Istio custom request/response header values with Envoy's %OPERATORS%

You can create custom request/response Header in Istio via VirtualService. apart from static value, we can use Envoy's command operators on headers.request.set and headers.response.set. the same %XXX% you use in access logs, so you can add connection metadata onto a request without writing an EnvoyFilter.

Pass the client's address through as a header:

apiVersion: networking.istio.io/v1
kind: VirtualService
metadata:
  name: echo
spec:
  hosts:
  - echo.example.com
  http:
  - route:
    - destination:
        host: echo.default.svc.cluster.local
    headers:
      request:
        set:
          X-Real-Client-IP: "%DOWNSTREAM_REMOTE_ADDRESS_WITHOUT_PORT%"

A few operators that are useful on a request header:

Operator	Value
%DOWNSTREAM_REMOTE_ADDRESS_WITHOUT_PORT%	client IP
%DOWNSTREAM_LOCAL_ADDRESS%	the address the client connected to
%PROTOCOL%	HTTP/1.1, HTTP/2
%REQ(header-name)%	copy another request header
%DOWNSTREAM_PEER_URI_SAN%	client cert SAN (under mTLS)
%START_TIME(%s)%	request start time

Docs: 1. Envoy custom request/response headers 2. access log command operators 3. Istio: VirtualService Headers.