#+date:        <2023-10-15 Sun 00:00:00>
#+title:       SSH Security Hardening on Alpine Linux: Configuration Procedures
#+description: Detailed guide to enhance Alpine Linux SSH server security through configuration adjustments and vulnerability mitigation.
#+slug:        alpine-ssh-hardening
#+filetags:    :security:ssh:alpine:

* 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.