To find what is calling a Fastify API, log request-scoped metadata such as request.id, request.ip, the HTTP method, route, and selected headers. To identify a verified user or service, use the identity established by your authentication middleware—not an IP address, user-agent, or request ID. If Fastify sits behind a proxy, configure proxy trust for the actual trusted chain before relying on forwarded addresses.
What Fastify can tell you about a caller
Fastify exposes several useful signals on each request, but they answer different questions. Its Request reference warns that request.ip, request.ips, request.host, request.hostname, request.port, and request.protocol come from socket or forwarding metadata and should be treated as untrusted input. See the Fastify Request reference.
| Signal | What it helps establish | What it does not establish |
|---|---|---|
request.id |
Correlates a request with its log entries; it can also help connect services if your application propagates a trustworthy correlation ID. | The identity of the person or service making the request. If request-ID headers are enabled, a client may supply an arbitrary value unless your application validates or controls it. |
request.ip |
The socket address by default, or a proxy-derived address when proxy trust is enabled. | A verified user or device. Shared NAT, gateways, and proxies may mean the address represents infrastructure or multiple callers. |
request.ips |
The forwarded address chain when proxy trust is enabled. | A reliable origin when forwarding metadata comes through untrusted or incorrectly configured proxies. |
| Incoming headers | Debugging clues such as a user-agent or a client-supplied request ID. | Verified identity. Header values are supplied by clients and can be spoofed. |
| Application authentication result | The verified account or service principal established by your authentication code, such as a validated API key or token subject. | Fastify does not determine this identity on its own; the exact mechanism depends on your application. |
For current behavior and configuration details, use documentation matching the Fastify major version installed by your application. The links here point to the project’s rolling latest documentation.
Log the requests you need to investigate
Enable Fastify logging
Fastify logging is off by default. Enable it when creating the instance with { logger: true } or a logger configuration such as { logger: { level: 'info' } }. Fastify uses Pino as its default logger when logging is enabled. The Fastify Logging guide describes the available options.
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Record a compact, intentional set of fields
Use the request-scoped request.log logger to keep an event associated with its request. For example, an onRequest hook can record the method, matched route, request ID, network address, and a selected client hint:
fastify.addHook('onRequest', async (request) => {
request.log.info({
method: request.method,
route: request.routeOptions.url,
requestId: request.id,
remoteIp: request.ip,
userAgent: request.headers['user-agent']
}, 'incoming request')
})Adapt the fields to your Fastify version and application. Treat the user-agent and every other incoming header as untrusted. Do not log the full headers or request body by default: credentials, personal data, and other secrets can end up in log storage. Fastify specifically warns that logging response headers may expose sensitive authentication data. If body logging is genuinely necessary, note that request bodies are not yet parsed when request serializers run; the Logging guide points to a preHandler hook for body logging, which should still be narrowly scoped and carefully controlled.
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Configure proxy trust before using forwarded IPs
Without proxy trust, request.ip is based on the socket address. With trustProxy enabled, Fastify can derive the address from X-Forwarded-For, and request.ips exposes the forwarded chain. Those values are only useful if the deployment’s trusted proxy path is configured correctly.
Set trustProxy to match known proxy addresses or use a trust function that validates the immediate peer. Do not trust every source if clients can also reach the Fastify origin directly: a client able to supply forwarding headers could otherwise influence the address Fastify reports. Consult the Fastify Server reference for proxy configuration and version-specific settings.
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Use authenticated identity to name a caller
If the question is “Which account or service made this call?”, inspect the verified result of your application’s authentication process. Log a suitable account or service identifier only after your middleware or handler has validated the relevant credential. Fastify’s request metadata can help explain where a request arrived from and correlate it across logs, but it does not authenticate the caller.
- Use
request.idto trace an individual request. - Use
request.ipand, when correctly configured,request.ipsto investigate network routing. - Use selected headers as diagnostic hints, not as proof.
- Use your authentication result to identify a verified user or service.
Keep request logs useful and safe
- Allow-list the fields you need instead of serializing every header.
- Redact authorization credentials and other secrets in logger configuration.
- Limit access to logs and their retention according to the sensitivity of the data recorded.
- Avoid logging bodies unless there is a specific operational need and an appropriate privacy and security policy.
Fastify’s Logging guide demonstrates redacting req.headers.authorization and cautions that header logging can expose sensitive information. Logging fewer, purposeful fields usually makes investigations clearer while reducing the chance of storing credentials or other private data.
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