Creating custom headers

Custom request and response headers allow you to specify additional headers that the external HTTP(S) load balancer adds to requests and responses. These headers can include information that the load balancer detects about the client connection, including the latency to the client, the geographic location of the client's IP address, and parameters of the TLS connection.

Custom request headers are supported for backend services, while custom response headers are supported for backend services and backend buckets.

HTTP(S) Load Balancing adds certain headers by default to all HTTP(S) requests and responses that it proxies between backends and clients. For more information, see Target proxies.

Before you begin

  • If necessary, update to the latest version of the Cloud SDK:

    gcloud components update
    

How custom headers work

Custom headers work as follows:

  • When the external HTTP(S) load balancer makes a request to the backend, the load balancer adds request headers.

  • The external HTTP(S) load balancer sets response headers before returning the responses to the client.

The request and response headers don't influence caching or load balancer behavior.

To enable custom headers, you specify a list of headers in a property of the backend service or backend bucket.

You specify each header as a header-name:header-value string. The header must contain a colon separating the header name and header value. Header names have the following properties:

  • The header name must be a valid HTTP header-field name definition (per RFC 7230).
  • The header name must not be X-User-IP or CDN-Loop.
  • The header name must not begin with X-Google, X-Goog-, X-GFE or X-Amz-
  • A header name must not appear more than once in the list of added headers.

Header values have the following properties:

  • The header value must be a valid HTTP header field definition (per RFC 7230, with obsolete forms disallowed).
  • The header value can be blank.
  • The header value can include one or more variables, enclosed by curly braces, that expand to values that the load balancer provides. A list of variables allowed in the header value is in the next section.

For example, you might specify a header with two variable names, for the client region and client city. The gcloud command-line tool has a flag for specifying request headers, --custom-request-header. For example:

    --custom-request-header 'X-Client-Geo-Location:{client_region},{client_city}'

For clients located in Mountain View, California, the load balancer adds a header as follows:

X-Client-Geo-Location:US,Mountain View

The general format for the flag is:

    --custom-request-header='HEADER_NAME:[HEADER_VALUE]'

The header values must be enclosed in curly braces. For example:

    --custom-request-header='X-PLACE:{client_city},{client_city_lat_long}'

Use only straight quotation marks (') in this command.

Variables that can appear in the header value

The following variables can appear in custom header values.

Variable Description Request Response (Beta)
cdn_cache_id Location code and ID of the cache instance used to serve the request. This is the same value populated in the jsonPayload.cacheId field of the Cloud CDN request logs in Logging.
cdn_cache_status Current cache status. Values can be hit, miss, revalidated, stale, uncacheable, or disabled for any object served by a Cloud CDN-enabled backend. N/A
origin_request_header Reflects the value of the Origin header in the request for Cross-Origin Resource Sharing (CORS) use cases.
client_rtt_msec Estimated round-trip transmission time between the load balancer and the HTTP(S) client, in milliseconds. This is the smoothed round-trip time (SRTT) parameter measured by the load balancer's TCP stack, per RFC 2988.
client_region The country (or region) associated with the client's IP address. This is a Unicode CLDR region code, such as US or FR. (For most countries, these codes correspond directly to ISO-3166-2 codes.)
client_region_subdivision Subdivision, e.g. a province or state, of the country associated with the client's IP address. This is a Unicode CLDR subdivision ID, such as USCA or CAON. (These Unicode codes are derived from the subdivisions defined by the ISO-3166-2 standard.)
client_city Name of the city from which the request originated, for example, Mountain View for Mountain View, California. There is no canonical list of valid values for this variable. The city names can contain US-ASCII letters, numbers, spaces, and the following characters: !#$%&'*+-.^_`|~.
client_city_lat_long Latitude and Longitude of the city from which the request originated, for example, 37.386051,-122.083851 for a request from Mountain View.
tls_sni_hostname Server name indication (as defined in RFC 6066), if provided by the client during the TLS or QUIC handshake. The hostname is converted to lowercase and with any trailing dot removed.
tls_version TLS version negotiated between client and load balancer during the SSL handshake. Possible values include: TLSv1, TLSv1.1, TLSv1.2, and TLSv1.3. If the client connects using QUIC instead of TLS, the value is QUIC.
tls_cipher_suite Cipher suite negotiated during the TLS handshake. The value is four hex digits defined by the IANA TLS Cipher Suite Registry, for example, 009C for TLS_RSA_WITH_AES_128_GCM_SHA256. This value is empty for QUIC and for unencrypted client connections.

The load balancer expands variables to empty strings when it cannot determine their values, for example for geographic location variables when the IP address's location is unknown, or for TLS parameters when TLS is not in use. For example:

  • Geographic location variables when the IP address's location is unknown
  • client_region_subdivision, client_city and client_city_lat_long in response headers
  • TLS parameters when TLS is not in use

Geographic values (regions, subdivisions, and cities) are estimates based on the client's IP address. From time to time, Google updates the data that provides these values in order to improve accuracy and to reflect geographic and political changes.

Headers added by the load balancer overwrite any existing headers that have the same name. Header names are case insensitive. When header names are passed to an HTTP/2 backend, the HTTP/2 protocol encodes header names as lower case.

In header values, leading whitespace and trailing whitespace are insignificant, and are not passed to the backend. To allow for curly braces in header values, the load balancer interprets two opening curly braces ({{) as a single opening brace ({), and two closing curly braces (}}) as a single closing brace (}).

Working with custom request headers

Console

To add custom request headers to an existing backend service:

  1. Go to the load balancing summary page.
    Go to the Load balancing page
  2. Click Backends.
  3. Click the name of a backend service.
  4. Click Edit .
  5. Click Advanced configurations (Session affinity, connection draining timeout, security policies).
  6. Under Custom request headers, click Add header.
  7. Enter the Header name and Header value for the custom request header.
  8. Enter any additional custom request headers.
  9. Click Save.

To remove a custom request header from a backend service:

  1. Go to the load balancing summary page.
    Go to the Load balancing page
  2. Click Backends.
  3. Click the name of a backend service.
  4. Click Edit .
  5. Click Advanced configurations (Session affinity, connection draining timeout, security policies).
  6. Click the X next to the name of the custom request header you want to remove.
  7. Click Save.

gcloud

To create a backend service with custom request headers:

gcloud compute backend-services create BACKEND_SERVICE_NAME \
  --global-health-checks \
  --global \
  --protocol HTTPS \
  --health-checks https-basic-check \
  --custom-request-header 'HEADER_NAME:[HEADER_VALUE]'

To add more request headers, specify a unique header name and value by repeating the --custom-request-header flag.

To add custom headers to an existing backend service:

gcloud compute backend-services update BACKEND_SERVICE_NAME \
  --global \
  --custom-request-header 'HEADER_NAME:[HEADER_VALUE]' \
  --custom-request-header 'HEADER_NAME:[HEADER_VALUE]'

Note that the above replaces any headers already in the backend service with the request headers you specify in the command.

To remove all headers from a backend service:

gcloud compute backend-services update BACKEND_SERVICE_NAME \
  --global \
  --no-custom-request-headers

API

Make a PATCH request to the backendServices.patch method.

PATCH https://www.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices/BACKEND_SERVICE_NAME
"customRequestHeaders": [
   "client_city:Mountain View"
]

Working with custom response headers

Console

The Cloud Console is currently unsupported.

gcloud

For backend services, use the gcloud beta compute backend-services create or gcloud beta compute backend-services update command with the --custom-response-header flag.

For backend buckets, use the gcloud beta compute backend-buckets create or gcloud beta compute backend-buckets update command with the --custom-response-header flag.

gcloud beta compute backend-services (create | update) BACKEND_SERVICE_NAME
    --custom-response-header='HEADER_NAME:[HEADER_VALUE]'
gcloud beta compute backend-buckets (create | update) BACKEND_BUCKET_NAME
    --custom-response-header='HEADER_NAME:[HEADER_VALUE]'

Example:

gcloud beta compute backend-buckets update gaming-lab \
    --custom-response-header='X-Frame-Options: DENY'

API

For backend buckets, use the Method: backendBuckets.insert or Method: backendBuckets.update API call.

For backend services, use the Method: backendServices.insert or Method: backendServices.update API call.

Use one of the following API calls:

POST https://compute.googleapis.com/compute/beta/projects/PROJECT_ID/global/backendBuckets

PUT https://compute.googleapis.com/compute/beta/projects/PROJECT_ID/global/backendBuckets/BACKEND_BUCKET_NAME

POST https://compute.googleapis.com/compute/beta/projects/PROJECT_ID/global/backendServices

PUT https://compute.googleapis.com/compute/beta/projects/PROJECT_ID/global/backendServices/BACKEND_SERVICE_NAME

Add the following snippet to the JSON request body:

"cdnPolicy": {
  "customResponseHeaders":HEADER_NAME:[HEADER_VALUE]
}

Setting response headers for Cloud Storage

If you need to set other HTTP headers on responses from Cloud Storage—such as Cross-Origin Resource Policy, X-Frame-Options, or X-XSS-Protection headers—Google Cloud offers two options to set custom headers for Cloud CDN with Cloud Storage:

Limitations

The following limitations apply to custom headers:

  • You can specify a maximum of 16 custom request headers for each backend service.
  • You can specify a maximum of 16 custom response headers for each backend service.
  • The total size of all custom request headers per backend service (name and value combined, before variable expansion) cannot exceed 8 KB.
  • The total size of all custom response headers per backend service (name and value combined, before variable expansion) cannot exceed 8 KB.

What's next