Symptoms: in default = "VPN" full-VPN mode external internet was dead even
though tunnel-internal ping (10.7.0.1) worked perfectly. The tunnel itself
was assembled and AEAD-encrypted (see TEST_CASES.md), but packets sent
through it died on the server side.
Root cause: server's `[server.nat]` was opt-in. On the production server
(187.77.67.17) deployed before v2, the section is absent in
/etc/aura/server.toml, so `aura server` never ran the iptables MASQUERADE
plan. Packets egressed to the upstream router with src = 10.7.0.10 (RFC1918),
which the provider's reverse-path filter dropped — full-VPN clients saw
"internet is dead". Tunnel-internal pool addresses worked because they
don't need NAT.
Fix:
* `server.rs`: when `[server.nat]` is absent in server.toml AND we are on
Linux, attempt auto-NAT with an auto-detected egress_iface. If detection
or the iptables call fails we DON'T bail — we log a loud error and let
the server come up so safe-mode clients keep working.
* `config.rs`: `ServerNatSection::default()` now defaults `auto = true`.
A bare `[server.nat]` header (no `auto =`) now means "yes, enable it"
instead of the silent-noop it used to be.
* New tests for both bare-header and explicit `auto = false` opt-out paths.
* `docs/server_nat_fix.md`: step-by-step instructions for fixing the
existing 187.77.67.17 server (binary upgrade vs. manual server.toml
patch vs. fully-manual sysctl + iptables).
* `docs/deployment.md`: replaces "manual mandatory step" wording with
the new auto-NAT story.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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