feat(sidecars): egress binds 127.0.0.1 when EGRESS_LISTEN_HOST is set (PRD 0023 chunk 3)

Egress's bind address is now env-driven via EGRESS_LISTEN_HOST.
Unset → mitmdump's default (all interfaces) — the docker
backend's behavior, unchanged. Set to `127.0.0.1` → mitmdump
binds localhost only.

The smolmachines launch sets EGRESS_LISTEN_HOST=127.0.0.1 in
the bundle's env unconditionally. TSI's allowlist is
`<bundle-ip>/32` (IP-only, not port-granular), which would
otherwise let the agent dial `<bundle-ip>:9099` and bypass
pipelock's DLP by talking to egress directly. Binding egress
to localhost inside the bundle closes that gap at the socket
level — the agent still reaches the IP (TSI permits it) but
egress refuses the connect because it's not listening on the
docker bridge interface.

The docker backend doesn't set the env var because its agent
dials egress directly via the docker network alias — egress
MUST be reachable from outside the bundle there. The
asymmetry is documented in the entrypoint script's comment.

Changes:
- egress_entrypoint.sh: read EGRESS_LISTEN_HOST, conditionally
  pass `--listen-host <host>` to mitmdump.
- smolmachines/launch.py: BundleLaunchSpec.environment now
  includes `EGRESS_LISTEN_HOST=127.0.0.1`.
- New unit tests (5): the entrypoint script's argv shape under
  various env combinations, verified via a fake mitmdump shim
  that prints its argv.

545 unit + 3 integration tests passing. The egress-port-bypass
probe from chunk 2d still passes (chunk 2d ran with daemons_csv=""
so no egress was up; chunk 3 makes the probe preserve its
property once egress IS up in chunk 4).

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

claude_bottle/backend/smolmachines/launch.py

@@ -48,6 +48,15 @@ def launch(
             # bringup with inner-Plan-driven env + volumes lands
             # in chunk 4 alongside provisioning.
             daemons_csv="",
+            # PRD 0023 chunk 3: pin egress to localhost INSIDE the
+            # bundle so the agent's TSI-permitted `<bundle-ip>:*`
+            # connect to :9099 refuses at the socket level. Always
+            # set in smolmachines mode — agent dials pipelock, not
+            # egress, so egress is bundle-internal regardless of
+            # whether routes are declared. The docker backend
+            # doesn't set this env (egress on 0.0.0.0 by default)
+            # since the docker agent goes via the egress alias.
+            environment=("EGRESS_LISTEN_HOST=127.0.0.1",),
         )
         _bundle.start_bundle(bundle_spec)
         stack.callback(_bundle.stop_bundle, plan.slug)

claude_bottle/egress_entrypoint.sh

@@ -36,6 +36,18 @@ if [ -n "$EGRESS_UPSTREAM_PROXY" ]; then
   MODE="--mode upstream:$EGRESS_UPSTREAM_PROXY --listen-port 9099"
 fi
 
+# Bind address. Docker backend wants `0.0.0.0` (agent dials egress
+# directly via the docker network alias). Smolmachines backend
+# wants `127.0.0.1` because the agent dials pipelock — not egress
+# — and egress is pipelock's localhost-only upstream inside the
+# bundle. TSI's IP-only allowlist would otherwise let the agent
+# reach `<bundle-ip>:9099` and bypass pipelock's DLP; binding
+# 127.0.0.1 inside the bundle closes that gap (PRD 0023 chunk 3).
+LISTEN_HOST_FLAG=""
+if [ -n "$EGRESS_LISTEN_HOST" ]; then
+  LISTEN_HOST_FLAG="--listen-host $EGRESS_LISTEN_HOST"
+fi
+
 TRUST_FLAG=""
 if [ -n "$EGRESS_UPSTREAM_CA" ] && [ -f "$EGRESS_UPSTREAM_CA" ]; then
   COMBINED=$CONFDIR/combined-trust.pem
@@ -57,4 +69,4 @@ if [ -n "$EGRESS_UPSTREAM_PROXY" ]; then
   export NO_PROXY="localhost,127.0.0.1"
 fi
 
-exec mitmdump $CONFDIR_FLAG $MODE $TRUST_FLAG -s /app/egress_addon.py
+exec mitmdump $CONFDIR_FLAG $MODE $LISTEN_HOST_FLAG $TRUST_FLAG -s /app/egress_addon.py

tests/unit/test_egress_entrypoint.py

@@ -0,0 +1,98 @@
+"""Unit: egress_entrypoint.sh argv construction (PRD 0023 chunk 3).
+
+The egress entrypoint is a small POSIX-sh script that builds
+the mitmdump argv from env vars. The smolmachines backend
+controls egress's bind address via EGRESS_LISTEN_HOST; the
+docker backend leaves it unset and gets mitmdump's default
+(all interfaces).
+
+We can't easily unit-test a shell script as Python, but we can
+run it under `sh -x` with mitmdump stubbed to print its argv,
+which is exactly what these tests do."""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import tempfile
+import unittest
+from pathlib import Path
+
+
+_SCRIPT = (
+    Path(__file__).resolve().parent.parent.parent
+    / "claude_bottle" / "egress_entrypoint.sh"
+)
+
+
+def _run_entrypoint(env: dict[str, str]) -> str:
+    """Run egress_entrypoint.sh with a stubbed mitmdump that
+    prints its argv. Returns the argv as a single string.
+
+    The script uses `exec mitmdump ...`. We shim by prepending a
+    fake `mitmdump` to PATH; the shim writes its argv to stdout
+    and exits 0."""
+    with tempfile.TemporaryDirectory() as tmp:
+        shim_dir = Path(tmp)
+        shim = shim_dir / "mitmdump"
+        shim.write_text(
+            "#!/bin/sh\n"
+            'printf "%s\\n" "$@"\n'
+        )
+        shim.chmod(0o755)
+        run_env = {
+            "PATH": f"{shim_dir}:{os.environ['PATH']}",
+            # cat needs to find ca-certificates.crt for the
+            # trust-bundle branch; we don't test that path here.
+            **env,
+        }
+        result = subprocess.run(
+            ["sh", str(_SCRIPT)],
+            capture_output=True, text=True, env=run_env,
+            check=True,
+        )
+        return result.stdout
+
+
+class TestEgressEntrypointArgv(unittest.TestCase):
+    def test_default_mode_regular_no_listen_host(self):
+        # No env set: --mode regular@9099 + no --listen-host.
+        argv = _run_entrypoint({})
+        self.assertIn("--mode\nregular@9099", argv)
+        self.assertNotIn("--listen-host", argv)
+        # Confdir always present.
+        self.assertIn(
+            "--set\nconfdir=/home/mitmproxy/.mitmproxy",
+            argv,
+        )
+
+    def test_listen_host_127_0_0_1_emits_flag(self):
+        # smolmachines backend sets EGRESS_LISTEN_HOST=127.0.0.1
+        # to scope egress to localhost inside the bundle.
+        argv = _run_entrypoint({"EGRESS_LISTEN_HOST": "127.0.0.1"})
+        self.assertIn("--listen-host\n127.0.0.1", argv)
+
+    def test_listen_host_unset_emits_no_flag(self):
+        # Docker backend leaves it unset and gets mitmdump's
+        # default bind address (all interfaces).
+        argv = _run_entrypoint({"EGRESS_LISTEN_HOST": ""})
+        self.assertNotIn("--listen-host", argv)
+
+    def test_upstream_mode_combined_with_listen_host(self):
+        # smolmachines mode also sets EGRESS_UPSTREAM_PROXY so
+        # both flags should compose correctly.
+        argv = _run_entrypoint({
+            "EGRESS_UPSTREAM_PROXY": "http://192.168.50.2:8888",
+            "EGRESS_LISTEN_HOST": "127.0.0.1",
+        })
+        self.assertIn("--mode\nupstream:http://192.168.50.2:8888", argv)
+        self.assertIn("--listen-port\n9099", argv)
+        self.assertIn("--listen-host\n127.0.0.1", argv)
+
+    def test_addon_always_loaded(self):
+        argv = _run_entrypoint({})
+        self.assertIn("-s\n/app/egress_addon.py", argv)
+
+
+if __name__ == "__main__":
+    unittest.main()