[jamesyong-42]

What

An app can choose the name its devices show on Tailscale's own pages. App membership moves off that name and onto a tag
the node announces in Hostinfo.App.

• NodeBuilder::hostname_prefix("vibefield") names the node vibefield-{slug} instead of truffle-{app_id}-{slug}.
  • It is validated as a single lowercase DNS label of 1–61 characters, leaving room for the hyphen and a slug.
  • It is exclusive with hostname(): build() refuses both.
• Every start announces Hostinfo.App = "truffle:{app_id}". tsnet:start carries app, and the sidecar calls
  hostinfo.SetApp before srv.Start(). SetApp is process-wide, and one sidecar process carries one node.
• Every peer row (tsnet:peers, tsnet:peerChanged) carries app, the peer's Hostinfo.App as control relays it.
  • ipnstate.PeerStatus has no App, so the sidecar reads it through LocalClient.PeerByID once per peer.
  • After that, the IPN bus keeps the entry current: a Hostinfo change arrives as a full node in PeersChanged, which
    the bus watch already subscribes to.
  • A failed lookup is not cached. A node sent without Hostinfo has no app.
• Admission is tag-first (is_app_row):
  • A tagged row is this app's only when its tag is exactly truffle:{app_id}, whatever its name. So a node of
    another app under the same prefix stays out.
  • An untagged row (a sidecar below 12, or a peer that announces nothing) falls back to the truffle-{app_id}-
    prefix, exactly as before.
  • The tag is the peer's own claim, just as its hostname is. It widens nothing a same-tailnet node could not already
    claim by its name.
• A hello-less peer named {prefix}-{slug} resolves by its slug under the node's own prefix. The convention still
  resolves, for a peer on an older release.
• Sidecar protocol 11 → 12. A sidecar below 12 ignores app. A node whose name is outside truffle-{app_id}-
  then logs a warning that its peers will not recognise it. The node still starts, as an explicit hostname() always
  has.

Additive. An app that sets no prefix keeps truffle-{app_id}-{slug} and today's discovery. TailscaleConfig is
unchanged: the tag is derived from its app_id.

Why: a real-tailnet probe first

The open question was whether control relays a peer's Hostinfo.App into the netmap a tsnet node receives.

• tsnet sets only Package "tsnet".
• On 2026-11-01, two ephemeral tsnet nodes on one tailnet answered it, using tailscale.com v1.104.1, the sidecar's
  own go.mod:
  • Node a called hostinfo.SetApp("truffle:vibefield-t8probe"); node b set nothing.
  • Each read the other through PeerByID.

[b]   Hostinfo.App     = "truffle:vibefield-t8probe"
[b]   Hostinfo.Package = ""
[b]   Hostinfo fields relayed: App, Hostname, OS, Services
[b] RESULT: peer truffle-t8probe-a Hostinfo.App RELAYED
[a]   Hostinfo fields relayed: Hostname, OS
[a] RESULT: peer truffle-t8probe-b Hostinfo.App NOT RELAYED (App empty)

So a peer's row keeps App, Hostname, OS and Services, and Package is stripped (no carrier). The tag shape stands.

Rollout hazard (read before turning a prefix on)

• A device on an earlier release reads the name only. Its is_app_peer admits truffle-{app_id}- alone, so it
  will not see a node with a custom prefix. An app should turn hostname_prefix on only once every shipped device
  reads the tag: two releases, or one upgrade whose builder call waits a version.
• The Apple plane is not in this PR. apple/Sources/Truffle/Identity/Slug.swift (Hostname.isAppPeer, used at
  MeshNode.swift:616 and :677) still filters by the prefix, and the Swift node announces no tag. A custom-prefix
  desktop is invisible to a Swift node until that plane reads and announces the tag, which is a follow-up.
• Existing devices are renamed. tsnet re-sends the hostname on every start, so a device takes its new name in the
  admin console at its next start. Release notes should say so.
• The napi option is a follow-up. NodeOptions.hostnamePrefix in truffle-napi regenerates index.d.ts, so it
  rides its own change.

Tests

The tests were written first and run red against stubs that ignored the tag and the prefix (logs kept):

• Rust: 6 of 8 failed, each on its assertion. The wire parse and the untagged fallback passed, as they should.
• Go: 3 of 4 failed. The start-data parse passed.

The new tests:

• is_app_row: a present tag decides whatever the name; a foreign tag under our prefix is refused; a same-prefix
  node of another app is refused; an absent or empty tag falls back to the prefix.
• classify_row admits a tagged custom-prefix row, and calls the same row untagged a stranger.
• A peer row reads app from the wire; a protocol-11 row has none.
• A scripted sidecar shows tsnet:start carries "app":"truffle:playground".
• hostname_prefix composes {prefix}-{slug}. It cuts a long device name to 63 characters, rejects bad and over-long
  prefixes, and refuses one together with hostname().
• hostname_slug learns the prefix and keeps the convention. A peer named vibefield-ec2-smoke shows ec2-smoke
  before hello and resolves by it.
• Go: applyApp reaches hostinfo.New().App and clears. The app cache fetches once, follows full-node notifies,
  forgets removed peers, never caches a failure, and tolerates a node without Hostinfo. An App change counts as a
  peer change.
  • The full suite found that last case: peer_watch tests feed nodes without Hostinfo, and a bare .App() panicked.
• The protocol pins moved to 12 (TestStatusEventAdvertisesProtocolVersion, the v11 status test).

Run locally at this commit, all green, with no tailnet key in the environment:

• cargo fmt --check.
• cargo clippy --locked --workspace --all-targets --exclude truffle-tauri-plugin --exclude truffle-napi --exclude truffle-bench-app -- -D warnings.
• RUSTDOCFLAGS='-D warnings' cargo doc --locked --no-deps --workspace (the same excludes).
• cargo test --locked --workspace (the same excludes): 21 suites, 773 passed, 0 failed, 30 ignored.
• In packages/sidecar-slim: gofmt -l is empty; go vet ./... and go test ./... pass; go test -race -run 'App|Peer|Watch|Status' passes.

Not run here: CI's real-tailnet integration jobs. They need TRUFFLE_TEST_AUTHKEY, and CI runs them on this PR.

Petition: VibeField's T8 (track TS, TS-D10).

🤖 Generated with Claude Code

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