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Enabling URL token validation

  Last updated September 04, 2018

Token validation allows you to create URLs that expire. Tokens are generated within your web application and appended to URLs in a query string. Requests are authenticated at Fastly's edge instead of your origin server. When Fastly receives a request for the URL, the token is validated before serving the content. After a configurable period of time, the token expires.

Adding custom VCL

To enable token validation, you'll need to create a Varnish configuration named vcl_recv and add the following example code to it.

/* only do this once per request */
if (req.restarts == 0) {
  declare local var.token_exp STRING;
  declare local var.token_sig STRING;

  /* make sure there is a token */
  if (req.url !~ ".+\?.*token=(\d{10,11})_([^&]+)") {
    error 403;
  }

  /* extract token expiration and signature */
  set var.token_exp = re.group.1;
  set var.token_sig = re.group.2;

  /* validate signature */
  if (var.token_sig == regsub(digest.hmac_sha1(digest.base64_decode("YOUR%SECRET%KEY%IN%BASE64%HERE"),
      req.url.path var.token_exp), "^0x", "")) {

    /* check that expiration time has not elapsed */
    if (time.is_after(now, std.integer2time(std.atoi(var.token_exp)))) {
      error 410;
    }
  } else {
    error 403;
  }
}

The custom VCL code above checks for two things:

If the signature is invalid, Varnish returns a 403 response. If the signature is valid but the expiration time has elapsed, Varnish returns a 410 response. The different response codes are helpful for debugging.

The token information

A token is expected in the ?token= GET parameter. Tokens take the format [expiration]_[signature] and look like this:

1441307151_4492f25946a2e8e1414a8bb53dab8a6ba1cf4615

The full request URL with the token looks like this:

http://www.example.com/foo/bar.html?token=1441307151_4492f25946a2e8e1414a8bb53dab8a6ba1cf4615

The signature validation

The key found in digest.hmac_sha1 can be any string. The one in this example was generated with the command openssl rand -base64 32. The example key YOUR%SECRET%KEY%IN%BASE64%HERE will intentionally cause an error if you use it. You must replace it with your own randomly generated secret key.

Configuring your application

You'll need to write custom code in your application to generate tokens and authenticate with Varnish. We provide examples in our token functions repository on GitHub. Review the examples in the repository to learn how to generate custom tokens within your application.

Testing

To test your configuration, append a token generated by your application to a URL in a query string. For example:

http://www.example.com/foo/bar.html?token=1441307151_4492f25946a2e8e1414a8bb53dab8a6ba1cf4615

If the token is valid, you will receive a normal response. If it is invalid, you will receive a 403 response.

Troubleshooting NUL bytes

You should verify that your secret key is devoid of NUL bytes. If the base64-decoded string contains a NUL byte (0x00), then that byte and any bytes following it will not be included in the response. See base64 decoding for more information.

Excluding the token query string from the cache key

If all tokens are dynamic and different, you may want to exclude the token query string from the cache key to avoid potentially affecting the cache hit ratio. To do this, you'll need to add the following code in custom VCL in addition to the example code above:

/* strip out the token querystring so Fastly does not vary the cache. */
set req.url = querystring.filter(req.url, "token");

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