feat(smolmachines): per-bottle loopback alias scopes TSI to single /32

PR #74's Docker-Desktop fix routed the agent through
`127.0.0.1:<random>` loopback forwards, but TSI filters by IP
only — so the allowlist `127.0.0.1/32` let the agent VM reach
**any** host service on macOS loopback (postgres, dev servers,
other bottles' published ports, mDNSResponder, ...). Real
downgrade vs the docker backend's `--internal` network.

Resolution: per-bottle loopback alias.

- New `loopback_alias` module manages a pool of
  `127.0.0.16` .. `127.0.0.31` on `lo0`. macOS only routes
  `127.0.0.1` by default; the extras need `sudo ifconfig lo0
  alias`. `ensure_pool()` lazily adds the missing entries via
  one sudo prompt on first launch per reboot — aliases persist
  on `lo0` until reboot, so subsequent launches skip the
  prompt entirely.
- `allocate(slug)` picks the lowest-numbered unused alias by
  inspecting running bundle containers' port-binding HostIps.
  No on-disk reservation — docker is the source of truth.
- Bundle bringup binds published ports to the allocated alias
  (`docker run -p <alias>::<port>`) instead of `127.0.0.1`.
- TSI allowlist becomes the alias's /32 — narrows reachability
  to this bottle's bundle only.
- Linux native daemons share the host's network namespace;
  `127.0.0.0/8` works without aliases, so the module no-ops on
  non-Darwin and returns `127.0.0.1` from `allocate`.

Tracking issue closed: gitea/issues/75.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/smolmachines/loopback_alias.py

@@ -0,0 +1,176 @@
+"""Per-bottle loopback alias allocation (PRD 0023, follow-up to
+the Docker-Desktop fix in PR #74).
+
+After the pivot to host-loopback port-forwards, the smolmachines
+TSI allowlist was `127.0.0.1/32` — which meant the agent VM could
+reach **any** service bound to macOS's loopback, not just the
+bundle's published ports. That's a real downgrade from the
+docker backend's `--internal` network isolation.
+
+This module narrows the allowlist by allocating each bottle a
+unique loopback alias (`127.0.0.16` .. `127.0.0.31` by default).
+The bundle's port-forwards bind to that alias, TSI's allowlist is
+the alias /32, and other host loopback services stay invisible to
+the bottle.
+
+macOS only configures `127.0.0.1` on `lo0` by default; the
+additional aliases require `sudo ifconfig lo0 alias`. We lazily
+sudo-add the missing pool on first use per boot — the aliases
+persist on `lo0` until reboot, so subsequent launches don't
+prompt.
+
+Linux native daemons share the host's network namespace; the
+whole `127.0.0.0/8` is reachable by default and aliases are
+unnecessary. The pool logic detects native-Linux and skips sudo
+entirely.
+
+Allocation is coordinated by inspecting running bundle
+containers' published host IPs — each bottle's bundle owns the
+alias appearing in its port bindings. The lowest-numbered free
+alias gets handed to a new bottle."""
+
+from __future__ import annotations
+
+import json
+import os
+import platform
+import re
+import subprocess
+from typing import Iterable
+
+from ...log import die, info
+
+
+# Sixteen aliases by default. Tunable for hosts that want more
+# concurrent bottles (each bottle reserves one alias for its
+# bundle bringup). The range is chosen to avoid the reserved
+# 127.0.0.1/2/3 ports (1 is the default, 2 is sometimes used by
+# CUPS, 3 by other macOS services) and stay well clear of
+# 127.0.0.53 (systemd-resolved) and 127.0.0.54 (libvirt).
+_POOL_START = 16
+_POOL_END = 31  # inclusive
+
+
+# Loopback aliases pool: 127.0.0.<start>..127.0.0.<end>.
+def _pool_addresses() -> list[str]:
+    return [f"127.0.0.{i}" for i in range(_POOL_START, _POOL_END + 1)]
+
+
+def _is_macos() -> bool:
+    return platform.system() == "Darwin"
+
+
+def ensure_pool() -> None:
+    """Make sure each address in the pool is up on `lo0`. Lazily
+    runs `sudo ifconfig lo0 alias <ip>/32 up` for missing entries
+    (sudo prompts once, then the aliases persist on lo0 until
+    reboot). No-op on non-macOS hosts."""
+    if not _is_macos():
+        return
+    missing = [ip for ip in _pool_addresses() if not _alias_present(ip)]
+    if not missing:
+        return
+    info(
+        f"smolmachines needs {len(missing)} loopback alias(es) on lo0 "
+        f"({', '.join(missing[:3])}{', ...' if len(missing) > 3 else ''}) "
+        f"to scope per-bottle TSI allowlists. sudo will prompt once; "
+        f"aliases persist until reboot."
+    )
+    for ip in missing:
+        result = subprocess.run(
+            ["sudo", "-p", "claude-bottle (loopback alias): ",
+             "ifconfig", "lo0", "alias", f"{ip}/32", "up"],
+            check=False,
+        )
+        if result.returncode != 0:
+            die(
+                f"sudo ifconfig lo0 alias {ip} failed (exit "
+                f"{result.returncode}). Re-run with sudo available, "
+                f"or add manually: sudo ifconfig lo0 alias {ip}/32 up"
+            )
+
+
+def allocate(slug: str) -> str:
+    """Pick the lowest-numbered alias from the pool not already
+    in use by a running smolmachines bundle. Bails when the pool
+    is exhausted — the caller should report the limit to the
+    operator. `slug` is logged for traceability; not otherwise
+    used (no on-disk reservation, allocation is purely
+    docker-state-driven).
+
+    On non-macOS the whole `127.0.0.0/8` is loopback by default;
+    `127.0.0.1` is fine to share and we skip the alias dance.
+    This still returns a deterministic address so launch.py's
+    callers don't have to branch on platform."""
+    if not _is_macos():
+        return "127.0.0.1"
+    in_use = _aliases_in_use()
+    for ip in _pool_addresses():
+        if ip not in in_use:
+            return ip
+    die(
+        f"smolmachines loopback alias pool exhausted "
+        f"({_POOL_END - _POOL_START + 1} aliases, all in use). "
+        f"Stop a running bottle (`smolvm machine ls --json`) or "
+        f"raise _POOL_END in loopback_alias.py."
+    )
+    return ""  # unreachable; die() never returns
+
+
+def _alias_present(ip: str) -> bool:
+    """True iff `ifconfig lo0` shows `<ip>` as an inet address.
+    Exact-match — `127.0.0.1` shouldn't match `127.0.0.16`."""
+    result = subprocess.run(
+        ["/sbin/ifconfig", "lo0"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return False
+    pattern = re.compile(rf"\binet {re.escape(ip)}\b")
+    return bool(pattern.search(result.stdout or ""))
+
+
+def _aliases_in_use() -> set[str]:
+    """Aliases already bound by another smolmachines bundle's
+    published-port mappings. We inspect every container whose
+    name matches the smolmachines bundle prefix and pull the
+    `HostIp` out of its port bindings."""
+    result = subprocess.run(
+        ["docker", "ps", "--format", "{{.Names}}",
+         "--filter", "name=claude-bottle-sidecars-"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return set()
+    names = [n.strip() for n in (result.stdout or "").splitlines() if n.strip()]
+    in_use: set[str] = set()
+    for name in names:
+        in_use.update(_host_ips_for_container(name))
+    return in_use
+
+
+def _host_ips_for_container(name: str) -> Iterable[str]:
+    """Yield the `HostIp` values across all port bindings on
+    container `name`. A bundle binds three or four ports and
+    they all share the same HostIp, so callers can take any."""
+    result = subprocess.run(
+        ["docker", "inspect", name,
+         "--format", "{{json .HostConfig.PortBindings}}"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return ()
+    try:
+        bindings = json.loads(result.stdout or "{}")
+    except json.JSONDecodeError:
+        return ()
+    seen: set[str] = set()
+    for _port, mappings in (bindings or {}).items():
+        for m in mappings or []:
+            host_ip = m.get("HostIp") or ""
+            if host_ip:
+                seen.add(host_ip)
+    return seen
+
+
+__all__ = ["allocate", "ensure_pool"]