dorfteich/docs/architecture/adr/0026-backup-target-restriction.md
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docs: VS-NfD readiness planning (ist-aufnahme, plan, ADRs 0019-0026, issue drafts)
Add docs/vs-nfd/: the analysis brief, the as-is assessment (42 findings,
all verified against the code), the prioritized action plan rev. 2 with
issue references written back to every checkbox, the two-stage issue/ADR
brief, and the full reviewed draft used to create the forge state.

Add eight proposed ADRs 0019-0026 covering the VS-NfD architecture
decisions: no security base functions (par. 52 VSA anchor), HKDF token
key separation, external authentication, page classification, read-access
audit trail (variant A), reproducible offline deployment, plugin trust
model, and backup target restriction.

Forge state created alongside this commit: 11 labels, milestones M24-M31,
issues #188-#236 (docs-only change, no code touched).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0168Ph5uBmHm8X28CSVpbpnJ
2026-07-30 01:48:05 +02:00

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2.4 KiB
Markdown

# ADR 0026: Backup target restriction
- Status: proposed
- Date: 2026-07-29
## Context
Backups are the largest single egress path in the system: the entire
content of the instance, in one artefact. Today the remote destination is a
freely configurable WebDAV/Nextcloud URL in `instance_settings`, validated
as a URL but not restricted to any host, plus an rsync mirror to a private
host (ADR 0015, issue #84). Anyone with Site-Admin can therefore direct a
full copy of the instance to an arbitrary server.
The tempting answer is to encrypt backups in the application. ADR 0019
rules that out: media protection is the platform's base function, and
implementing it here would move Dorfteich into the certification
obligation under §51 VSA.
## Decision
1. **A deploy-level allowlist constrains permissible backup
destinations.** Deploy-level, not a runtime setting, so a compromised
Site-Admin account cannot widen it.
2. **An empty allowlist disables every remote target** — WebDAV and rsync
mirror alike. "Local only" is the VS-NfD reference configuration.
3. **The admin UI distinguishes "unavailable" from "unconfigured"**, so an
operator is never left guessing whether a missing backup is a
misconfiguration or policy.
4. **No application-side backup encryption**, following ADR 0019. Backup
media are protected by the platform.
5. **Integrity of backup artefacts is in scope**, unlike their
confidentiality: checksums let a restore be verified, which is an
application concern because only we know what the artefact should
contain (see #199 for the same reasoning on attachments).
## Consequences
- Existing deployments that use a remote target must have it added to the
allowlist, or backups stop. This is a breaking change and is called out
in the release notes.
- The delimitation statement (#226) must state plainly that backups leave
the application unencrypted and that media protection is the operator's
duty. That sentence will be read closely; it is the correct one.
- Off-site backup in an airgapped deployment becomes an operator process
(media handling), not an application feature.
- Restore stays unchanged, including the maintenance-mode interlock that
closes collab sessions during a restore.
## Implementing issues
#192 (allowlist + deploy-level disable). Related: #199 (integrity
hashes), #229 (backup/restore chapter of the operations manual).