fix: allow NAT64-wrapped public IPs through SSRF filter

[gantoine]

Summary

Fixes #3668. On DNS64/NAT64 networks (e.g. OPNsense + Tayga), the resolver synthesizes IPv6 answers by embedding a public IPv4 inside the RFC 6052 well-known NAT64 prefix 64:ff9b::/96. For example neoclone.screenscraper.fr resolves to 64:ff9b::8d5f:accd, which wraps the public IPv4 141.95.172.205.

RomM's SSRF classifier is_forbidden_ip rejected these because the well-known NAT64 range is classified as is_reserved (and not is_global on some Python versions), producing:

SSRF prevention: hostname 'neoclone.screenscraper.fr' resolves to forbidden IP 64:ff9b::8d5f:accd
Unable to fetch cover at https://neoclone.screenscraper.fr/...

This broke all outbound metadata and cover fetching on any NAT64-only deployment.

Change

is_forbidden_ip now detects a NAT64-wrapped address, extracts the embedded IPv4, and runs the forbidden check against that IPv4:

• A wrapped public IPv4 (64:ff9b::8d5f:accd → 141.95.172.205) is now allowed.
• A wrapped private/internal IPv4 (64:ff9b::7f00:1 → 127.0.0.1, 64:ff9b::a00:1 → 10.0.0.1, cloud metadata, etc.) is still blocked.

Because is_forbidden_ip is the shared classifier used by both the DNS-resolution backends and the syntactic URL validator, this single change covers every code path. The RFC 8215 local-use prefix 64:ff9b:1::/48 is marked private by Python, so it is not unwrapped and remains blocked.

Scope: only the RFC 6052 well-known prefix 64:ff9b::/96 is unwrapped (the default used by the reporter and the vast majority of NAT64/DNS64 setups). No new configuration.

Tests

• New TestIsForbiddenIp cases for NAT64-wrapped public (allowed) and private/loopback/link-local (blocked) addresses, plus an RFC 8215 sanity case.
• New async-backend regression tests: a hostname resolving to 64:ff9b::8d5f:accd connects to the pinned NAT64 address; one resolving to 64:ff9b::7f00:1 still raises ConnectError.

uv run pytest tests/utils/test_ssrf.py → 62 passed. trunk fmt + trunk check clean. No API surface change, so no OpenAPI/type regen needed.

AI assistance disclosure

This change was written with AI assistance (PostHog Code / Claude). The root-cause analysis, code changes, tests, and this description were AI-generated and reviewed before submission.

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Created with PostHog Code

[Copilot]

Pull request overview

Updates RomM’s SSRF IP classifier to correctly handle DNS64/NAT64 environments by unwrapping RFC 6052 well-known NAT64 IPv6 addresses (64:ff9b::/96) and applying SSRF rules to the embedded IPv4, preventing false blocks of legitimate public destinations while still blocking wrapped internal/private targets.

Changes:

• Add NAT64 well-known prefix detection and embedded-IPv4 extraction to is_forbidden_ip.
• Extend SSRF unit tests to cover NAT64-wrapped public vs. private/loopback/link-local behavior.
• Add async-backend regression tests to ensure NAT64-wrapped public addresses connect and NAT64-wrapped private addresses are rejected.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
backend/utils/ssrf.py	Unwraps RFC 6052 well-known NAT64 IPv6 addresses and evaluates SSRF forbidden-ness based on the embedded IPv4.
backend/tests/utils/test_ssrf.py	Adds unit + async-backend regression coverage for NAT64-wrapped addresses (public allowed, private/loopback still blocked).

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