NLog.Targets.OpenTelemetryHttp 6.2.3

NLog.Targets.OpenTelemetryHttp

NLog OpenTelemetry target for exporting log events to an OpenTelemetry Collector or any OTLP/HTTP-compatible endpoint.

Designed for applications that already use NLog and want to export log events directly using the OTLP protobuf format, without depending on the OpenTelemetry SDK logging pipeline.

The target automatically captures TraceId and SpanId for distributed tracing correlation and exports NLog event properties as structured OpenTelemetry log attributes.

If you have trouble with output, check NLog InternalLogger for clues. See also Troubleshooting NLog.

See also the OpenTelemetry specifications for OTLP/HTTP and exporters.

Register Extension

The OpenTelemetry type alias is available in an NLog.config file after registering the extension:

<extensions>
    <add assembly="NLog.Targets.OpenTelemetryHttp"/>
</extensions>

Alternatively, register it in code using the fluent configuration API:

LogManager.Setup().SetupExtensions(ext => {
    ext.RegisterTarget<NLog.Targets.OpenTelemetryHttpTarget>();
});

Configuration Examples

The typical OTLP/HTTP endpoint URL is http://localhost:4318/v1/logs. The target also reads OpenTelemetry environment variables as fallbacks.

NLog.config

<nlog xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<extensions>
    <add assembly="NLog.Targets.OpenTelemetryHttp"/>
</extensions>

<targets>
    <target xsi:type="OpenTelemetry"
            name="otel"
            url="http://localhost:4318/v1/logs" />
</targets>

<rules>
    <logger name="*" minlevel="Info" writeTo="otel" />
</rules>
</nlog>

AppSettings.json

When using NLog.Extensions.Logging the target can be configured from appsettings.json:

{
  "NLog": {
    "extensions": [
      { "assembly": "NLog.Targets.OpenTelemetryHttp" }
    ],
    "targets": {
      "otel": {
        "type": "OpenTelemetry",
        "url": "http://localhost:4318/v1/logs"
      }
    },
    "rules": [
      { "logger": "*", "minLevel": "Info", "writeTo": "otel" }
    ]
  }
}

Fluent Configuration API

NLog.LogManager.Setup().LoadConfiguration(builder =>
{
    builder.ForLogger()
        .FilterMinLevel(NLog.LogLevel.Info)
        .WriteTo(new NLog.Targets.OpenTelemetryHttpTarget
        {
            Url = "http://localhost:4318/v1/logs"
        });
});

Parameters

Resource Attributes

Additional OpenTelemetry resource attributes can be configured using resourceAttribute entries:

<target xsi:type="OpenTelemetry" name="otel">

    <resourceAttribute name="service.namespace" layout="Backend" />
    <resourceAttribute name="deployment.environment" layout="Production" />

</target>

Structured Log Properties

NLog event properties are exported as structured OpenTelemetry log attributes using the OTLP AnyValue format:

• Primitive values: strings, Booleans, integers, and floating-point numbers.
• Enum values: serialized as strings.
• DateTime values: DateTime and DateTimeOffset serialized using ToString("o") for round-trip ISO 8601 format.
• Dictionary values: IDictionary and IEnumerable<KeyValuePair<string, object>> represented as OpenTelemetry KeyValueList values.
• Arrays and collections: serialized as OpenTelemetry ArrayValue, with default max of 1000 items, but can be adjusted with setting maxItemsPerCollection
• Nested collections: Max Recursion Depth of default two, but can be adjusted with setting maxRecursionDepth.

Unknown or unrecognized object types fall back to string formatting. Values beyond the supported nesting depth are formatted as strings. String values longer than 65,536 characters are truncated.

Azure Application Insights

Microsoft Application Insights ver. 3.x supports log ingestion using the OpenTelemetry Protocol (OTLP).

NLog.Targets.OpenTelemetryHttp supports custom HTTP headers, allowing NLog to send logs directly to Application Insights using HTTP authentication headers:

<target xsi:type="OpenTelemetry" name="applicationInsights"
        url="https://your-app-insights-endpoint/v1/logs">
    <header name="x-otlp-api-key" layout="${configsetting:name=ApplicationInsights.InstrumentationKey}" />
</target>

Note The official approach for sending logs and telemetry to Application Insights is to use: builder.Services.AddOpenTelemetry().UseAzureMonitor(opt => ...).

OpenTelemetry Environment Variables

The target automatically resolves standard OpenTelemetry environment variables when equivalent target properties have not been explicitly configured.

Endpoint

Order of precedence:

1. OTEL_EXPORTER_OTLP_LOGS_ENDPOINT
2. OTEL_EXPORTER_OTLP_ENDPOINT

When OTEL_EXPORTER_OTLP_ENDPOINT is used, /v1/logs is automatically appended.

Headers

Order of precedence:

1. OTEL_EXPORTER_OTLP_LOGS_HEADERS
2. OTEL_EXPORTER_OTLP_HEADERS

Example:

OTEL_EXPORTER_OTLP_HEADERS=api-key=my-secret-key

Compression

Order of precedence:

1. OTEL_EXPORTER_OTLP_LOGS_COMPRESSION
2. OTEL_EXPORTER_OTLP_COMPRESSION

Supported values:

• gzip
• none

Timeout

Order of precedence:

1. OTEL_EXPORTER_OTLP_LOGS_TIMEOUT
2. OTEL_EXPORTER_OTLP_TIMEOUT

Timeout values are specified in milliseconds.

Service Name

OTEL_SERVICE_NAME

Overrides the service.name value from OTEL_RESOURCE_ATTRIBUTES.

Resource Attributes

OTEL_RESOURCE_ATTRIBUTES

Example:

OTEL_RESOURCE_ATTRIBUTES=service.namespace=Backend,deployment.environment=Production

The special attribute:

service.name

is automatically mapped to the serviceName property.

Client Certificate

Order of precedence:

1. OTEL_EXPORTER_OTLP_LOGS_CLIENT_CERTIFICATE
2. OTEL_EXPORTER_OTLP_CLIENT_CERTIFICATE

Used to configure mutual TLS authentication.