diff --git a/apps/backup/Dockerfile b/apps/backup/Dockerfile
index 49d7634..df287e6 100644
--- a/apps/backup/Dockerfile
+++ b/apps/backup/Dockerfile
@@ -22,8 +22,9 @@ ENV NODE_ENV=production APP_VERSION=${APP_VERSION} \
     BACKUPS_DIR=/backups UPLOADS_DIR=/data/uploads PLUGINS_DIR=/data/plugins \
     SECRETS_FILE=/data/secrets/secrets.env
 # pg_dump/pg_restore matching the stack's postgres:17 server, GNU tar for the
-# volume archives, tzdata so BACKUP_TIME honors a configured TZ.
-RUN apk add --no-cache postgresql17-client tar tzdata \
+# volume archives, tzdata so BACKUP_TIME honors a configured TZ, and
+# rsync + ssh for the optional off-host mirror (issue #84).
+RUN apk add --no-cache postgresql17-client tar tzdata rsync openssh-client \
   && mkdir -p /backups && chown node:node /backups
 WORKDIR /app
 COPY --from=build --chown=node:node /out /app
diff --git a/apps/backup/src/index.ts b/apps/backup/src/index.ts
index e19c9f9..2372209 100644
--- a/apps/backup/src/index.ts
+++ b/apps/backup/src/index.ts
@@ -8,6 +8,7 @@ import { createArchive } from './archive.js';
 import { createCommandListener } from './commands.js';
 import { loadBackupEnv } from './config.js';
 import { sendFailureMail } from './mail.js';
+import { mirrorSets, resolveMirrorConfig } from './mirror.js';
 import { performRestore } from './perform-restore.js';
 import { pgDump } from './pg.js';
 import { fetchRemoteSet, resolveRemoteTarget, uploadDue, uploadSet } from './remote.js';
@@ -43,6 +44,7 @@ function readSecrets(): Record<string, string> {
 async function buildRunnerDeps(trigger: 'scheduled' | 'manual'): Promise<RunnerDeps> {
   const settings = await readBackupDbSettings(env.DATABASE_URL);
   const target = resolveRemoteTarget(settings, readSecrets());
+  const mirrorConfig = resolveMirrorConfig(env);
   return {
     backupsDir: env.BACKUPS_DIR,
     retentionDays: settings.localRetentionDays ?? env.BACKUP_RETENTION_DAYS,
@@ -76,6 +78,17 @@ async function buildRunnerDeps(trigger: 'scheduled' | 'manual'): Promise<RunnerD
           });
         }
       : undefined,
+    mirror: mirrorConfig
+      ? ({ previous }) =>
+          mirrorSets({
+            env,
+            config: mirrorConfig,
+            backupsDir: env.BACKUPS_DIR,
+            previous,
+            now: () => new Date(),
+            log,
+          })
+      : undefined,
     log,
   };
 }
diff --git a/apps/backup/src/mail.ts b/apps/backup/src/mail.ts
index 27459ca..4306a3a 100644
--- a/apps/backup/src/mail.ts
+++ b/apps/backup/src/mail.ts
@@ -28,7 +28,7 @@ export interface RenderedFailureMail {
 const CATALOGS = { de: deMails, en: enMails } as const;
 
 /** Both alert mails share one key shape; the namespace picks the texts. */
-type AlertNamespace = 'backupFailed' | 'backupUploadFailed';
+type AlertNamespace = 'backupFailed' | 'backupUploadFailed' | 'backupMirrorFailed';
 
 function t(
   locale: 'de' | 'en',
@@ -100,6 +100,17 @@ export async function sendUploadFailureMail(
   return sendAlertMail(env, renderUploadFailureMail(input, env.BACKUP_MAIL_LOCALE, label(env)));
 }
 
+/** Same delivery path for the rsync-mirror alert (issue #84). */
+export async function sendMirrorFailureMail(
+  env: BackupEnv,
+  input: FailureMailInput,
+): Promise<boolean> {
+  return sendAlertMail(
+    env,
+    renderAlertMail('backupMirrorFailed', input, env.BACKUP_MAIL_LOCALE, label(env)),
+  );
+}
+
 function label(env: BackupEnv): string {
   return env.BACKUP_INSTANCE_LABEL || 'Dorfteich';
 }
diff --git a/apps/backup/src/mirror.test.ts b/apps/backup/src/mirror.test.ts
new file mode 100644
index 0000000..e4f2e70
--- /dev/null
+++ b/apps/backup/src/mirror.test.ts
@@ -0,0 +1,148 @@
+import { execFileSync } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { mkdtemp, readdir, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+import type { BackupEnv } from '@dorfteich/shared';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+
+import {
+  buildRsyncArgs,
+  mirrorSets,
+  parseTransferredFiles,
+  resolveMirrorConfig,
+} from './mirror.js';
+
+const silentLog = { info: () => {}, warn: () => {}, error: () => {} };
+const noMailEnv = { BACKUP_MAIL_TO: undefined } as unknown as BackupEnv;
+
+function hasRsync(): boolean {
+  try {
+    execFileSync('rsync', ['--version'], { stdio: 'ignore' });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+describe('resolveMirrorConfig', () => {
+  it('requires both target and key; port defaults to 22', () => {
+    const base = { BACKUP_MIRROR_SSH_PORT: 22 } as unknown as BackupEnv;
+    expect(resolveMirrorConfig(base)).toBeNull();
+    expect(
+      resolveMirrorConfig({ ...base, BACKUP_MIRROR_TARGET: 'u@h:/x/' } as BackupEnv),
+    ).toBeNull();
+    expect(
+      resolveMirrorConfig({
+        ...base,
+        BACKUP_MIRROR_TARGET: 'u@h:/x/',
+        BACKUP_MIRROR_SSH_KEY: '/data/secrets/key',
+      } as BackupEnv),
+    ).toEqual({ target: 'u@h:/x/', sshKeyFile: '/data/secrets/key', sshPort: 22 });
+  });
+});
+
+describe('buildRsyncArgs', () => {
+  it('transfers only set files, deletes within the filter, uses the pinned ssh', () => {
+    const args = buildRsyncArgs(
+      { target: 'u@h:/backups/', sshKeyFile: '/k', sshPort: 2222 },
+      '/backups',
+    );
+    expect(args).toContain('--delete');
+    expect(args).toContain('--include=db-*.dump');
+    expect(args).toContain('--include=files-*.tar.gz');
+    expect(args).toContain('--exclude=*');
+    const ssh = args[args.indexOf('-e') + 1]!;
+    expect(ssh).toContain('-i /k');
+    expect(ssh).toContain('-p 2222');
+    expect(ssh).toContain('BatchMode=yes');
+    expect(args.at(-2)).toBe('/backups/');
+    expect(args.at(-1)).toBe('u@h:/backups/');
+  });
+});
+
+describe('parseTransferredFiles', () => {
+  it('reads the rsync stats line, tolerating thousands separators', () => {
+    expect(parseTransferredFiles('Number of regular files transferred: 4\n')).toBe(4);
+    expect(parseTransferredFiles('Number of regular files transferred: 1,234\n')).toBe(1234);
+    expect(parseTransferredFiles('no stats here')).toBeUndefined();
+  });
+});
+
+describe.skipIf(!hasRsync())('mirrorSets (real rsync, local target)', () => {
+  let source: string;
+  let target: string;
+  let keyFile: string;
+
+  beforeEach(async () => {
+    source = await mkdtemp(join(tmpdir(), 'dorfteich-mirror-src-'));
+    target = await mkdtemp(join(tmpdir(), 'dorfteich-mirror-dst-'));
+    // rsync to a local path ignores -e ssh; a dummy key satisfies the check.
+    keyFile = join(source, '.dummy-key');
+    await writeFile(keyFile, 'dummy');
+    await writeFile(join(source, 'db-20260712-030000.dump'), 'dump-1');
+    await writeFile(join(source, 'files-20260712-030000.tar.gz'), 'files-1');
+    await writeFile(join(source, 'db-20260711-030000.dump'), 'dump-0');
+    await writeFile(join(source, 'files-20260711-030000.tar.gz'), 'files-0');
+    await writeFile(join(source, 'status.json'), '{}');
+  });
+
+  afterEach(async () => {
+    await rm(source, { recursive: true, force: true });
+    await rm(target, { recursive: true, force: true });
+  });
+
+  const run = () =>
+    mirrorSets({
+      env: noMailEnv,
+      config: { target: `${target}/`, sshKeyFile: keyFile, sshPort: 22 },
+      backupsDir: source,
+      previous: undefined,
+      now: () => new Date('2026-07-12T03:05:00Z'),
+      log: silentLog,
+    });
+
+  it('transfers set files only, is idempotent, and aligns retention', async () => {
+    const first = await run();
+    expect(first.lastRun.outcome).toBe('succeeded');
+    expect(first.lastRun.transferredFiles).toBe(4);
+    expect(first.lastSuccessAt).not.toBeNull();
+    expect((await readdir(target)).sort()).toEqual([
+      'db-20260711-030000.dump',
+      'db-20260712-030000.dump',
+      'files-20260711-030000.tar.gz',
+      'files-20260712-030000.tar.gz',
+    ]);
+
+    // Idempotent re-run: nothing travels.
+    const second = await run();
+    expect(second.lastRun.outcome).toBe('succeeded');
+    expect(second.lastRun.transferredFiles).toBe(0);
+
+    // A locally pruned set disappears remotely too (retention alignment).
+    await rm(join(source, 'db-20260711-030000.dump'));
+    await rm(join(source, 'files-20260711-030000.tar.gz'));
+    await run();
+    expect((await readdir(target)).sort()).toEqual([
+      'db-20260712-030000.dump',
+      'files-20260712-030000.tar.gz',
+    ]);
+  });
+
+  it('reports a failure without throwing and carries the last success', async () => {
+    const good = await run();
+    const failed = await mirrorSets({
+      env: noMailEnv,
+      config: { target: `${target}/`, sshKeyFile: '/nonexistent-key', sshPort: 22 },
+      backupsDir: source,
+      previous: good,
+      now: () => new Date('2026-07-12T03:10:00Z'),
+      log: silentLog,
+    });
+    expect(failed.lastRun.outcome).toBe('failed');
+    expect(failed.lastRun.error).toContain('not found');
+    expect(failed.lastSuccessAt).toBe(good.lastSuccessAt);
+    expect(existsSync(join(target, 'db-20260712-030000.dump'))).toBe(true);
+  });
+});
diff --git a/apps/backup/src/mirror.ts b/apps/backup/src/mirror.ts
new file mode 100644
index 0000000..7164a81
--- /dev/null
+++ b/apps/backup/src/mirror.ts
@@ -0,0 +1,130 @@
+import { execFile } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { join } from 'node:path';
+import { promisify } from 'node:util';
+
+import type { BackupEnv, BackupMirrorStatus } from '@dorfteich/shared';
+
+import { sendMirrorFailureMail } from './mail.js';
+import type { RemoteLogger } from './remote.js';
+
+const execFileAsync = promisify(execFile);
+
+/**
+ * The rsync mirror to a private host (issue #84, ADR 0015) — the operator
+ * extra beside the admin-configured Nextcloud target (#103). After every
+ * successful local run the set artifacts are rsynced to
+ * `BACKUP_MIRROR_TARGET`; `--delete` keeps the remote retention aligned
+ * with the local prune (the newest-set guarantee therefore carries over).
+ * Only set files travel — status files and staging artifacts stay local.
+ * rsync's delta transfer makes re-runs idempotent (0 files transferred).
+ */
+
+export interface MirrorConfig {
+  target: string;
+  sshKeyFile: string;
+  sshPort: number;
+}
+
+/** The mirror configuration, or null when the env does not enable it. */
+export function resolveMirrorConfig(env: BackupEnv): MirrorConfig | null {
+  if (!env.BACKUP_MIRROR_TARGET || !env.BACKUP_MIRROR_SSH_KEY) return null;
+  return {
+    target: env.BACKUP_MIRROR_TARGET,
+    sshKeyFile: env.BACKUP_MIRROR_SSH_KEY,
+    sshPort: env.BACKUP_MIRROR_SSH_PORT,
+  };
+}
+
+/**
+ * The rsync invocation: only complete-set artifacts (and the remote bundle
+ * naming is local-only, so just dumps + archives), `--delete` inside that
+ * filter for retention alignment. The known-hosts file lives on the
+ * backups volume so the host key pins across container recreations;
+ * `accept-new` covers the very first contact (inside the WireGuard tunnel).
+ */
+export function buildRsyncArgs(config: MirrorConfig, backupsDir: string): string[] {
+  const ssh = [
+    'ssh',
+    `-i ${config.sshKeyFile}`,
+    `-p ${config.sshPort}`,
+    '-o StrictHostKeyChecking=accept-new',
+    `-o UserKnownHostsFile=${join(backupsDir, '.mirror_known_hosts')}`,
+    '-o BatchMode=yes',
+  ].join(' ');
+  return [
+    '--archive',
+    // Fixed modes on the mirror host (dirs 750, files 640) — `--archive`
+    // would otherwise copy the container-side modes onto the target.
+    // Symbolic form: octal `--chmod` needs rsync ≥ 3, which not every
+    // dev machine has (macOS ships 2.6.9); the symbolic one works on both.
+    '--chmod=Du=rwx,Dg=rx,Do-rwx,Fu=rw,Fg=r,Fo-rwx',
+    '--delete',
+    '--stats',
+    '--include=db-*.dump',
+    '--include=files-*.tar.gz',
+    '--exclude=*',
+    '-e',
+    ssh,
+    `${backupsDir}/`,
+    config.target,
+  ];
+}
+
+/** The transferred-file count from `rsync --stats` output. rsync 3 prints
+ * "Number of regular files transferred", the ancient 2.6 (macOS) drops
+ * "regular" — accept both. */
+export function parseTransferredFiles(stats: string): number | undefined {
+  const match = /Number of (?:regular )?files transferred:\s*([\d,.]+)/.exec(stats);
+  if (!match) return undefined;
+  return Number(match[1]!.replace(/[,.]/g, ''));
+}
+
+/**
+ * Runs one mirror pass and returns the new mirror status. Failures are
+ * reported in the status and alert by mail — never thrown: the local
+ * backup succeeded and must count (issue #84 acceptance criteria).
+ */
+export async function mirrorSets(deps: {
+  env: BackupEnv;
+  config: MirrorConfig;
+  backupsDir: string;
+  previous: BackupMirrorStatus | undefined;
+  now(): Date;
+  log: RemoteLogger;
+}): Promise<BackupMirrorStatus> {
+  const previousSuccessAt = deps.previous?.lastSuccessAt ?? null;
+  try {
+    if (!existsSync(deps.config.sshKeyFile)) {
+      throw new Error(`mirror ssh key not found: ${deps.config.sshKeyFile}`);
+    }
+    const { stdout } = await execFileAsync('rsync', buildRsyncArgs(deps.config, deps.backupsDir), {
+      maxBuffer: 16 * 1024 * 1024,
+    });
+    const finishedAt = deps.now().toISOString();
+    const transferredFiles = parseTransferredFiles(stdout);
+    deps.log.info({ target: deps.config.target, transferredFiles }, 'mirror run succeeded');
+    return {
+      lastRun: { finishedAt, outcome: 'succeeded', transferredFiles },
+      lastSuccessAt: finishedAt,
+    };
+  } catch (error) {
+    const stderr = (error as { stderr?: string }).stderr?.trim();
+    const message = stderr || (error instanceof Error ? error.message : String(error));
+    deps.log.error({ target: deps.config.target, error: message }, 'mirror run failed');
+    try {
+      const sent = await sendMirrorFailureMail(deps.env, {
+        backupId: 'mirror',
+        error: message,
+        lastSuccessAt: previousSuccessAt,
+      });
+      if (!sent) deps.log.warn({}, 'no BACKUP_MAIL_TO configured, mirror alert not sent');
+    } catch (mailError) {
+      deps.log.error({ error: String(mailError) }, 'mirror alert could not be sent');
+    }
+    return {
+      lastRun: { finishedAt: deps.now().toISOString(), outcome: 'failed', error: message },
+      lastSuccessAt: previousSuccessAt,
+    };
+  }
+}
diff --git a/apps/backup/src/runner.ts b/apps/backup/src/runner.ts
index 63929f9..4cf602d 100644
--- a/apps/backup/src/runner.ts
+++ b/apps/backup/src/runner.ts
@@ -2,7 +2,7 @@ import { mkdir, readdir, rename, rm, stat } from 'node:fs/promises';
 import { join } from 'node:path';
 
 import { archiveFileName, dumpFileName, expiredSets, listSets, newBackupId } from './backup-set.js';
-import type { BackupRemoteStatus } from '@dorfteich/shared';
+import type { BackupMirrorStatus, BackupRemoteStatus } from '@dorfteich/shared';
 
 import {
   readStatus,
@@ -43,6 +43,14 @@ export interface RunnerDeps {
     backupId: string;
     previous: BackupRemoteStatus | undefined;
   }): Promise<BackupRemoteStatus | undefined>;
+  /**
+   * rsync mirror to a private host (issue #84, mirror.ts). Runs after the
+   * prune so the remote retention aligns with the local one. Failures are
+   * reported inside the returned status, never thrown.
+   */
+  mirror?(input: {
+    previous: BackupMirrorStatus | undefined;
+  }): Promise<BackupMirrorStatus | undefined>;
   log: {
     info(details: object, message: string): void;
     error(details: object, message: string): void;
@@ -55,6 +63,7 @@ export async function runBackup(deps: RunnerDeps): Promise<BackupStatus> {
   const previous = readStatus(deps.backupsDir);
   const lastSuccess = previous?.lastSuccess ?? null;
   let remote = previous?.remote;
+  let mirror = previous?.mirror;
   await mkdir(deps.backupsDir, { recursive: true });
 
   const dumpFile = join(deps.backupsDir, dumpFileName(backupId));
@@ -111,6 +120,18 @@ export async function runBackup(deps: RunnerDeps): Promise<BackupStatus> {
 
   await prune(deps);
 
+  // After the prune, so `--delete` aligns the remote retention with the
+  // local one — including the newest-complete-set guarantee.
+  if (run.outcome === 'succeeded' && deps.mirror) {
+    try {
+      mirror = (await deps.mirror({ previous: mirror })) ?? mirror;
+    } catch (mirrorError) {
+      // Defensive like the upload hook: mirror.ts reports failures in its
+      // return value; a throw here must never fail the local run.
+      deps.log.error({ error: String(mirrorError) }, 'mirror hook threw unexpectedly');
+    }
+  }
+
   const status: BackupStatus = {
     schemaVersion: 1,
     updatedAt: deps.now().toISOString(),
@@ -118,6 +139,7 @@ export async function runBackup(deps: RunnerDeps): Promise<BackupStatus> {
     lastRun: run,
     lastSuccess: success,
     ...(remote ? { remote } : {}),
+    ...(mirror ? { mirror } : {}),
   };
   await writeStatus(deps.backupsDir, status);
   return status;
diff --git a/apps/web/src/pages/AdminBackupSection.tsx b/apps/web/src/pages/AdminBackupSection.tsx
index 98562e2..70d3330 100644
--- a/apps/web/src/pages/AdminBackupSection.tsx
+++ b/apps/web/src/pages/AdminBackupSection.tsx
@@ -121,6 +121,23 @@ function StatusCard({ view }: { view: SystemBackupView }): React.JSX.Element {
                 </>
               )}
             </dd>
+            {view.status.mirror && (
+              <>
+                <dt>{t('backup.mirror.title')}</dt>
+                <dd>
+                  {t('backup.mirror.lastSuccess')}:{' '}
+                  {view.status.mirror.lastSuccessAt
+                    ? new Date(view.status.mirror.lastSuccessAt).toLocaleString()
+                    : t('backup.never')}
+                  {view.status.mirror.lastRun.outcome === 'failed' && (
+                    <>
+                      {' — '}
+                      {t('backup.mirror.failed')}: <code>{view.status.mirror.lastRun.error}</code>
+                    </>
+                  )}
+                </dd>
+              </>
+            )}
           </dl>
           <p className="system-backup__retention">
             {t('backup.retention', { days: view.status.retentionDays })}
diff --git a/deploy/backup-basel.md b/deploy/backup-basel.md
new file mode 100644
index 0000000..906908b
--- /dev/null
+++ b/deploy/backup-basel.md
@@ -0,0 +1,95 @@
+# Backup mirror to BASEL (issue #84)
+
+Nightly rsync of the Prod backup sets to the private BASEL host over the
+WireGuard tunnel (ADR 0015) — the operator-level extra beside the
+admin-configured Nextcloud target (#103). The sidecar mirrors the set
+files (`db-*.dump`, `files-*.tar.gz`) after every successful local run;
+`--delete` keeps the remote retention aligned with the local prune, so
+the newest-complete-set guarantee carries over. Mirror outcome lands in
+`status.json` (`mirror` block, shown on the admin backup card); failures
+alert through the backup failure mail while local backups continue.
+
+## BASEL-side setup (once, as root on BASEL)
+
+A **dedicated user with a home under `/home/`** — deliberately NOT the
+Debian `backup` system user (UID 34), whose `/var/backups` home and
+`nologin` shell are documented foot-guns in the operator conventions:
+
+```sh
+useradd --create-home --shell /bin/bash dorfteich-backup
+mkdir -p /home/dorfteich-backup/.ssh
+# authorized_keys: the public half of the key generated below
+install -m 600 -o dorfteich-backup -g dorfteich-backup authorized_keys \
+  /home/dorfteich-backup/.ssh/authorized_keys
+chmod 700 /home/dorfteich-backup/.ssh
+chown dorfteich-backup:dorfteich-backup /home/dorfteich-backup/.ssh
+
+mkdir -p /home/RAID/BACKUPS/dorfteich-prod
+chown dorfteich-backup:dorfteich-backup /home/RAID/BACKUPS/dorfteich-prod
+chmod 750 /home/RAID/BACKUPS/dorfteich-prod
+```
+
+**Pitfalls (learned on the wochenplan setup):**
+
+- The login shell MUST be `/bin/bash` (or `/bin/sh`) — with
+  `/usr/sbin/nologin` sshd rejects every session, including rsync's.
+  Security comes from the key-only login, not from nologin; if you want
+  to lock it down further, prefix the `authorized_keys` line with
+  `command="rsync --server ..."` restrictions.
+- `.ssh` must be mode 700 and owned by the user; `authorized_keys` 600.
+- The home directory itself may be root-owned but must not be
+  group/world-writable (sshd's StrictModes).
+
+## Stack-side setup (per stage that mirrors)
+
+1. **Generate a keypair** (on the docker host, never in the repo):
+
+   ```sh
+   ssh-keygen -t ed25519 -N '' -C dorfteich-backup-mirror -f backup_mirror_ed25519
+   # the .pub half goes into BASEL's authorized_keys (above)
+   ```
+
+2. **Put the private key on the `secrets` volume** (the sidecar mounts it
+   read-only at `/data/secrets`; copy through the api container, which
+   mounts it read-write):
+
+   ```sh
+   docker compose cp backup_mirror_ed25519 api:/data/secrets/backup_mirror_ed25519
+   docker compose exec api chmod 600 /data/secrets/backup_mirror_ed25519
+   shred -u backup_mirror_ed25519
+   ```
+
+3. **Configure the stage `.env`** and recreate the sidecar:
+
+   ```sh
+   BACKUP_MIRROR_TARGET=dorfteich-backup@172.30.1.10:/home/RAID/BACKUPS/dorfteich-prod/
+   BACKUP_MIRROR_SSH_KEY=/data/secrets/backup_mirror_ed25519
+   #BACKUP_MIRROR_SSH_PORT=22   # default
+   ```
+
+   ```sh
+   docker compose up -d backup
+   ```
+
+4. **Verify** with an on-demand run — the first mirror pins BASEL's host
+   key (`accept-new`) into `.mirror_known_hosts` on the backups volume:
+
+   ```sh
+   docker compose run --rm -e BACKUP_RUN_ONCE=1 backup
+   docker compose exec backup sh -c \
+     'grep -A4 \"mirror\" /backups/status.json'
+   # re-run: "transferredFiles": 0 proves idempotency
+   ```
+
+## Behaviour
+
+- Mirror runs after the local prune of every **successful** run (nightly,
+  manual button, `BACKUP_RUN_ONCE`); a failed local run never mirrors.
+- A mirror failure sets `status.json → mirror.lastRun.outcome = "failed"`
+  (visible on the admin backup card) and sends the `backupMirrorFailed`
+  alert mail — the local run still counts as succeeded.
+- Restoring from the mirror: copy the set pair back into the stack's
+  backups volume and run `./restore.sh <backup-id>` — same procedure as
+  the Nextcloud path in `docs/operations/restore-runbook.md`.
+- The mirror carries only the raw set files; `status.json` and the
+  Nextcloud bundles stay local.
diff --git a/deploy/compose/.env.example b/deploy/compose/.env.example
index 13a8357..c244da7 100644
--- a/deploy/compose/.env.example
+++ b/deploy/compose/.env.example
@@ -79,6 +79,13 @@ SMTP_FROM=Dorfteich <wiki@example.com>
 #BACKUP_MAIL_TO=ops@example.com
 #BACKUP_MAIL_LOCALE=en
 #BACKUP_INSTANCE_LABEL=dorfteich-test
+# Optional rsync mirror of the backup sets to a private host (issue #84):
+# rsync-over-ssh target plus the private key file INSIDE the container —
+# put the key on the secrets volume (docker compose cp), never in the repo.
+# Full setup walkthrough: deploy/backup-basel.md. Unset = no mirror.
+#BACKUP_MIRROR_TARGET=dorfteich-backup@172.30.1.10:/home/RAID/BACKUPS/dorfteich-prod/
+#BACKUP_MIRROR_SSH_KEY=/data/secrets/backup_mirror_ed25519
+#BACKUP_MIRROR_SSH_PORT=22
 
 # --- first-run setup (optional pre-seeding, issue #80) ------------------------
 # A fresh (empty) database makes the instance require the browser setup
diff --git a/deploy/compose/docker-compose.yml b/deploy/compose/docker-compose.yml
index 2e5f343..9467eff 100644
--- a/deploy/compose/docker-compose.yml
+++ b/deploy/compose/docker-compose.yml
@@ -156,6 +156,11 @@ services:
       BACKUP_MAIL_TO: ${BACKUP_MAIL_TO:-}
       BACKUP_MAIL_LOCALE: ${BACKUP_MAIL_LOCALE:-}
       BACKUP_INSTANCE_LABEL: ${BACKUP_INSTANCE_LABEL:-${COMPOSE_PROJECT_NAME:-dorfteich}}
+      # Optional rsync mirror to a private host (issue #84); the SSH key
+      # lives on the secrets volume (see deploy/backup-basel.md).
+      BACKUP_MIRROR_TARGET: ${BACKUP_MIRROR_TARGET:-}
+      BACKUP_MIRROR_SSH_KEY: ${BACKUP_MIRROR_SSH_KEY:-}
+      BACKUP_MIRROR_SSH_PORT: ${BACKUP_MIRROR_SSH_PORT:-}
       # Same SMTP resolution as the api: explicit env wins, the wizard-written
       # secret store fills the gaps (issue #80).
       SMTP_HOST: ${SMTP_HOST:-}
diff --git a/docs/architecture/operations.md b/docs/architecture/operations.md
index 0eb105d..4b1be00 100644
--- a/docs/architecture/operations.md
+++ b/docs/architecture/operations.md
@@ -50,7 +50,11 @@ monitoring, structured logs, backup alerting — no dedicated metrics stack.
   7 Test+Int; a Site-Admin setting overrides the env; the newest complete
   set always survives) → `status.json` on the `backups` volume → on
   failure a mail directly via the instance SMTP to `BACKUP_MAIL_TO`.
-  Mirror to BASEL is issue #84.
+- **Private mirror** (issue #84): with `BACKUP_MIRROR_TARGET` +
+  `BACKUP_MIRROR_SSH_KEY` set (Prod: BASEL over WireGuard), every
+  successful run rsyncs the set files after the prune (`--delete` aligns
+  the remote retention); outcome in `status.json → mirror`, failures mail
+  like run failures. Setup: `deploy/backup-basel.md`.
 - **Off-host copies** (issue #103): with a Nextcloud target configured in
   the admin UI (WebDAV base URL + username + folder in instance settings,
   app password in the secret store), each successful set is bundled into
diff --git a/packages/shared/i18n/de/mails.json b/packages/shared/i18n/de/mails.json
index 6149d19..88f0942 100644
--- a/packages/shared/i18n/de/mails.json
+++ b/packages/shared/i18n/de/mails.json
@@ -40,6 +40,15 @@
     "lastSuccessNever": "Letzter erfolgreicher Upload: noch keiner",
     "hint": "Prüfe die Nextcloud-Verbindungseinstellungen im Admin-Bereich (Verbindungstest) und die Sidecar-Logs (docker compose logs backup)."
   },
+  "backupMirrorFailed": {
+    "subject": "[{{instance}}] Backup-Spiegelung zum privaten Host fehlgeschlagen",
+    "intro": "Das lokale Backup auf {{instance}} war erfolgreich, aber die Spiegelung der Sets per rsync ist fehlgeschlagen.",
+    "backupId": "Komponente: rsync-Mirror (Issue #84)",
+    "error": "Fehler: {{error}}",
+    "lastSuccess": "Letzte erfolgreiche Spiegelung: {{finishedAt}}",
+    "lastSuccessNever": "Letzte erfolgreiche Spiegelung: noch keine",
+    "hint": "Prüfe den Tunnel zum Mirror-Host, den SSH-Key im Secrets-Volume und die Sidecar-Logs (docker compose logs backup). Einrichtung: deploy/backup-basel.md."
+  },
   "digest": {
     "subject": "Dorfteich: {{count}} Neuigkeiten für dich",
     "intro": "Das ist auf von dir beobachteten Seiten passiert ({{count}} Neuigkeiten):",
diff --git a/packages/shared/i18n/de/system.json b/packages/shared/i18n/de/system.json
index 8cc3c21..4c6c3e8 100644
--- a/packages/shared/i18n/de/system.json
+++ b/packages/shared/i18n/de/system.json
@@ -111,6 +111,11 @@
         "succeeded": "Wiederherstellung von {{id}} war erfolgreich ({{finishedAt}}).",
         "failed": "Wiederherstellung von {{id}} ist fehlgeschlagen: {{error}}"
       }
+    },
+    "mirror": {
+      "title": "Privater Spiegel",
+      "lastSuccess": "Letzte erfolgreiche Spiegelung",
+      "failed": "letzter Lauf fehlgeschlagen"
     }
   },
   "audit": {
diff --git a/packages/shared/i18n/en/mails.json b/packages/shared/i18n/en/mails.json
index b75793b..3fb69b8 100644
--- a/packages/shared/i18n/en/mails.json
+++ b/packages/shared/i18n/en/mails.json
@@ -40,6 +40,15 @@
     "lastSuccessNever": "Last successful upload: none yet",
     "hint": "Check the Nextcloud connection settings in the admin panel (test connection) and the sidecar logs (docker compose logs backup)."
   },
+  "backupMirrorFailed": {
+    "subject": "[{{instance}}] Backup mirror to the private host failed",
+    "intro": "The local backup on {{instance}} succeeded, but mirroring the sets via rsync failed.",
+    "backupId": "Component: rsync mirror (issue #84)",
+    "error": "Error: {{error}}",
+    "lastSuccess": "Last successful mirror: {{finishedAt}}",
+    "lastSuccessNever": "Last successful mirror: none yet",
+    "hint": "Check the tunnel to the mirror host, the SSH key in the secrets volume, and the sidecar logs (docker compose logs backup). Setup: deploy/backup-basel.md."
+  },
   "digest": {
     "subject": "Dorfteich: {{count}} updates for you",
     "intro": "Here is what happened on pages you watch ({{count}} updates):",
diff --git a/packages/shared/i18n/en/system.json b/packages/shared/i18n/en/system.json
index d56b8a3..bb4d9e6 100644
--- a/packages/shared/i18n/en/system.json
+++ b/packages/shared/i18n/en/system.json
@@ -111,6 +111,11 @@
         "succeeded": "Restore of {{id}} succeeded ({{finishedAt}}).",
         "failed": "Restore of {{id}} failed: {{error}}"
       }
+    },
+    "mirror": {
+      "title": "Private mirror",
+      "lastSuccess": "Last successful mirror",
+      "failed": "last run failed"
     }
   },
   "audit": {
diff --git a/packages/shared/src/backup-status.ts b/packages/shared/src/backup-status.ts
index fee579b..7a0ff2f 100644
--- a/packages/shared/src/backup-status.ts
+++ b/packages/shared/src/backup-status.ts
@@ -53,6 +53,8 @@ export interface BackupStatus {
    * visible.
    */
   remote?: BackupRemoteStatus;
+  /** rsync mirror state (issue #84); absent until a target is configured. */
+  mirror?: BackupMirrorStatus;
 }
 
 /** One WebDAV upload attempt of a complete restore set (issue #103). */
@@ -72,6 +74,25 @@ export interface BackupRemoteStatus {
   lastSuccessfulUpload: { backupId: string; finishedAt: string; sizeBytes: number } | null;
 }
 
+/**
+ * The rsync mirror to a private host (issue #84, ADR 0015) — an
+ * operator-level extra beside the admin-configured Nextcloud target
+ * (#103). Present once a mirror target is configured; carried across
+ * failed runs like the other blocks.
+ */
+export interface BackupMirrorStatus {
+  lastRun: {
+    finishedAt: string;
+    outcome: 'succeeded' | 'failed';
+    /** Present on failure: the first error the mirror hit. */
+    error?: string;
+    /** Present on success: files rsync actually transferred (0 = idempotent re-run). */
+    transferredFiles?: number;
+  };
+  /** Carried across failed runs — when the mirror last matched the local sets. */
+  lastSuccessAt: string | null;
+}
+
 /**
  * The remote bundle naming scheme (issue #103): one self-contained archive
  * per restore set, everything needed to rebuild an instance after total
diff --git a/packages/shared/src/env.ts b/packages/shared/src/env.ts
index 64c1957..0c236e7 100644
--- a/packages/shared/src/env.ts
+++ b/packages/shared/src/env.ts
@@ -172,6 +172,17 @@ export const backupEnvSchema = z.object({
   BACKUP_MAIL_LOCALE: z.enum(['de', 'en']).default('en'),
   /** Instance label in the mail subject, e.g. "dorfteich-test". */
   BACKUP_INSTANCE_LABEL: z.string().optional(),
+  /**
+   * Optional rsync mirror to a private host (issue #84, ADR 0015), e.g.
+   * `dorfteich-backup@172.30.1.10:/home/RAID/BACKUPS/dorfteich-prod/`.
+   * Unset disables the mirror entirely. Deliberately env-only (operator
+   * territory), unlike the admin-configured Nextcloud target (#103).
+   */
+  BACKUP_MIRROR_TARGET: z.string().optional(),
+  /** Private SSH key file for the mirror; mount it via the secrets volume
+   * (e.g. `/data/secrets/basel_ed25519`), mode 600, never in the image. */
+  BACKUP_MIRROR_SSH_KEY: z.string().optional(),
+  BACKUP_MIRROR_SSH_PORT: z.coerce.number().int().min(1).max(65535).default(22),
   ...smtpFields,
   /** Read-only view of the wizard-written secret store (issue #80). */
   SECRETS_FILE: z.string().min(1).default('./data/secrets.env'),