Templated Host Rewrite From Path Match

[zhaohuabing]

Description:

Problem

Some backends have predictable hostnames, and the desired upstream host can be derived from part of the request path.

Example:

https://example.com/node/1/api
-> backend-1.service.namespace.svc.cluster.local

https://example.com/node/2/api
-> backend-2.service.namespace.svc.cluster.local

Request

Today, Envoy Gateway supports host rewrite, and routes can match paths with regular expressions. However, there does not seem to be a native way to reuse a regex path capture when rewriting the upstream Host header.

apiVersion: gateway.envoyproxy.io/v1alpha1
kind: HTTPRouteFilter
metadata:
  name: host-rewrite-from-path
spec:
  urlRewrite:
    hostname:
      type: PathRegex
      pathRegex:
        pattern: "^/node/([0-9]+)/api.*"
        substitution: "backend-\\1.service.namespace.svc.cluster.local"

This would allow a single route to map path segments to predictable backend hostnames without requiring Lua, ExtProc, Wasm, or one route per backend instance.

Security Considerations

The API should avoid arbitrary host injection. Possible safeguards:

• Require the path match to be a regular expression with explicit capture groups.
• Allow only captured values in a fixed hostname template.
• Support validation such as numeric-only captures or min/max bounds.
• Reject requests where the capture is missing or invalid.
• Document interaction with DynamicResolver if the rewritten host is resolved dynamically.

[optional Relevant Links:]

Any extra documentation required to understand the issue.

[github-actions]

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