Google Cloud Functions SSL Expiry Monitoring Tutorial

Google Cloud Functions are a powerful way to run serverless code, handling everything from API backends to event-driven processing. While Google manages much of the underlying infrastructure, including the default SSL/TLS certificates for its REGION.cloudfunctions.net domains, relying solely on that "managed" aspect can be a pitfall. If you're using custom domains – and most production applications do – the responsibility for ensuring those SSL/TLS certificates remain valid often falls back to you. An expired certificate on a critical function can lead to application downtime, frustrated users, and a scramble to restore service.

This article will walk you through understanding how SSL/TLS works with Cloud Functions, how to identify your relevant endpoints, and the various approaches to monitoring certificate expiry, from manual checks to building your own automated system, and finally, leveraging dedicated tools.

The Challenge with Cloud Functions and SSL/TLS

When you deploy a Google Cloud Function, it's typically assigned a default URL like https://REGION-PROJECT_ID.cloudfunctions.net/FUNCTION_NAME. For these default URLs, Google Cloud automatically provisions and manages the SSL/TLS certificates. This is great; you don't have to worry about them expiring or renewing them.

However, in many real-world scenarios, you'll want to expose your Cloud Function via a custom domain (e.g., api.your-service.com). This is commonly done through various Google Cloud services acting as proxies or frontends:

  • Cloud Load Balancer (External HTTPS Load Balancer): Directs traffic to your Cloud Function (or Function URL) via a serverless NEG. The Load Balancer handles the SSL/TLS certificate for your custom domain.
  • Cloud CDN: Can sit in front of a Load Balancer, also managing SSL/TLS.
  • API Gateway: Provides a managed API layer with custom domain support and can route to Cloud Functions. It handles its own SSL/TLS.
  • Firebase Hosting: Can proxy requests to Cloud Functions and manages SSL/TLS for its custom domains.

In these custom domain setups, while Google often provides managed certificates for services like Load Balancers or API Gateway, these are different certificates than the ones for the default Cloud Functions URLs. More importantly, it's your responsibility to ensure the custom domains are correctly configured and that the managed certificates are in a healthy state (or to provision and renew your own if you're using self-managed certificates). Even Google-managed custom domain certificates can sometimes face issues, and you need to be aware of their expiry or provisioning status to prevent outages.

The core challenge is identifying all the SSL/TLS endpoints that serve your Cloud Functions and establishing a reliable, automated system to monitor their expiry.

Identifying Your Cloud Functions' SSL/TLS Endpoints

Before you can monitor, you need to know what to monitor. This involves listing your Cloud Functions and understanding how they're exposed.

  1. Default Cloud Function URLs: These are the easiest to identify. You can list all your functions and their default HTTPS trigger URLs using the gcloud CLI:

    bash gcloud functions list --format="table(name,entryPoint,status,httpsTrigger.url)"

    This command will output a table similar to this:

    NAME ENTRY_POINT STATUS URL my-function-v1 helloWorld ACTIVE https://us-central1-my-project-12345.cloudfunctions.net/my-function-v1 another-func anotherFunc ACTIVE https://europe-west1-my-project-12345.cloudfunctions.net/another-func

    Each URL in the httpsTrigger.url column is an SSL/TLS endpoint that Google manages. While less critical to monitor for expiry (as Google handles renewal), it's good to keep track of them for completeness, especially if you're using them internally.

  2. Custom Domain URLs: These are the critical ones to monitor. How you find them depends on how you've set them up:

    • Cloud Load Balancer:
      • Check your HTTP(S) Load Balancers. The frontend configuration will show the IP addresses or hostnames used. The SSL certificates are attached to the frontend proxy.
      • You can list load balancers and their frontend configurations: bash gcloud compute url-maps list gcloud compute target-https-proxies list gcloud compute ssl-certificates list
      • Then, you'd trace which SSL certificates are attached to which proxies, and which domains those certificates cover.
    • API Gateway:
      • If you're using API Gateway, list your gateways and their associated custom domains. bash gcloud api-gateway gateways list --location=global gcloud api-gateway apis list # Then inspect API configs and gateway details for custom domains
      • The custom domain you've configured for your API Gateway is the endpoint you need to monitor.
    • Firebase Hosting:
      • If you're using Firebase Hosting to proxy to your Cloud Functions, check your firebase.json configuration for rewrites rules pointing to functions.
      • The custom domains configured for your Firebase project are the ones to monitor. You can find these in the Firebase console or via firebase hosting:site get your-site-id.

    The key is to compile a definitive list of all hostnames (default and custom) that serve your Cloud Functions.

Manual Monitoring: A Recipe for Disaster (or at least, Tedium)

You could manually check the SSL/TLS certificate for each endpoint. For a single domain, it's straightforward:

echo | openssl s_client -servername api.your-service.com -connect api.your-service.com:443 2>/dev/null | openssl x509 -noout -dates

This command will output something like:

``` notBefore=Jan 1 00:00:00 2023 GMT notAfter=Jan 1 23:59:59 2024