PaperClipAI/packages/tailscale-https-broker

Paperclip Tailscale HTTPS broker

Least-privilege host broker that manages only Paperclip-owned, tailnet-only, same-number HTTPS-to-loopback listeners for managed branch runtimes.

It exists so the Paperclip app/agent account never gains Tailscale operator authority (see PAP-16989) while still getting automatic trusted HTTPS previews per branch runtime. Design: PAP-17049 plan; security contract: PAP-17050 threat-model verdict.

Runtime services opt in explicitly; existing services and the primary :443 route are unchanged:

{
  "port": { "type": "auto", "envKey": "PORT" },
  "expose": {
    "type": "tailscale_https",
    "hostname": "auto",
    "publicPort": "same",
    "includePaperclipViteHmr": true,
    "failurePolicy": "fail_closed"
  }
}

What it can and cannot do

Supported operations (over a Unix socket, one runtime-service at a time):

• list — the caller's own exposures (never returns lease handles).
• reserve — atomically reserve an app/HMR pair before either backend binds, returning an unguessable, short-lived lease handle bound to the caller, runtime ID, ports, purposes, and generation.
• expose — redeem that reservation only after /proc proves both listeners are loopback-only and owned by the configured managed-runtime UID.
• remove — remove the caller's own listeners, proven by exact lease handle.

Hard-denied, deny-by-default: Funnel, certificates, Tailscale Services, serve reset / set-config, path handlers, arbitrary targets, non-loopback or wildcard/dual-stack backends, port 443, privileged/reserved ports, ports outside the dedicated runtime range, unknown fields, and removal of any mapping not matching an exact registry + lease + live Serve entry. The primary :443 → 127.0.0.1:3100 route is verified structurally before and after every mutation and is never modified.

The socket transport reads Linux SO_PEERCRED before admission, admits at most 8 concurrent sockets per resolved UID, and reserves 4 of its 32 global slots for the configured Paperclip service UID. Connection deadlines destroy the socket so timed-out peers cannot retain kernel-level connection slots. Missing or invalid native credentials fail closed; socket permissions are not used as a substitute identity.

One-time host installation (paperclip-dev)

These steps require root and must be run by CloudOps/host owner, not the Paperclip agent account. They install the broker as a dedicated Tailscale-operator service account distinct from the Paperclip app account.

1. Preconditions. Tailscale is installed and up on the node, the node has an HTTPS-capable trusted cert (MagicDNS + HTTPS enabled), and the existing :443 → 127.0.0.1:3100 Serve mapping is present.

2. Create the dedicated operator account and socket group.

   sudo useradd --system --home /var/lib/paperclip-tailscale-broker \
     --shell /usr/sbin/nologin paperclip-tsbroker
   sudo groupadd --system paperclip-tsbroker-sock
   # The Paperclip *app* service account must have this as its PRIMARY group so
   # its SO_PEERCRED gid matches the socket group (supplemental membership is
   # intentionally NOT accepted).
   sudo usermod -g paperclip-tsbroker-sock <paperclip-app-account>
   

3. Grant Tailscale operator authority to the broker account only.

   sudo tailscale set --operator=paperclip-tsbroker
   

   Do not grant --operator to the Paperclip app/agent account (that grant was explicitly rejected in PAP-16989).

4. Create state directories (not writable by the Paperclip app). The packaged unit creates these automatically; for a manual install use:

   sudo install -d -o paperclip-tsbroker -g paperclip-tsbroker-sock -m 0750 /run/paperclip-tailscale-broker
   sudo install -d -o paperclip-tsbroker -g paperclip-tsbroker-sock -m 0700 /var/lib/paperclip-tailscale-broker
   sudo install -d -o paperclip-tsbroker -g paperclip-tsbroker-sock -m 0700 /var/log/paperclip-tailscale-broker
   

   The broker refuses to start if the registry path's parent is group/other writable.

5. Build, install the package under /opt/paperclip, and install the packaged systemd unit. The unit's ExecStart (and the doctor command below) run the build output from /opt/paperclip/packages/tailscale-https-broker/dist, so copy it there explicitly. The Linux build requires a C compiler and Node.js headers to compile the dependency-free N-API SO_PEERCRED addon. The output is self-contained (Node builtins plus the compiled addon; no node_modules needed).

   pnpm --filter @paperclipai/tailscale-https-broker build
   sudo install -d -m 0755 /opt/paperclip/packages/tailscale-https-broker
   sudo cp -r packages/tailscale-https-broker/dist \
     /opt/paperclip/packages/tailscale-https-broker/
   sudo install -D -m 0644 \
     packages/tailscale-https-broker/deploy/paperclip-tailscale-https-broker.service \
     /etc/systemd/system/paperclip-tailscale-https-broker.service
   sudo install -d -m 0750 /etc/paperclip
   sudoedit /etc/paperclip/tailscale-https-broker.env
   

   The packaged unit is equivalent to:

   [Unit]
   Description=Paperclip Tailscale HTTPS broker
   After=tailscaled.service
   Requires=tailscaled.service
   
   [Service]
   Type=simple
   User=paperclip-tsbroker
   # Socket must end up 0660 paperclip-tsbroker:paperclip-tsbroker-sock. Set the group here and
   # the broker chmods the socket to 0660 on bind.
   Group=paperclip-tsbroker-sock
   EnvironmentFile=/etc/paperclip/tailscale-https-broker.env
   ExecStart=/usr/bin/node /opt/paperclip/packages/tailscale-https-broker/dist/main.js
   Restart=on-failure
   NoNewPrivileges=true
   ProtectSystem=strict
   ReadWritePaths=/run/paperclip-tailscale-broker /var/lib/paperclip-tailscale-broker /var/log/paperclip-tailscale-broker
   
   [Install]
   WantedBy=multi-user.target
   

   Put the BROKER_* values from the table below in the environment file. Set PAPERCLIP_TAILSCALE_BROKER_SOCKET=/run/paperclip-tailscale-broker/broker.sock on the Paperclip service only if overriding its default.

   Environment variables (defaults in src/config.ts):

   Var	Required	Default	Meaning
   BROKER_NODE_IDENTITY	yes	—	hostname + boot id; a change forces quarantine + operator reconciliation
   BROKER_SERVICE_UID	yes	—	UID of the Paperclip app account allowed to connect
   BROKER_SERVICE_GID	yes	—	GID of the dedicated socket group (caller's primary GID)
   BROKER_RUNTIME_UID	yes	—	UID that owns Paperclip-managed runtime processes (normally the Paperclip app service account); only its loopback listeners are eligible
   BROKER_TAILSCALE_BIN	no	/usr/bin/tailscale	absolute path to the Tailscale CLI
   BROKER_SOCKET_PATH	no	/run/paperclip-tailscale-broker/broker.sock	Unix socket path
   BROKER_REGISTRY_PATH	no	/var/lib/paperclip-tailscale-broker/registry.json	root-owned 0600 ownership registry
   BROKER_AUDIT_PATH	no	/var/log/paperclip-tailscale-broker/audit.log	append-only security audit log
   BROKER_PROTECTED_PORTS	no	(empty)	comma/space separated ports the broker must never create, remove, or reclaim — even when its own registry holds a valid lease for them (see below)

   BROKER_PROTECTED_PORTS — operator-declared preservation (PAP-17285)

   The long-standing "unknown/manual entries are never modified" invariant is provenance-blind: it protects only entries the broker has no lease for. It therefore could not protect the 42000/52000 mappings, because the broker had itself created them for a canary lane that was later retired — so its registry still called them owned, while operators had reclassified them as must-preserve after failing to attribute them to any live lane. Both views were internally consistent, they disagreed, and a fully authorized, shape-valid, :443-preserving removal destroyed them with no guard able to object.

   A protected port is an operator assertion that outranks the broker's own ownership record. Enforcement is fail-closed and layered: refused during argv construction, denied in reserve/expose/remove with protected_port, excluded from the allocatable allowlist so no lane can acquire one, and asserted byte-unchanged across every before/after snapshot (protected_entry_violation). A malformed list makes the broker refuse to start rather than silently protect nothing; 443 is rejected because the primary route already has a stronger, non-optional invariant.

   BROKER_PROTECTED_PORTS=42000,52000
   

   Confirm it took effect before trusting it — --doctor echoes the parsed set:

   sudo -u paperclip-tsbroker \
     env $(cat /etc/paperclip/tailscale-https-broker.env | xargs) \
     node /opt/paperclip/packages/tailscale-https-broker/dist/main.js --doctor
   

6. Preflight (read-only, no mutation).

   sudo -u paperclip-tsbroker \
     BROKER_NODE_IDENTITY=$(hostname) BROKER_SERVICE_UID=... BROKER_SERVICE_GID=... BROKER_RUNTIME_UID=... \
     node /opt/paperclip/packages/tailscale-https-broker/dist/main.js --doctor
   

   Verifies: supported Tailscale CLI version, Serve status is readable, the primary :443 route is intact, the registry path is safe, and prints the node identity. Exit 0 = ready. It never mutates Serve state.

7. Enable. sudo systemctl daemon-reload && sudo systemctl enable --now paperclip-tailscale-https-broker. Confirm the socket is 0660 paperclip-tsbroker:paperclip-tsbroker-sock.

Upgrade

Deploy new package output to /opt/paperclip/packages/tailscale-https-broker/dist, then sudo systemctl restart paperclip-tailscale-https-broker. On restart the broker re-reads its root-owned registry and adopts only exact-lease matches; a changed BROKER_NODE_IDENTITY (host reimage / boot-id change) forces quarantine and operator reconciliation rather than silently re-adopting.

Uninstall / rollback / opt-out

Rollback disables new exposure and removes only broker-owned listeners; it never resets Serve or changes the primary route.

1. Disable the exposure flag on the project runtime (Paperclip stops requesting expose). Existing previews drain on runtime stop.
2. Drain owned listeners: stop each managed runtime so Paperclip issues remove for its own leases (proven by handle).
3. sudo systemctl disable --now paperclip-tailscale-https-broker.
4. Optional cleanup: remove the state dirs and sudo tailscale set --operator= to drop the operator grant. Do not run tailscale serve reset — remove only the specific per-port Serve entries if any remain.

Recovery

If a mutation fails partway, the broker removes only the exact listeners it applied; if exact cleanup cannot be proven it quarantines the affected ports and reports cleanup_pending (partial app+HMR exposure is never reported healthy). Quarantined ports are not reused until an operator clears them. The append-only audit log at BROKER_AUDIT_PATH records every allow/deny and mutation outcome (peer UID/GID/PID, operation, runtime UUID, ports, decision reason, before/after state digests, quarantine/recovery) with lease handles and raw CLI output redacted.

Tests

pnpm --filter @paperclipai/tailscale-https-broker test        # 72 tests
pnpm --filter @paperclipai/tailscale-https-broker typecheck
pnpm --filter @paperclipai/tailscale-https-broker build