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Headers

Background

All HTTP request and response headers are available through the Headers API.

When a header name possesses multiple values, those values will be concatenated as a single, comma-delimited string value. This means that Headers.get will always return a string or a null value. This applies to all header names except for Set-Cookie, which requires Headers.getAll. This is documented below in Differences.

let headers = new Headers;
headers.get('x-foo'); //=> null
headers.set('x-foo', '123');
headers.get('x-foo'); //=> "123"
headers.set('x-foo', 'hello');
headers.get('x-foo'); //=> "hello"
headers.append('x-foo', 'world');
headers.get('x-foo'); //=> "hello, world"

Differences

Cloudflare headers

Cloudflare sets a number of its own custom headers on incoming requests and outgoing responses. While some may be used for its own tracking and bookkeeping, many of these can be useful to your own applications – or Workers – too.

Request headers

  • CF-Connecting-IP: In same-zone Worker subrequests, the value of CF-Connecting-IP reflects the value of x-real-ip (the client’s IP). x-real-ip can be altered by the user in their Worker script.

    In cross-zone subrequests from one Cloudflare customer zone to another Cloudflare customer zone, the CF-Connecting-IP value will be set to the Worker client IP address '2a06:98c0:3600::103' for security reasons.

    For Worker subrequests destined for a non-Cloudflare customer zone, the CF-Connecting-IP and x-real-ip headers will both reflect the client's IP address, with only the x-real-ip header able to be altered.

    When no Worker subrequest is triggered, cf-connecting-ip reflects the client's IP address and the x-real-ip header is stripped.

  • CF-Worker: Added to all Worker subrequests sent via fetch(). Set to the name of the zone which owns the Worker making the subrequest. For example, a Worker script on route for foo.example.com/* from example.com will have all subrequests with the header:

    CF-Worker: example.com

    The intended purpose of this header is to provide a means for recipients (e.g., origins, load balancers, other Workers) to recognize, filter, and route traffic generated by Workers on specific zones.

  • CF-EW-Via: Used for loop detection, similar to the CDN-Loop header.