Create custom headers in URL maps

This page describes how custom headers work in URL maps used by regional internal Application Load Balancers and cross-region internal Application 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 that the load balancer detects, these headers can include the following information:

• Latency to the client

• Parameters of the TLS connection

Important: Global external Application Load Balancers support header transformations in the URL map as well as in the backend service. However, the regional internal Application Load Balancers and cross-region internal Application Load Balancers only support header transformations in URL maps.

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 makes 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 a response to the client.

To enable custom headers for regional internal Application Load Balancers and cross-region internal Application Load Balancers, you specify a list of header names and header values in the URL map configuration file.

Header names must 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.
• The header name must not begin with X-Google, X-Goog-, X-GFE, or X-Amz-.
• The following hop-by-hop headers must not be used: Keep-Alive, Transfer-Encoding, TE, Connection, Trailer, and Upgrade. In accordance with RFC 2616, these headers are not stored by caches or propagated by the target proxies.
• 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. 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 headers.

Header names are case-insensitive. When header names are passed to an HTTP/2 backend, the HTTP/2 protocol encodes header names as lowercase.

Header values must 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't be blank. Blank headers are rejected.
• The header value can include one or more variables, enclosed by curly braces, that expand to values that the load balancer provides. For a complete list of variables allowed in the header value, see Variables that can appear in the header value.

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 (}).

Add request or response headers

To add request or response headers, use the gcloud CLI to edit the URL map as follows:

Note: If you are using the Google Cloud console to add the request and response headers in the URL map, you need to enter the host address in the Hosts field and the path matcher section of the following YAML in the Path matcher field.

Regional

    gcloud compute url-maps edit URL_MAP_NAME \
        --region=REGION
    

Following is a sample YAML file that shows you how to use variables in custom headers:

   defaultService: regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
               headerAction:
                 requestHeadersToAdd:
                 - headerName: X-header-1-client-region
                   headerValue: "{client_region}"
                 - headerName: X-header-2-client-ip-port
                   headerValue: "{client_ip_address}, {client_port}"
                   replace: True
                 requestHeadersToRemove:
                 - header-3-name
                 responseHeadersToAdd:
                 - headerName: X-header-4-server-ip-port
                   headerValue: "{server_ip_address}, {server_port}"
                   replace: True
                 responseHeadersToRemove:
                 - header-5-name
                 - header-6-name
    

Cross-region

    gcloud compute url-maps edit URL_MAP_NAME \
        --global
    

Following is a sample YAML file that shows you how to use variables in custom headers:

   defaultService: global/backendServices/BACKEND_SERVICE_1
   name: global-lb-map
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: global/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: global/backendServices/BACKEND_SERVICE_1
               weight: 100
               headerAction:
                 requestHeadersToAdd:
                 - headerName: X-header-1-client-region
                   headerValue: "{client_region}"
                 - headerName: X-header-2-client-ip-port
                   headerValue: "{client_ip_address}, {client_port}"
                   replace: True
                 requesteHeadersToRemove:
                 - header-3-name
                 responseHeadersToAdd:
                 - headerName: X-header-4-server-ip-port
                   headerValue: "{server_ip_address}, {server_port}"
                   replace: True
                 responseHeadersToRemove:
                 - header-5-name
                 - header-6-name
    

Note the following behaviors:

• If a response header with custom variables resolves to an empty string, it is removed.
• If a request header with custom variables resolves to an empty string, it is retained with an empty string placeholder.
• If a custom request header includes a custom variable, and an incoming client request also includes the same header, the client request header value will be replaced with the new value provided by the load balancer's custom header.

Variables that can appear in the header value

The following variables can appear in custom header values.

Variable	Description
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_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.
origin_request_header	Reflects the value of the Origin header in the request for Cross-Origin Resource Sharing (CORS) use cases.
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 can't determine their values. For example:

• TLS parameters when TLS is not in use

• The {origin_request_header} when the request does not include an Origin header

Geographic values 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.

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 string client_cert_serial_number_exceeded_size_limit is added to client_cert_error, 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 string client_cert_spiffe_id_exceeded_size_limit is added to client_cert_error.
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 client_cert_uri_sans field. If the client_cert_uri_sans is longer than 512 bytes, the string client_cert_uri_sans_exceeded_size_limit is added to client_cert_error, and the comma-separated list is set to an empty string.
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_dnsname_sans is longer than 512 bytes, the string client_cert_dnsname_sans_exceeded_size_limit is added to client_cert_error, and the comma-separated list is set to an empty string.
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 full Issuer field from the certificate. If the client_cert_issuer_dn is longer than 512 bytes, the string client_cert_issuer_dn_exceeded_size_limit is added to client_cert_error, and client_cert_issuer_dn is set to an empty string.
client_cert_subject_dn	Base64-encoded full Subject field from the certificate. If the client_cert_subject_dn is longer than 512 bytes, the string client_cert_subject_dn_exceeded_size_limit is added to client_cert_error, and client_cert_subject_dn is set to an empty string.
client_cert_leaf	The client leaf certificate for an established mTLS connection where the certificate passed validation. Certificate encoding is compliant with RFC 9440: the binary DER certificate is encoded using Base64 (without line breaks, spaces, or other characters outside the Base64 alphabet) and delimited with colons on either side. If client_cert_leaf exceeds 16 KB unencoded, the string client_cert_validated_leaf_exceeded_size_limit is added to client_cert_error, and client_cert_leaf is set to an empty string.
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 client_cert_leaf and client_cert_chain before Base64 encoding exceeds 16 KB, the string client_cert_validated_chain_exceeded_size_limit is added to client_cert_error, and client_cert_chain is set to an empty string.

Limitations

The following limitations apply:

• You can't configure custom headers on backend services used by regional internal Application Load Balancers and cross-region internal Application Load Balancers.