docs: add preflight installation-check page plan and roadmap entry

Captures the design for a /preflight page that verifies instance-level infrastructure (rDNS, TLS, ports, HELO banner, proxy headers, DKIM socket, test email). Status: candidate. Co-authored-by: Cursor <cursoragent@cursor.com>
2026-08-19 14:57:03 +03:00
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+# Plan: preflight (installation check page)
+
+**Status:** candidate  
+**Date:** 2026-08-19  
+**Version:** `1.x` MINOR; `candidate` until explicitly agreed.
+
+---
+
+## Goal
+
+A dedicated `/preflight` page (global-admin only) that runs infrastructure-level
+checks and helps the administrator confirm the instance is fully configured and
+ready to deliver mail. Each failed check shows an actionable recommendation
+(what to do or where to look).
+
+## Scope
+
+**In:**
+
+- rDNS / PTR verification (reuse `dnscheck.Checker.checkServer`)
+- TLS certificate validity + hostname match (CN/SAN contains `SELFPOST_HOSTNAME`)
+- Port connectivity: 25, 465, and 587 (if `SUBMISSION_ENABLE`) — dial own
+  public IP from inside the container, expect a 220 banner
+- HELO/EHLO banner — connect to localhost:25, verify Postfix announces
+  `SELFPOST_HOSTNAME` (not `localhost` or a container ID)
+- Trusted proxy headers:
+  - If `TRUSTED_PROXY_CIDR` set: verify peer IP is in range and
+    `X-Forwarded-For` is present
+  - If `PANEL_COOKIE_SECURE=true`: verify `X-Forwarded-Proto: https`
+  - If `TRUSTED_PROXY_CIDR` empty: informational warning (panel exposed
+    directly)
+- DKIM signing — OpenDKIM milter socket reachable (reuse `health.CheckSocket`)
+- Send test email — admin enters recipient address, system sends via
+  localhost:25, shows queued/error result
+
+**Out:**
+
+- Per-domain DNS checks (SPF, DKIM TXT, DMARC) — those live on domain detail
+  pages.
+- Delivery tracking of test email (only confirms local MTA accepted).
+
+## Done when
+
+An administrator can open `/preflight`, see a traffic-light summary of all
+infrastructure prerequisites, read specific fix instructions for any failures,
+and send a test email to verify end-to-end flow.
+
+## Risks
+
+- Port-connectivity check from inside the container may not reflect external
+  reachability if NAT hairpin is unsupported (document limitation).
+- Test email requires at least one configured domain (or falls back to
+  `postmaster@SELFPOST_HOSTNAME`).
+
+## Implementation checklist
+
+Target version cut: TBD (MINOR). One commit per step; code only after roadmap
+status is **agreed**. See [development.md](../development.md) § Plan checklists.
+
+- [ ] Port-connectivity helper (`internal/health/port.go`) — dial own IP, read
+      SMTP banner — **Sonnet**
+- [ ] HELO banner check (connect localhost:25, parse 220 line) — **Sonnet**
+- [ ] Hostname-in-certificate check (extend or wrap `health.CheckCertificate`)
+      — **Sonnet**
+- [ ] Handler `handlers_preflight.go` (assemble all checks, actionable
+      recommendations) — **Sonnet**
+- [ ] Test-email handler (`POST /preflight/test-email`, `net/smtp` to
+      localhost:25) — **Sonnet**
+- [ ] Template `preflight.html` (traffic-light cards + test-email form) —
+      **Sonnet**
+- [ ] Routes in `web.go`, nav link — **Sonnet**