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diff --git a/content/blog/2023-10-15-alpine-ssh-hardening.org b/content/blog/2023-10-15-alpine-ssh-hardening.org new file mode 100644 index 0000000..a3164fa --- /dev/null +++ b/content/blog/2023-10-15-alpine-ssh-hardening.org @@ -0,0 +1,67 @@ +#+date: <2023-10-15> +#+title: SSH Hardening for Alpine Linux +#+description: + + +* Overview + +This guide follows the standard +[[https://www.ssh-audit.com/hardening_guides.html][ssh-audit]] hardening +guide, tweaked for Alpine Linux. + +* Hardening Guide + +These steps must be performed as root. You can try to use =doas= or +=sudo=, but there may be issues. + +1. Re-generate the RSA and ED25519 keys + +#+begin_src sh +rm /etc/ssh/ssh_host_* +ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N "" +ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N "" +#+end_src + +2. [@2] Remove small Diffie-Hellman moduli + +#+begin_src sh +awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe +mv /etc/ssh/moduli.safe /etc/ssh/moduli +#+end_src + +3. [@3] Enable the RSA and ED25519 HostKey directives in the + /etc/ssh/sshd_{config} file + +#+begin_src sh +sed -i 's/^\#HostKey \/etc\/ssh\/ssh_host_\(rsa\|ed25519\)_key$/HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_config +#+end_src + +4. [@4] Restrict supported key exchange, cipher, and MAC algorithms + +#+begin_src sh +echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\nHostKeyAlgorithms 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" > /etc/ssh/sshd_config.d/ssh-audit_hardening.conf +#+end_src + +5. [@5] Include the /etc/ssh/sshd_{config}.d directory + +#+begin_src sh +echo -e "Include /etc/ssh/sshd_config.d/*.conf" > /etc/ssh/sshd_config +#+end_src + +6. [@6] Restart OpenSSH server + +#+begin_src sh +rc-service sshd restart +#+end_src + +* Testing SSH + +You can test the results with the =ssh-audit= python script. + +#+begin_src sh +pip3 install ssh-audit +ssh-audit localhost +#+end_src + +If everything succeeded, the results will show as all green. If anything +is yellow, orange, or red, you may need to tweak additional settings. |