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When handling HTTP/1.1, Envoy will normalize the header keys to be all lowercase.
While this is compliant with the HTTP/1.1 spec, in practice this can result in issues when migrating existing systems that might rely on specific header casing.
In our case a problem was detected in the SDC client implementation, thich relies on uppercase header values.
To solve this problem in general

we add a EnvoyFilter to keep the uppercase header in the istio-config namespace to apply for all namespaces.
but set the context to SIDECAR_INBOUND to avoid problems in the connection between Istio-Gateway and Services

Create a EnvoyFilter file (e.g. envoyfilter-case.yaml)

Code Block

apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: header-casing
  namespace: istio-config
spec:
  configPatches:
  - applyTo: CLUSTER
    match:
      context: ANYSIDECAR_INBOUND
    patch:
      operation: MERGE
      value:
        typed_extension_protocol_options:
          envoy.extensions.upstreams.http.v3.HttpProtocolOptions:
            '@type': type.googleapis.com/envoy.extensions.upstreams.http.v3.HttpProtocolOptions
            use_downstream_protocol_config:
              http_protocol_options:
                header_key_format:
                  stateful_formatter:
                    name: preserve_case
                    typed_config:
                      '@type': type.googleapis.com/envoy.extensions.http.header_formatters.preserve_case.v3.PreserveCaseFormatterConfig
  - applyTo: NETWORK_FILTER
    match:
      listener:
        filterChain:
          filter:
            name: envoy.filters.network.http_connection_manager
    patch:
      operation: MERGE
      value:
        typed_config:
          '@type': type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
          http_protocol_options:
            header_key_format:
              stateful_formatter:
                name: preserve_case
                typed_config:
                  '@type': type.googleapis.com/envoy.extensions.http.header_formatters.preserve_case.v3.PreserveCaseFormatterConfig

Apply the change to Istio

Code Block
$ kubectl apply -f envoyfilter-case.yaml

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Old Version 11

New Version 12

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