roles/common: sshd overrides for Debian 13

roles/common/tasks/sshd.yml

@@ -1,6 +1,7 @@
 ---
-# SSH configs don't change in Debian minor versions
+# Only override the system sshd configuration on older Debian.
 - name: Reconfigure /etc/ssh/sshd_config
+  when: ansible_distribution_version is version('12', '<=')
   ansible.builtin.template:
     src: "sshd_config_{{ ansible_distribution }}-{{ ansible_distribution_major_version }}.j2"
     dest: /etc/ssh/sshd_config
@@ -9,6 +10,18 @@
     mode: "0600"
   notify: Reload sshd
 
+# Newer OpenSSH versions support including extra configuration. The includes
+# happen at the beginning of the file and the first value to be read is used.
+- name: Configure sshd_config.d overrides
+  when: ansible_distribution_version is version('13', '>=')
+  ansible.builtin.template:
+    src: etc/ssh/sshd_config.d/01-{{ ansible_distribution }}-{{ ansible_distribution_major_version }}.conf.j2
+    dest: /etc/ssh/sshd_config.d/01-custom.conf
+    owner: root
+    group: root
+    mode: "0600"
+  notify: Reload sshd
+
 # See: WeakDH (2015): https://weakdh.org/sysadmin.html
 - name: Remove small Diffie-Hellman SSH moduli
   block:

roles/common/templates/etc/ssh/sshd_config.d/01-Debian-13.conf.j2

@@ -0,0 +1,34 @@
+{{ ansible_managed | comment }}
+
+HostKey /etc/ssh/ssh_host_ed25519_key
+
+# LogLevel VERBOSE logs user's key fingerprint on login. Needed to have a clear
+# audit track of which key was using to log in.
+LogLevel VERBOSE
+
+MaxAuthTries 4
+
+AuthorizedKeysFile	.ssh/authorized_keys
+
+# To disable tunneled clear text passwords, change to no here!
+{% if ssh_password_authentication == 'disabled' %}
+PasswordAuthentication no
+{% else %}
+PasswordAuthentication yes
+{% endif %}
+
+X11Forwarding no
+
+# Based on the ssh-audit profile for Debian 13, but with but with all algos with
+# less than 256 bits removed, as NSA's Suite B removed them years ago and the
+# new (2018) CNSA suite is 256 bits and up.
+#
+# See: ssh-audit.py -P "Hardened Debian 13 (version 1)"
+# See: https://en.wikipedia.org/wiki/Commercial_National_Security_Algorithm_Suite
+Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr
+MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com
+KexAlgorithms mlkem768x25519-sha256,sntrup761x25519-sha512,sntrup761x25519-sha512@openssh.com
+
+{% if ssh_allowed_users is defined and ssh_allowed_users %}
+AllowUsers {{ ssh_allowed_users|join(" ") }} {{ provisioning_user.name }}
+{% endif %}