon which comes with the FreeBSD Base System.
When you want to use the openssh-portable port instead, skip Step 3, the rest should be identical.
Step 1) Delete existing host keys, generate new rsa and ed25519 key:
rm /etc/ssh/ssh_host_* ssh-keygen -q -t rsa -b 4096 -f ssh_host_rsa_key -N "" ssh-keygen -q -t ed25519 -f ssh_host_ed25519_key -N ""
Step 2) Create new Diffie-Hellman groups and avoid small moduli
ssh-keygen -G moduli-3072.candidates -b 3072 ssh-keygen -T moduli-3072 -f moduli-3072.candidates mv moduli-3072 /etc/ssh/moduli rm moduli-3072.candidates
Step 3) Disable DSA and ECDSA host keys, only use RSA and ED25519
sysrc sshd_dsa_enable="NO" sysrc sshd_ecdsa_enable="NO" sysrc sshd_ed25519_enable="YES" sysrc sshd_rsa_enable="YES"
Step 4) Optimize /etc/ssh/sshd_config, improve security, restrict allowed key exchange, cipher and MAC algorithms
# Hardening HostKey /etc/ssh/ssh_host_rsa_key HostKey /etc/ssh/ssh_host_ed25519_key KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-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 # Security PermitRootLogin no AuthenticationMethods publickey ChallengeResponseAuthentication no UsePAM no VersionAddendum none X11Forwarding no AuthorizedKeysFile .ssh/authorized_keys Subsystem sftp /usr/libexec/sftp-server
Run service sshd restart to apply the new settings, to verify the results of your hardening,
you can use the CLI Tool ssh-audit
which is also available as Online Version.
Following a Custom Policy for ssh-audit based on the above recommendations:
name = "Custom Policy - FreeBSD 13 Base System OpenSSH Daemon (2022/03/17)" version = 1 dh_modulus_size_diffie-hellman-group-exchange-sha256 = 2048 host keys = ssh-ed25519 key exchanges = curve25519-sha256, curve25519-sha256@libssh.org, diffie-hellman-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
Copy the policy into a file and run ssh-audit -P=<policy.txt> <servername>.
The result should be similar to following example without warnings or errors:
# general
(gen) banner: SSH-2.0-OpenSSH_8.8
(gen) software: OpenSSH 8.8
(gen) compatibility: OpenSSH 7.4+, Dropbear SSH 2018.76+
(gen) compression: enabled (zlib@openssh.com)
# key exchange algorithms
(kex) curve25519-sha256 -- [info] available since OpenSSH 7.4, Dropbear SSH 2018.76
(kex) curve25519-sha256@libssh.org -- [info] available since OpenSSH 6.5, Dropbear SSH 2013.62
(kex) diffie-hellman-group16-sha512 -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73
(kex) diffie-hellman-group18-sha512 -- [info] available since OpenSSH 7.3
(kex) diffie-hellman-group-exchange-sha256 (2048-bit) -- [info] available since OpenSSH 4.4
# host-key algorithms
(key) ssh-ed25519 -- [info] available since OpenSSH 6.5
# encryption algorithms (ciphers)
(enc) chacha20-poly1305@openssh.com -- [info] available since OpenSSH 6.5
`- [info] default cipher since OpenSSH 6.9.
(enc) aes256-gcm@openssh.com -- [info] available since OpenSSH 6.2
(enc) aes128-gcm@openssh.com -- [info] available since OpenSSH 6.2
(enc) aes256-ctr -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
(enc) aes192-ctr -- [info] available since OpenSSH 3.7
(enc) aes128-ctr -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
# message authentication code algorithms
(mac) hmac-sha2-256-etm@openssh.com -- [info] available since OpenSSH 6.2
(mac) hmac-sha2-512-etm@openssh.com -- [info] available since OpenSSH 6.2
(mac) umac-128-etm@openssh.com -- [info] available since OpenSSH 6.2
# fingerprints
(fin) ssh-ed25519: SHA256:uN9Oton+VmLL793KirVFB+ilD3Bndra4I/3yFntgX8k
# algorithm recommendations (for OpenSSH 8.8)
(rec) +diffie-hellman-group14-sha256 -- kex algorithm to append
(rec) +rsa-sha2-256 -- key algorithm to append
(rec) +rsa-sha2-512 -- key algorithm to append
Sources: