netbird_client: manage the wt0 MTU in default.json

netbird_client/tasks/config.yml

@@ -36,6 +36,43 @@
         cfg: "{{ netbird_config_stopped.content | b64decode | from_json }}"
       when: netbird_stopped is changed
 
+- name: Read netbird config for the MTU
+  slurp:
+    src: /var/lib/netbird/default.json
+  register: netbird_config_mtu_file
+  when: netbird_client.mtu | default('') | string | length > 0
+  tags: netbird_client_config
+
+- name: Set the wt0 MTU in the netbird config
+  when: netbird_config_mtu_file.content is defined
+  vars:
+    netbird_config_mtu: "{{ netbird_config_mtu_file.content | b64decode | from_json }}"
+  tags: netbird_client_config
+  block:
+    # netbird writes its in-memory config back on exit, so edit only while it is stopped
+    - name: Stop netbird to edit its MTU
+      systemd:
+        name: netbird
+        state: stopped
+      when: (netbird_config_mtu.MTU | default(0)) != (netbird_client.mtu | int)
+      notify: restart_netbird
+      register: netbird_mtu_stopped
+
+    - name: Re-read netbird config after stop
+      slurp:
+        src: /var/lib/netbird/default.json
+      register: netbird_config_mtu_stopped
+      when: netbird_mtu_stopped is changed
+
+    - name: Write MTU
+      copy:
+        dest: /var/lib/netbird/default.json
+        content: "{{ cfg | combine({'MTU': netbird_client.mtu | int}) | to_nice_json(indent=4, sort_keys=False) }}\n"
+        mode: "0600"
+      vars:
+        cfg: "{{ netbird_config_mtu_stopped.content | b64decode | from_json }}"
+      when: netbird_mtu_stopped is changed
+
 - name: Set netbird log level
   copy:
     dest: /etc/systemd/system/netbird.service.d/20-log-level.conf