Envoy Status Code 431 "Request Header Fields Too Large" Is Returned

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Envoy Status Code 431 "Request Header Fields Too Large" Is Returned

Last updated: 2023-12-26 14:32:01

For an HTTP request in Istio, Envoy returns abnormal status code 431.
HTTP/1.1 431 Request Header Fields Too Large
Cause Analysis
This status code indicates that the size of the HTTP request header exceeds the limit. The default limit is 60 KiB. The limit is determined by the max_request_headers_kb field in the HttpConnectionManager configuration, and its maximum value can be set to 96 KiB.
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Solution
The header size limit can be increased by adjusting the max_request_headers_kb field through the EnvoyFilter.
An EnvoyFilter example (verification in Istio 1.6 has passed):
apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: max-header
  namespace: istio-system
spec:
  configPatches:
  - applyTo: NETWORK_FILTER
    match:
      context: ANY
      listener:
        filterChain:
          filter:
            name: "envoy.http_connection_manager"
    patch:
      operation: MERGE
      value:
        typed_config:
          "@type": "type.googleapis.com/envoy.config.filter.network.http_connection_manager.v2.HttpConnectionManager"
          max_request_headers_kb: 96
A later version is compatible with the v2 configurations above, but it is recommended to use v3 configurations (verification in Istio 1.8 has passed).
apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: max-header
  namespace: istio-system
spec:
  configPatches:
  - applyTo: NETWORK_FILTER
    match:
      context: ANY
      listener:
        filterChain:
          filter:
            name: "envoy.http_connection_manager"
    patch:
      operation: MERGE
      value:
        typed_config:
          "@type": "type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager"
          max_request_headers_kb: 96
If the size of a header exceeds 96 KiB, it is recommended to place this part of data in the body.