Create custom headers in backend services

This page describes how to configure custom headers in backend services used by global external HTTP(S) load balancers.

Custom request and response headers let you specify additional headers that the load balancer can add to HTTP(S) requests and responses. Depending on the information detected by the load balancer, these headers can include the following information:

• Latency to the client
• Geographic location of the client's IP address
• 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.

Important: Global external HTTP(S) load balancers also support header transformations in the URL map. You can manage custom headers in backend services (as described on this page) or in URL maps. The capabilities are similar. We recommend putting your header transformations in the URL map so that all of your header transformations are maintained in a single location. If custom header configurations are present in both the URL map and the backend service, both configurations are applied. However, if the same header is set in both the backend service and the URL map, the URL map takes precedence and the backend service setting is ignored. For a URL map-based example, see Setting up traffic management for global external HTTP(S) load balancers.

Before you begin

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

  gcloud components update
  

  Note: Make sure that you're using gcloud CLI version 309.0.0 or later.

How custom headers work

Custom headers work as follows:

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

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

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-
• The header name must not be Host or authority. Both Host and authority are special keywords reserved by Google Cloud. You can't modify these headers for Envoy-based load balancers such as the global external HTTP(S) load balancer. Instead, we recommend that you create other custom headers (for example, MyHost) so that you don't interfere with the reserved header names.
• 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]'

Any variable names 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.

Note: We announced a change in behavior for custom headers configured on URL maps and backend services. This change does not affect projects that had existing traffic that used custom headers before November 14, 2022. For details, see the release note for November 14, 2022.

Variables that can appear in the header value

The following variables can appear in custom header values.

Variable	Description
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.
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. Smoothed RTT is an algorithm that deals with variations and anomalies that may occur in RTT measurements.
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.
client_ip_address	The client's IP address. This is the same as the client IP address that is the next-to-last address in the X-Forwarded-For header.
client_port	The client's source port.
client_encrypted	Set to true if the connection between the client and the load balancer is encrypted (using HTTPS, HTTP/2 or HTTP/3), set to false otherwise.
client_protocol	The HTTP protocol used for communication between the client and the load balancer. One of HTTP/1.0, HTTP/1.1, HTTP/2, or HTTP/3.
server_ip_address	The IP address of the load balancer that the client connects to. This can be useful when multiple load balancers share common backends. This is the same as the last IP address in the X-Forwarded-For header.
server_port	The destination port number that the client connects to.
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.
tls_ja3_fingerprint	JA3 TLS/SSL fingerprint if the client connects using HTTPS, HTTP/2 or HTTP/3.

The load balancer expands variables to empty strings when it cannot determine their values. For example:

• Geographic location variables when the IP address's location is unknown
• TLS parameters when TLS is not in use
• The {origin_request_header} when the request does not include an Origin header
• The {cdn_cache_status} header when included in a request header

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 specify custom request headers, use the gcloud compute backend-services create or gcloud compute backend-services update command with the --custom-request-header flag.

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_1_NAME:[HEADER_1_VALUE]' \
    --custom-request-header='HEADER_2_NAME:[HEADER_2_VALUE]'

The previous step replaces any headers already in the backend service with the request headers that 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://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices/BACKEND_SERVICE_NAME
"customRequestHeaders": [
   "client_city:Mountain View"
]

Working with custom response headers

Console

To add custom response 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 response headers, click Add header.
7. Enter the Header name and Header value for the custom response header.
8. Enter any additional custom response headers.
9. Click Save.

To remove a custom response 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 response header you want to remove.
7. Click Save.

gcloud

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

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

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

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

Example:

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

Example:

The following example shows you how to add a custom response header to support HTTP Strict Transport Security (HSTS):

gcloud compute backend-services update customer-bs-name \
    --global \
    --custom-response-header='Strict-Transport-Security: max-age=63072000'

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/v1/projects/PROJECT_ID/global/backendBuckets

PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendBuckets/BACKEND_BUCKET_NAME

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices

PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices/BACKEND_SERVICE_NAME

Add the following snippet to the JSON request body:

"customResponseHeaders":HEADER_NAME:[HEADER_VALUE]

Setting response headers for Cloud Storage

If you need to set HTTP headers on responses from Cloud Storage—such as Cross-Origin Resource Policy, X-Frame-Options, or X-XSS-Protection headers—Google Cloud offers the option to use custom headers for Cloud CDN with Cloud Storage. You can do this by configuring custom headers at the load balancer backend bucket level, as described on this page.

Custom response headers configured at the backend bucket level are only added to responses if the client request is sent to the load balancer IP address. Custom headers are not added to responses if the client's request was made directly to the Cloud Storage API.

Working with Google Cloud Armor

When you configure a Google Cloud Armor security policy, you can configure Google Cloud Armor to insert a custom header and value. If your Google Cloud Armor security policy is configured to insert the same custom header name as your global external HTTP(S) load balancer or global external HTTP(S) load balancer (classic)'s custom headers, then the header value populated by the load balancer is overwritten by the value specified in your Google Cloud Armor security policy. If you do not want Google Cloud Armor to overwrite the header value, ensure that you do not use the same name.

Limitations

The following limitations apply to custom headers:

• Custom headers are not supported for regional external HTTP(S) load balancers.
• 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.