> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/nats-io/nats-server/llms.txt
> Use this file to discover all available pages before exploring further.

# Logging Options

> Configure NATS server logging, debugging, and tracing

Logging options control how NATS server outputs operational information, debug messages, and protocol traces.

## Command Syntax

```bash theme={null}
nats-server [logging-options]
```

## Logging Options Reference

<ParamField path="-l, --log" type="string">
  File to redirect log output. By default, logs are written to stdout.

  **Examples:**

  ```bash theme={null}
  # Log to file
  nats-server -l /var/log/nats-server.log

  # Log to file with rotation
  nats-server -l /var/log/nats.log --log_size_limit 100MB
  ```

  <Note>
    The log file can be rotated using the `reopen` signal. See [Signals](/cli/signals) for details.
  </Note>
</ParamField>

<ParamField path="-T, --logtime" type="boolean" default="true">
  Timestamp log entries. Enabled by default.

  **Examples:**

  ```bash theme={null}
  # Timestamps enabled (default)
  nats-server -T

  # Disable timestamps
  nats-server -T=false
  ```

  With timestamps:

  ```
  [1] 2024-03-04 10:15:23.123456 [INF] Starting nats-server
  ```

  Without timestamps:

  ```
  [1] [INF] Starting nats-server
  ```
</ParamField>

<ParamField path="-s, --syslog" type="boolean">
  Log to syslog or Windows event log. Routes log output to the system's logging facility.

  **Examples:**

  ```bash theme={null}
  # Log to local syslog
  nats-server -s

  # On Linux, logs appear in /var/log/syslog or /var/log/messages
  # On macOS, logs appear in system log (viewable with Console.app)
  # On Windows, logs appear in Event Viewer
  ```
</ParamField>

<ParamField path="-r, --remote_syslog" type="string">
  Syslog server address. Send logs to a remote syslog server.

  **Format:** `udp://hostname:port`

  **Examples:**

  ```bash theme={null}
  # Default remote syslog
  nats-server -r udp://localhost:514

  # Custom remote syslog server
  nats-server -r udp://syslog.example.com:514

  # Remote syslog on custom port
  nats-server -r udp://10.0.1.5:5140
  ```
</ParamField>

<ParamField path="-D, --debug" type="boolean">
  Enable debugging output. Provides detailed information about server operations.

  **Examples:**

  ```bash theme={null}
  # Enable debug logging
  nats-server -D

  # Debug with log file
  nats-server -D -l debug.log
  ```

  Debug output includes:

  * Client connections and disconnections
  * Subscription details
  * Route and gateway connections
  * Internal server state changes
</ParamField>

<ParamField path="-V, --trace" type="boolean">
  Trace the raw protocol. Shows the actual NATS protocol messages exchanged with clients (excluding system account).

  **Examples:**

  ```bash theme={null}
  # Enable protocol tracing
  nats-server -V

  # Trace with debug
  nats-server -DV
  ```

  Trace output example:

  ```
  [2] 2024-03-04 10:15:23.456789 [TRC] 192.168.1.5:54321 - cid:1 - <<- [PUB foo 5]
  [2] 2024-03-04 10:15:23.456790 [TRC] 192.168.1.5:54321 - cid:1 - <<- MSG_PAYLOAD: ["hello"]
  ```

  <Warning>
    Trace logging can generate significant output and impact performance. Use only for debugging.
  </Warning>
</ParamField>

<ParamField path="-VV" type="boolean">
  Verbose trace. Traces system account as well. Includes all protocol messages including system account traffic.

  **Examples:**

  ```bash theme={null}
  # Verbose trace
  nats-server -VV

  # Debug with verbose trace
  nats-server -DVV
  ```

  Use this when debugging system account issues or JetStream internals.
</ParamField>

<ParamField path="-DV" type="boolean">
  Debug and trace. Convenient combination of debug and trace flags.

  **Examples:**

  ```bash theme={null}
  # Debug and trace
  nats-server -DV

  # Equivalent to
  nats-server -D -V
  ```
</ParamField>

<ParamField path="-DVV" type="boolean">
  Debug and verbose trace. Combination of debug and verbose trace flags.

  **Examples:**

  ```bash theme={null}
  # Debug and verbose trace
  nats-server -DVV

  # Equivalent to
  nats-server -D -VV
  ```
</ParamField>

<ParamField path="--log_size_limit" type="string" default="auto">
  Logfile size limit. When the log file reaches this size, it is automatically rotated.

  **Format:** Number with optional unit (B, KB, MB, GB)

  **Examples:**

  ```bash theme={null}
  # Limit to 100 megabytes
  nats-server -l nats.log --log_size_limit 100MB

  # Limit to 1 gigabyte
  nats-server -l nats.log --log_size_limit 1GB

  # Limit to 50 megabytes
  nats-server -l nats.log --log_size_limit 52428800
  ```

  When the limit is reached, the current log file is renamed with a timestamp suffix and a new log file is created.
</ParamField>

<ParamField path="--max_traced_msg_len" type="number" default="unlimited">
  Maximum printable length for traced messages. Limits the payload size shown in trace output.

  **Examples:**

  ```bash theme={null}
  # Limit traced payload to 1KB
  nats-server -V --max_traced_msg_len 1024

  # Limit to 100 bytes
  nats-server -V --max_traced_msg_len 100
  ```

  Useful when tracing messages with large payloads to prevent log file bloat.

  Output example with limit:

  ```
  [TRC] - cid:1 - <<- MSG_PAYLOAD: ["very long message content..."] (truncated, 5000 bytes total)
  ```
</ParamField>

## Complete Examples

### Development Debugging

```bash theme={null}
# Debug with trace to stdout
nats-server -DV
```

### Production Logging

```bash theme={null}
# Log to file with rotation
nats-server \
  -l /var/log/nats-server.log \
  --log_size_limit 100MB
```

### Remote Syslog

```bash theme={null}
# Send logs to centralized syslog server
nats-server -r udp://syslog.internal.com:514
```

### Protocol Debugging

```bash theme={null}
# Trace protocol with limited message length
nats-server -V --max_traced_msg_len 256 -l trace.log
```

### Full Debug Mode

```bash theme={null}
# Debug, verbose trace, and log to file
nats-server -DVV -l debug.log --log_size_limit 50MB
```

### Production with Info Logging

```bash theme={null}
# Standard logging with timestamps
nats-server \
  -c /etc/nats/nats-server.conf \
  -l /var/log/nats-server.log \
  -T \
  --log_size_limit 100MB
```

## Log Levels

NATS server uses the following log levels:

| Level | Description            | Enabled By        |
| ----- | ---------------------- | ----------------- |
| `ERR` | Error messages         | Always            |
| `WRN` | Warning messages       | Always            |
| `INF` | Informational messages | Always            |
| `DBG` | Debug messages         | `-D` or `--debug` |
| `TRC` | Protocol trace         | `-V` or `--trace` |

## Log Rotation

Logs can be rotated in two ways:

1. **Automatic rotation** - When `--log_size_limit` is reached
2. **Manual rotation** - Send `reopen` signal to the server

```bash theme={null}
# Trigger log rotation
nats-server --signal reopen=/var/run/nats-server.pid
```

See [Signals](/cli/signals) for more details.

## Related Options

* [Server Options](/cli/server-options) - Core server configuration
* [Signals](/cli/signals) - Log rotation and reload signals