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+#+title: SSH Hardening for Alpine Linux
+#+date: 2023-10-15
+#+description: A quick guide to harden SSH configuration on Alpine.
+#+filetags: :linux:
+
+* 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.
+
+#+begin_src txt
+,#+caption: ssh audit
+#+end_src
+
+#+caption: ssh-audit
+[[https://img.cleberg.net/blog/20231015-ssh-hardening/ssh-audit.png]]