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.
The load balancer 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 Google Cloud CLI:
gcloud components update
How custom headers work
Custom headers work as follows:
When the load balancer forwards a request to the backend, the load balancer adds request headers.
The load balancer adds custom request headers only to the client requests, not to the health check probes. If your backend requires a specific header for authorization that is missing from the health check packet, the health check might fail.
The 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 must fulfill the following requirements:
- The header name must be a valid HTTP header-field name definition (per RFC 7230).
- The header name must not be
X-User-IPorCDN-Loop. - The following hop-by-hop headers must not be used:
Keep-Alive,Transfer-Encoding,TE,Connection,Trailer,Upgrade,Proxy-Authorization, andProxy-Authenticate. In accordance with RFC 2616, these headers are not stored by caches or propagated by the target proxies. - The header name must not begin with
X-Google,X-Goog-,X-GFEorX-Amz-. - A header name must not appear more than once in the list of added headers.
Header values must fulfill the following requirements:
- 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.
The gcloud command-line tool has a flag for specifying
request headers, which is --custom-request-header. Make sure to enclose the
header name and header value in straight single quotes (') with this flag.
The general format for the flag is:
--custom-request-header='HEADER_NAME:[HEADER_VALUE]'
The following is an example of a header value with two variables,
client_region and client_city, enclosed in curly braces.
--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
Variables supported with header values
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, for example, 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 usually the same as the client IP
address that is the next-to-last address in the
X-Forwarded-For
header, unless the client is using a proxy or the
X-Forwarded-For header has been tampered with. |
| client_port | The client's source port. |
| client_encrypted | true if the connection between the client and the
load balancer is encrypted (using HTTPS, HTTP/2 or HTTP/3); otherwise,
false.
|
| 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 anOriginheader - 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. Even if the original X-Forwarded-For header contains valid
location information, Google estimates client locations by using the source IP
address information contained in packets received by the load balancer.
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 (}).
Mutual TLS custom headers
The following additional header variables are available if mutual TLS (mTLS) is configured on the load balancer's TargetHttpsProxy.
| Variable | Description |
|---|---|
| client_cert_present | true if the
client has provided a certificate during the TLS handshake;
otherwise, false.
|
| client_cert_chain_verified | true if the
client certificate chain is verified against a configured
TrustStore; otherwise, false.
|
| client_cert_error | Predefined strings representing the error conditions. For more information about the error strings, see mTLS client validation modes. |
| client_cert_sha256_fingerprint | Base64-encoded SHA-256 fingerprint of the client certificate. |
| client_cert_serial_number | The serial number of the client certificate.
If the serial number is longer than 50 bytes, the client_cert_error
is set to client_cert_serial_number_exceeded_size_limit, and the
serial number is set to an empty string. |
| client_cert_spiffe_id | The SPIFFE ID from the Subject Alternative Name (SAN) field. If the value is not valid or exceeds 2048 bytes, the SPIFFE ID is set to an empty string. If the SPIFFE ID is longer than 2048 bytes, the |
| client_cert_uri_sans | Comma-separated Base64-encoded list of the SAN extensions of type URI.
The SAN extensions are extracted from the client certificate.
The SPIFFE ID is not
included in the If the |
| client_cert_dnsname_sans | Comma-separated Base64-encoded list of the SAN extensions of type DNSName. The SAN extensions are extracted from the client certificate. If the |
| client_cert_valid_not_before | Timestamp (RFC 3339
date string format) before which the client certificate is not valid.
For example, 2022-07-01T18:05:09+00:00.
|
| client_cert_valid_not_after | Timestamp (RFC 3339
date string format) after which the client certificate is not valid.
For example, 2022-07-01T18:05:09+00:00.
|
| client_cert_issuer_dn | Base64-encoded DER encoding of the full Issuer field from the certificate. If the |
| client_cert_subject_dn | Base64-encoded DER encoding of the full Subject field from the certificate. If the |
| client_cert_leaf | The client leaf certificate for an established mTLS connection where the certificate passed validation. Certificate encoding is compliant with RFC 9440. This means the binary DER certificate is encoded using Base64 and delimited with colons on either side. If |
| client_cert_chain | The comma-delimited list of certificates, in standard TLS order, of the client certificate chain for an established mTLS connection where the client certificate passed validation, not including the leaf certificate. Certificate encoding is compliant with RFC 9440. If the combined size of |
Configure custom request headers
Console
To add custom request headers to an existing backend service:
- Go to the load balancing summary page.
Go to the Load balancing page - Click Backends.
- Click the name of a backend service.
- Click Edit .
- Click Advanced configurations (Session affinity, connection draining timeout, security policies).
- Under Custom request headers, click Add header.
- Enter the Header name and Header value for the custom request header.
- Enter any additional custom request headers.
- Click Save.
To remove a custom request header from a backend service:
- Go to the load balancing summary page.
Go to the Load balancing page - Click Backends.
- Click the name of a backend service.
- Click Edit .
- Click Advanced configurations (Session affinity, connection draining timeout, security policies).
- Click the X next to the name of the custom request header you want to remove.
- 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" ]
Terraform
For a Terraform script that creates a load balancer with custom headers, see Terraform examples for global external Application Load Balancers.
Configure custom response headers
Console
To add custom response headers to an existing backend service:
- Go to the load balancing summary page.
Go to the Load balancing page - Click Backends.
- Click the name of a backend service.
- Click Edit .
- Click Advanced configurations (Session affinity, connection draining timeout, security policies).
- Under Custom response headers, click Add header.
- Enter the Header name and Header value for the custom response header.
- Enter any additional custom response headers.
- Click Save.
To remove a custom response header from a backend service:
- Go to the load balancing summary page.
Go to the Load balancing page - Click Backends.
- Click the name of a backend service.
- Click Edit .
- Click Advanced configurations (Session affinity, connection draining timeout, security policies).
- Click the X next to the name of the custom response header you want to remove.
- 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 with X-Frame-Options header:
gcloud compute backend-buckets update gaming-lab \
--custom-response-header='X-Frame-Options: DENY'
Example with Strict-Transport-Security header:
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]
Terraform
For a Terraform script that creates a load balancer with custom headers, see Terraform examples for global external Application Load Balancers.
Set 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.
Use custom headers 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 Application Load Balancer or classic Application Load Balancer's custom headers, then the header value specified in your Google Cloud Armor security policy is overwritten by the value populated by the load balancer. If you don't want the Google Cloud Armor policy to be overwritten, ensure that you don't use the same name.
Limitations
The following limitations apply to custom headers used with global load balancers:
- The total size of all custom request headers (name and value combined, before variable expansion) for each backend service must not exceed 8 KB or 16 request headers.
- The total size of all custom response headers (name and value combined, before variable expansion) for each backend service must not exceed 8 KB or 16 response headers.
What's next
- For an example use case, see Configuring a load balancer with an external backend.