Headers middleware configuration for conditional response header injection

Answer

The Traefik Headers middleware does **not** evaluate request attributes to decide whether to inject a response header; it only applies static header maps.

Why (likely explanation)

The middleware runs for every request, but its configuration consists of fixed key/value pairs for request and response headers. Traefik’s expression language is reserved for router rules (Header(), PathPrefix(), etc.) and is not exposed inside the Headers middleware. Therefore, conditional logic based on an incoming header cannot be expressed directly in a single Headers definition.

Confirmed fact

According to the Traefik v3.0 reference, the Headers middleware supports fields such as customRequestHeaders, customResponseHeaders, accessControlAllowMethods, etc., all of which are static values. No conditional or templating mechanism is documented.

Recommended approach (plugin‑based)

1. Select a community plugin that can read request headers and set response headers (e.g., traefik/header-plugin or develop a simple Go plugin).
2. Declare the plugin in your static configuration (traefik.yml) or via a Kubernetes Middleware CRD:

# traefik.yml
experimental:
  plugins:
    header-condition:
      moduleName: github.com/traefik/header-plugin
      version: v0.1.0

3. Create a middleware instance that defines the condition:

# dynamic config (YAML)
http:
  middlewares:
    inject-cond:
      plugin:
        header-condition:
          # Example: if request header X-Foo equals "bar", set response header X-Custom to "baz"
          requestHeader: "X-Foo"
          requestValue: "bar"
          responseHeader: "X-Custom"
          responseValue: "baz"

4. Attach this middleware to the router(s) that serve the target services:

http:
  routers:
    myservice:
      rule: Host(`example.com`)
      service: myservice
      middlewares:
        - inject-cond

5. Apply the configuration and reload Traefik; the plugin will evaluate the request header per request and inject the response header only when the condition matches.

Avoiding router proliferation

If you prefer not to use a plugin, you can create separate routers each with a Header() matcher (e.g., Header(`X-Foo`, `bar`)) and reuse the same Headers middleware. This works but leads to a linear increase in routers for each distinct header/value pair, which may be undesirable at scale.

Missing diagnostic detail

To decide whether a single router with a Header() matcher is sufficient, please confirm the exact request header name and the value (or set of values) you need to match.