---
id: implementation/local-providers
name: Local (the socket-backed seam providers, tcp_dial / udp_egress / bind_accept)
status: completed
depends_on: []
scope: moderate
risk: medium
impact: component
level: implementation
---

## Description

The first batch-2 task: the `local` feature's first-party socket-backed
providers, the three seams' faithful-RFC constructors (ADR-008's
local variants, ADR-009's feature gate). These are the assembly-layer
convenience constructors over the landed seam types
(`src/egress.rs`); the protocol layer never sees them.

Deliverables:

1. **`Cargo.toml` feature wiring**: `local = ["tokio/net"]` (the
   feature's own socket providers; `socket2` only if a provider
   actually needs it, though the POCs ran tokio-only, so default to NOT
   adding it). tokio stays `default-features = false` in
   `[dependencies]`; the `net` feature rides `local` alone (ADR-009).
   Verify the default graph stays net-free (`cargo tree` spot-check)
   and the default wasm checks stay green.
2. **`src/local/mod.rs`** (feature `local`): the provider
   constructors, named per producer.md §Wiring:
   - `tcp_dial()` → `DialFn`: `TcpStream::connect` per CONNECT target;
     domain targets resolve here (the ssh `-D` semantic; per ADR-012
     resolution is the provider's job); the connected stream splits
     into the `TargetHandle` halves (`Box<dyn AsyncRead + Send + Sync
     + Unpin>`; `TcpStream::into_split` halves are Send+Sync, so verify
     the boxing compiles). Dial failures map to `SocksDialError`
     faithfully (`ConnectionRefused` for connect-refused,
     `NetworkUnreachable`/`HostUnreachable` per `io::ErrorKind`,
     `NotAllowed` never synthesized by this provider).
   - `udp_egress()` → `EgressFactory`: one real `UdpSocket` per
     association (bind `127.0.0.1:0`, the assembly-layer bind
     decision, made explicit here), per-datagram `send_to`/`recv_from`, no
     flow table; domain destinations resolve at the egress
     (`resolve_target`-shaped; port the POC #2 `LocalEgress`
     helper). **The readiness contract is load-bearing**: `next_reply`
     parks on `sock.readable().await` then `try_recv_from`
     (`WouldBlock` → the `Ok(None)` spurious-wakeup shape; a real async
     park, never a hot `Ok(None)`); replies carry *inner* bytes
     (`TargetAddr::to_be_bytes() + data`, the sender's address per
     the RFC's per-datagram source semantics; framing-once, so the
     length prefix is the wire layer's). `send` takes payload only.
     Egress failures map to `EgressError` (bind failure → `General`;
     resolve failure → `Unreachable`).
   - `bind_accept()` → `BindAcceptFactory`: a real `TcpListener` per
     BIND session (the faithful RFC shape, ADR-007 variant 1): the
     requested target's port honored when non-zero (FTP active mode),
     port 0 → ephemeral; `local_addr()` is reply#1's `bind_addr`; the
     accept future owns the listener end-to-end (the quiet wait's
     drop IS the client-gone teardown, and the listener lives inside the
     future, never in a separate binding); `accept()`'s peer is
     reply#2's `BindTargetHandle.peer` (byte-exact). Factory failures
     → `BindAcceptFactoryError` (bind refused/held port → `Refused`
     or `Shortage` per the io error; accept-stage failures →
     `BindAcceptError`). Address policy is the caller's: the
     constructor takes the bind host (loopback default is the POC's
     posture; a wildcard bind is the caller's explicit choice).
   - Module doc: names the ADRs (008/009), the never-binds posture
     (the providers ARE the explicit bind decisions), and the seam
     contracts each constructor must honor (readiness, ownership,
     accept-future-owns, framing-once).
3. **Tests** (`src/local/` unit + a `tests/local-providers.rs`
   integration file): real sockets (dev-deps already carry full
   tokio). Echo TCP server for `tcp_dial` round-trips; a real UDP
   echo for the egress (datagram round-trip through the producer
   stage, the POC #2 shape); a real BIND accept (dial the listener,
   reply#1/reply#2 carry the real addresses); the readiness contract
   re-pinned with the real socket egress (park-then-datagram, no
   spin); refusal mappings (connect-refused → 0x05 through the
   producer's reply).

## Acceptance Criteria

- [x] `cargo test` passes; `cargo test --all-features` passes
- [x] `cargo clippy --all-targets -- -D warnings` clean; `cargo fmt
      --check` clean
- [x] Default wasm checks stay green (`cargo check`/`clippy --target
      wasm32-unknown-unknown -- -D warnings`), meaning `local` code is
      cfg-gated out of the default build
- [x] Default dependency graph has no `tokio/net`/`mio`/`socket2`
      (`cargo tree` spot-check); `local = ["tokio/net"]` wired
- [x] `tcp_dial`/`udp_egress`/`bind_accept` land with the exact
      producer.md §Wiring names and the landed seam types
- [x] The egress readiness contract honored (real async park; no hot
      `Ok(None)`), pinned by test
- [x] The accept future owns its listener end-to-end, pinned by a
      drop-witness test (client-gone → listener closed)
- [x] Refusal mappings table-tested against the landed error types'
      `to_reply_error()`

## References

- docs/architecture/decisions/008-provider-defaults.md (the local
  variants), 009-feature-gates.md (the feature layout), 004 (the
  seams), 007 (BIND variant 1), 012 (resolution at the egress)
- docs/architecture/producer.md §Wiring, §The Three Seams
- POC reference: `/workspace/alksocks-udp-poc/src/egress.rs`
  (`LocalEgress`, the readiness shape), `/workspace/alksocks-bind-
  poc/src/accept.rs` (`local_bind_accept`)
- Landed seams: `src/egress.rs` (the contracts' normative doc
  comments)

## Notes

> Filled by the implementation agent (2026-09-25).

- `Cargo.toml`: `local = ["tokio/net"]` (the feature's own socket
  providers; no `socket2`, because the POCs ran tokio-only and nothing here
  needs it). tokio stays `default-features = false` with the
  wasm-clean subset; the `net` feature rides `local` alone
  (ADR-009). `cargo tree` spot-checks: the default graph
  (`-e normal,features --no-default-features`) carries no
  `tokio/net`/`mio`/`socket2` (inverting `-i mio` prints nothing);
  with `--features local --target all` the `net` feature appears
  rooted at `alksocks feature "local"`, dragging `mio` + `socket2`
  in through tokio (the dev-dep `full` also appears in dev builds,
  pre-existing and test-only, not the feature's doing).
- `src/local/mod.rs` (feature `local`, one module file + a
  `tests` submodule per convention 15):
  - `tcp_dial()` → `DialFn`: `TcpStream::connect` per CONNECT
    target; domain targets resolve here (the ssh `-D` semantic,
    with ADR-012 in play), iterating the resolved address set (the `-D`
    connect-multi-address posture: first success wins, the last
    failure is the reported error); `into_split` halves boxed as
    `Send + Sync + Unpin` (compiles, pinned by the round-trip test).
    Refusal mapping (`dial_error_of`): `ConnectionRefused` →
    `SocksDialError::Refused`, `NetworkUnreachable`/`TimedOut` →
    `NetworkUnreachable`, `HostUnreachable` → `HostUnreachable`,
    resolution failure → `HostUnreachable`, everything else →
    `General`; `NotAllowed` never synthesized.
  - `udp_egress()` → `EgressFactory`: one real `UdpSocket` per
    association (bind `127.0.0.1:0`, the explicit assembly-layer
    bind decision, `LocalEgress::bind`); per-datagram
    `send_to`/`recv_from`, no flow table; domain destinations
    resolve at the egress (`resolve_target`, porting the POC #2
    `LocalEgress` helper ported). The readiness contract is the
    load-bearing shape: `next_reply` parks on `sock.readable().await`
    (a real async park on receive readiness, since `writable()` is always
    ready for UDP and would spin), then `try_recv_from`
    (`WouldBlock` → the spurious-wakeup `Ok(None)`); replies carry
    *inner* bytes (`TargetAddr::to_be_bytes() + data`, the sender's
    address per the RFC's per-datagram source semantics;
    framing-once); `send` takes payload only. Bind failure →
    `General`; resolve failure → `Unreachable`.
  - `bind_accept(bind_host)` → `BindAcceptFactory`: a real
    `TcpListener` per BIND session (ADR-007 variant 1); the
    requested port honored when non-zero, port 0 → ephemeral;
    `local_addr()` is reply#1's `bind_addr`; the accept future owns
    the listener end-to-end (the listener lives inside the future, so
    the quiet wait's drop IS the client-gone teardown);
    `accept()`'s peer is reply#2's `BindTargetHandle.peer`
    (byte-exact). Factory failures → `BindAcceptFactoryError`
    (`PermissionDenied`/`AddrInUse` → `Shortage`,
    `AddrNotAvailable` → `Refused`, else `General`);
    accept-stage failures → `BindAcceptError` (`General`). Address
    policy is the caller's: the constructor takes the bind host
    (`IpAddr`-convertible; loopback is the POC's posture, and the
    wildcard bind is the caller's explicit `0.0.0.0`).
  - Module doc names ADR-008/009, the never-binds posture (the
    providers ARE the explicit bind decisions), and the four seam
    contracts (readiness, ownership-rides-the-handle, framing-once,
    accept-future-owns).
- `src/lib.rs`: `#[cfg(feature = "local")] pub mod local;` + the
  `tcp_dial`/`udp_egress`/`bind_accept`/`LocalEgress` re-exports;
  the crate doc's "not yet present" paragraph updated (the composed
  providers remain pending).
- Tests: 12 unit tests (`src/local/tests.rs`: the TCP echo
  round-trip incl. domain resolution, connect-refused/unresolvable
  mappings, the UDP echo round-trips (inner bytes + sender address,
  in-order, empty payload), the park-then-datagram readiness probe,
  the BIND accept (real reply#1/reply#2, requested-port honored,
  held-port refusal, the drop-closes-the-listener witness)) + 6
  integration tests (`tests/local-providers.rs`, feature-gated with
  `#![cfg(feature = "local")]`: CONNECT through a real echo target,
  connect-refused → 0x05 through the producer's reply, ASSOCIATE
  through the producer's datagram stage with the real egress, the
  readiness contract re-pinned through the stage (park-then-datagram,
  no spin), BIND end-to-end with real addresses both directions, and
  the client-gone-mid-quiet-wait drop-witness (the listener's port
  frees)).
- Deliberate deviations from the POC code: the dial iterates the
  full resolved address set (the POC resolved to the first address);
  `TimedOut` maps to `NetworkUnreachable` and `PermissionDenied` to
  `General` in the dial table (the POC had no io-error mapping at
  all); `bind_accept` takes the bind host as a parameter (the POC
  hardcoded `127.0.0.1`) so the wildcard bind is the caller's
  spelled-out choice; the held-port factory refusal maps to
  `Shortage`/`Refused` per the io kind (the POC mapped everything to
  `Shortage`).

## Summary

> Filled on completion (2026-09-25).

Implemented the `local` feature's socket-backed seam providers
(ADR-008's local variants, ADR-009's feature gate), the three seams'
faithful-RFC assembly-layer constructors over the landed seam types
(`src/egress.rs`): `tcp_dial()` (real `TcpStream` per CONNECT, domain
resolution at the provider, faithful `SocksDialError` mapping),
`udp_egress()` (one real `UdpSocket` per association bound
`127.0.0.1:0`, per-datagram send/recv, the POC #2 `LocalEgress`
readiness shape: park on `readable()`, `try_recv_from`, inner bytes
`to_be_bytes() + data`, framing-once), and `bind_accept(bind_host)`
(a real `TcpListener` per BIND session, requested port honored, the
accept future owning the listener end-to-end, two-stage failure
mapping). The protocol layer never sees them; these are feature-gated
(`local = ["tokio/net"]`), the default graph stays
net/mio/socket2-free (`cargo tree` spot-checked both ways), and the
wasm checks stay green. 18 new tests (12 unit + 6 integration) pin:
the real-socket round-trips through the producer's actual dispatch
path (echo TCP, UDP echo through the framed datagram stage, BIND
end-to-end with byte-exact real addresses), the readiness contract
with the real socket (park-then-datagram, no spin), the
accept-future-owns-its-listener contract (drop-witness: client-gone
→ the advertised port frees), and the refusal mappings
table-tested against the landed error types' `to_reply_error()`
(connect-refused → 0x05 through the producer's reply, held port →
`Shortage`/`Refused` → the RFC catch-all, unresolvable domain →
`HostUnreachable`/`NetworkUnreachable`).

Verification: `cargo test` (157 green across the 9 targets) +
`cargo test --all-features` green, `cargo clippy --all-targets -- -D
warnings` clean (default + `--all-features`), `cargo fmt --check`
clean, both wasm checks (`cargo check` + `cargo clippy -D warnings`
on `wasm32-unknown-unknown`) clean, `cargo tree` spot-checks: the
default graph carries no `tokio/net`/`mio`/`socket2` and
`--features local --target all` roots the `net` feature at
`alksocks feature "local"`.