Troubleshooting Cloudflare Access Denied Errors from GCP Instances

Apr 2, 2024 ยท 3 min read

Have you ever tried accessing a website hosted behind Cloudflare from your Google Cloud Platform (GCP) instance, only to be greeted by a cryptic "Access Denied" error message? This frustrating problem can happen due to various reasons, but the good news is it can usually be easily fixed with some targeted troubleshooting.

Why Access Might Be Blocked

There are a few common culprits for getting blocked by Cloudflare when trying to access a site from GCP:

  • IP reputation - Cloudflare maintains a database of IP addresses tied to malicious activity like spam or attacks. If your GCP instance gets assigned an IP that is on this blacklist, Cloudflare will automatically block requests from it.
  • Geography - Some website owners configure Cloudflare to only allow traffic from certain countries or regions. Your GCP instance might be in a data center that is blocked.
  • Bot protection - Cloudflare's bot management can sometimes misidentify scripts and automation tools as bots. If you are programmatically accessing the site from your instance, this could trigger an access denial.
  • Troubleshooting Steps

    Here are some things you can try to regain access:

    Check IP Reputation

    Use a tool like IPVoid to check if your GCP instance's public IP address has been flagged for abuse. If so, you may need to restart your instance to get a clean IP assigned.

    # Restart GCP instance
    gcloud compute instances reset [INSTANCE_NAME]

    Verify Location Access

    Some websites block traffic from certain physical locations. Use a geolocation tool like IPLocation to check if your instance's region is being blocked. Consider launching in a different zone if access is limited.

    Confirm Not a Bot

    Cloudflare may be blocking automated scraping or scripted access attempts. Try manually accessing the site from your instance through curl or browser to verify you are not being flagged as a bot.

    # Test site access from command line
    curl -I

    Check Cloudflare WAF Rules

    The site owner may have set up Web Application Firewall (WAF) rules in Cloudflare that are blocking your instance. Ask them to review their WAF configuration and whitelist your instance's IP if needed.

    Use a VPN/Proxy

    As a last resort, you can mask your GCP IP and geography by connecting your instance to a VPN service or proxy server. This makes your traffic appear to come from the VPN/proxy provider instead.

    Gaining Insights from Errors

    While frustrating, "Access Denied" errors often provide clues to the underlying block reason if you look closely.

    # Example Cloudflare Block Page
    Access Denied
    You don't have permission to access "" on this server.
    Reference #18.adff185.1594934.1c7364b

    The reference # provides a unique identifier for the block instance that can be used when troubleshooting with Cloudflare support. The first section gives context on the specific policy triggering it.

    Digging into these details can reveal whether it is an IP-based block, bot related, or other firewall policy causing issues.

    Preventing Future Access Problems

    Once you regain access, it is smart to take steps to prevent ending up blocked again in the future:

  • Whitelist instance IP - Have the site owner add your instance's IP to a Cloudflare allow list.
  • Automate IP refreshing - Script instance restarts/recreation on a schedule to cycle IPs automatically.
  • Maintain user-agent - Mimic a real browser's user-agent string if possible when programmatically accessing pages.
  • Enable Cloudflare cache - Cached page access is less likely to trigger bot detection.
  • With some targeted troubleshooting and preventative measures, you can get Cloudflare to stop wrongly blocking your important GCP instance traffic.

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