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Override HTTP Host headers

When your application needs specialized routing (CNAME setup or custom hosts like Heroku), you can customize the Host header used in health checks on a per-origin or per-monitor level.

Per origin Host header override

To balance traffic across multiple hosts, add Host headers to individual origins within the same pool.

For example, you might have a pool with origins hosted in multiple AppEngine projects or Amazon S3 buckets. You also might want to set up specific failover origins within a pool.

Since these examples require specific hostnames per origin, your load balancer will not properly route traffic without a Host header override.

If you need an origin Host header override, add it when creating or editing a pool. For security reasons, this header must meet one of the following criteria:

  • Is a subdomain of a zone associated with this account
  • Matches the origin address
  • Publicly resolves to the origin address

Host header prioritization

If you set a header override on an individual origin, it will take precedence over a header override set on a monitor during health checks.

For example, you might have a load balancer for www.example.com with the following setup:

  • Origin Pools:

    • Pool 1:

      • Origin 1 (Host header set to lb-app-a.example.com)
      • Origin 2
    • Pool 2:

      • Origin 3
      • Origin 4 (Host header set to lb-app-b.example.com)
  • Monitor (Host header set to www.example.com)

In this scenario, health checks for Origin 1 would use lb-app-a.example.com, health checks for Origin 4 would use lb-app-b.example.com, and all other health checks would default to www.example.com. For more information on updating your custom host configuration to be compatible with Cloudflare, see Configure Cloudflare and Heroku over HTTPS.

For a list of origins that override a monitor's Host header:

  1. On a monitor, select Edit.
  2. Select Advanced health check settings.
  3. If you have origin overrides, you will see Origin host header overrides.

List of origin host header overrides