Add docs for logging env vars and log rotation troubleshooting

src/pages/client/environment-variables.mdx

@@ -112,6 +112,14 @@ These control the exponential backoff when the client loses its connection to th
 |---|---|---|
 | `NB_MANAGEMENT_GRPC_MAX_MSG_SIZE` | All | Maximum size (in bytes) of a single gRPC message from the management server. Increase this when the network map (peers, firewall rules, routes, DNS config) exceeds the default 4 MB gRPC limit. |
 
+## Logging
+
+| Variable | Platform | Description |
+|---|---|---|
+| `NB_LOG_FORMAT` | All | Override the log output format. Accepted values: `json` (one JSON object per line) or `syslog` (compact format used when logging to syslog). Anything else falls back to the default human-readable text formatter. The `syslog` value is also selected automatically when the log destination is set to `syslog`. |
+| `NB_LOG_MAX_SIZE_MB` | All | Maximum size in megabytes of a single log file before NetBird's built-in rotation triggers (default: `15`). Only effective when the built-in rotation is active, see `NB_LOG_DISABLE_ROTATION` and [Log rotation](/help/troubleshooting-client#log-rotation). |
+| `NB_LOG_DISABLE_ROTATION` | All | Set to `true` to disable NetBird's built-in log rotation. NetBird will write to the configured log file directly and leave rotation to an external tool (e.g. system `logrotate` on Linux). On Linux, NetBird also auto-disables its own rotation when it detects a `logrotate` config that mentions `netbird`, this variable lets you force the same behavior on other platforms or when the auto-detection misses your config. Also see [Log rotation troubleshooting](/help/troubleshooting-client#log-rotation). |
+
 ## Other
 
 | Variable | Platform | Description |

src/pages/help/troubleshooting-client.mdx

@@ -94,6 +94,32 @@ By default, client logs are located in the `/var/log/netbird/client.log` file on
 You can analyze the logs to identify the root cause of the problem. If you need help, open
 a [github issue](https://github.com/netbirdio/netbird/issues/new/choose) and attach the logs.
 
+### Log rotation
+
+NetBird rotates `client.log` itself by default, old files are written next to the active log as gzipped archives (e.g. `client-2025-04-01T12-00-00.000.log.gz`). The maximum size of the active file is controlled by [`NB_LOG_MAX_SIZE_MB`](/client/environment-variables#logging) (default: 15 MB).
+
+Running NetBird's built-in rotation **and** the `logrotate` service for Linux on the same file causes problems where the log file might become corrupted and logs lost. To avoid this, NetBird scans `/etc/logrotate.conf` and `/etc/logrotate.d/*` on Linux startup. If any non-comment line mentions `netbird`, the client logs a warning and disables its own rotation, letting `logrotate` own the file. You will see a line like this in `netbird.err` or the console:
+
+```text
+log rotation conflict detected in: "/etc/logrotate.d/netbird", rotation is disabled
+```
+
+<Note>
+To use `logrotate`, we require `copytruncate` and not `create` to be set in the config file, otherwise the daemon needs to be restarted for the new log file to be opened after rotation.
+
+Netbird supports `compress`, `delaycompress` and `nocompress`.
+</Note>
+
+If you want to force disable log rotation (for example if your `logrotate` config lives outside `/etc/logrotate.d/` or does not contain the literal string `netbird`), set [`NB_LOG_DISABLE_ROTATION=true`](/client/environment-variables#logging) on the daemon. When the env flag is set, `netbird.err` or the console will show:
+
+```text
+log rotation is disabled by env flag
+```
+
+<Warning>
+  If you disable built-in rotation without configuring an external rotator, `client.log` will grow without bound. Make sure something is rotating the file before turning rotation off.
+</Warning>
+
 ### Debug bundle
 
 A debug archive containing the recent logs and the status at the time of execution can be generated with the following