How a sidecar proxy intercepts pod traffic using iptables without app changes

A sidecar proxy can handle features like telemetry, mTLS, circuit breaking, and retries for many applications without reconfiguring each one, but only if the app never realizes traffic is being routed through it. The article explains that this is achieved by making the proxy transparent, using iptables rules in the Linux kernel to redirect inbound and outbound traffic to the sidecar. Outbound traffic is redirected to a proxy port, while the sidecar's own traffic, localhost, and API server calls are exempted to avoid loops. A common pitfall is an infinite redirect loop caused by the sidecar's own requests, which is solved by exempting traffic owned by its user ID. The init container that applies these rules needs elevated network privileges, since modifying kernel netfilter rules is a restricted operation.