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#+date: [2023-10-15 Sun 00:00:00]
#+title: Alpine Linux SSH Hardening
#+description: Hardening SSH on Alpine Linux with config changes and key-only auth.
#+slug: alpine-ssh-hardening
#+filetags: :linux:security:
* 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 [email protected],curve25519-sha256,[email protected],diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr\nMACs [email protected],[email protected],[email protected]\nHostKeyAlgorithms ssh-ed25519,[email protected],[email protected],[email protected],rsa-sha2-512,[email protected],rsa-sha2-256,[email protected]" > /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.
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