From c7c9f6d5d16b15c74d3d91a61824b56dbcde0ef9 Mon Sep 17 00:00:00 2001
From: Ruakij <ruakij@ruekov.eu>
Date: Mon, 2 Nov 2026 22:35:03 +0000
Subject: [PATCH] netbird_client: manage the wt0 MTU in default.json

---
 netbird_client/defaults/netbird_client.yml |  2 ++
 netbird_client/tasks/config.yml            | 37 ++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/netbird_client/defaults/netbird_client.yml b/netbird_client/defaults/netbird_client.yml
index 237e759..c135eaa 100644
--- a/netbird_client/defaults/netbird_client.yml
+++ b/netbird_client/defaults/netbird_client.yml
@@ -9,3 +9,5 @@ netbird_client:
   iface_blacklist_extra: []
   # Overrides the --log-level of the netbird service when set
   log_level:
+  # MTU of wt0, written to /var/lib/netbird/default.json (netbird restarts on change); leave empty to keep the config as is
+  mtu:
diff --git a/netbird_client/tasks/config.yml b/netbird_client/tasks/config.yml
index 96c38fd..e0e25fc 100644
--- a/netbird_client/tasks/config.yml
+++ b/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