Use TCP redirection in the proxy

[mattbates]

Adapted the nftables rules to redirect localhost-destined TCP traffic to the proxy and added parametrisation to the nftables script template.

[gemini-code-assist]

Summary of Changes

Hello @mattbates, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the proxy's network interception capabilities by introducing new nftables rules. These rules are specifically designed to redirect localhost-destined TCP traffic to the proxy, ensuring that local applications can be properly routed through the proxy. The changes also involve refining the nftables script structure for better maintainability and updating associated container image versions.

Highlights

• Implementation of TCP Redirection for Localhost Traffic: The core change involves adapting the nftables rules to redirect all TCP traffic destined for localhost (127.0.0.1/8 and ::1/128) to the proxy. This enhances the proxy's ability to intercept and manage local network traffic.
• Refinement of nftables Rules and Structure: The nftables script has been refined, including renaming the table and chain, and adding explicit rules to ensure Envoy's own traffic and traffic already directed to the Envoy port are not re-redirected. This improves the robustness and correctness of the network interception.
• Introduction of Configurable nftables Parameters: A new Go struct, NftablesParams, was introduced to pass configuration parameters like Envoy UID, Envoy Port, and DNS Proxy Port to the nftables script template. This makes the script more dynamic and configurable.
• Container Image Version Updates: The pull request includes minor version updates for the DefaultDebugUIImage and InitHelperImage constants, moving them from v0.2.3 to v0.2.4.

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[gemini-code-assist]

Code Review

This pull request adapts the nftables rules to redirect localhost-destined TCP traffic to the proxy, in addition to the existing DNS traffic redirection. The changes are well-implemented, using Go templates to make the nftables script more configurable and readable. I've identified one critical issue in the updated script that would cause it to fail during execution. My review includes a comment with details on how to fix it.

[mattbates]

in preparation for the next release

[markgoddard]

Have you checked that applications are still able to use the loopback interface for other purposes?