[WSLC] Proxy Pt. 2: Enable session host loopback by theelliotm · Pull Request #1714 · microsoft/mxc

Elliot (theelliotm) · 2026-11-09T23:02:23Z

📖 Description

Second PR in the WSLC built-in proxy stack (#1625). Turns on WSL host loopback, so hostLoopback: allow reaches Windows 127.0.0.1. Before this, it was accepted but had no effect.

Every session MXC creates (create-and-run and the lifecycle daemon) sets the host loopback hostname host.wslc.internal, which WSL's CLI also uses.
On WSL older than 3.0.5, session creation returns WSLC_E_RUNTIME_UPDATE_NEEDED; MXC fails with the "WSLC runtime unavailable" error telling the user to install WSL 3.0.5 or newer.
Isolated (NONE) containers in a session with host loopback still can't reach Windows loopback, by name or by the relay address.

Behavior change: existing all-allow configs gain outbound access to every Windows loopback TCP port. There's no per-port filter; that comes with the controller later in the stack.

🔗 References

Refs #1462
Spec: #1625
Stacked on #1679

🔍 Validation

On ARM64 with WSL 3.0.5.0 and WSLC_SDK_PATH set to the local 3.0.5 SDK build:

cargo build -p wxc --features wslc --release and cargo build -p mxc-sdk --bin wxc-wslc-daemon --features wslc --release: passed
cargo fmt --all -- --check: passed
cargo clippy -p mxc-sdk --features wslc --all-targets -- -D warnings: passed
cargo test -p mxc-sdk --features wslc --lib wslc: passed (434 passed, 7 ignored)
tests/scripts/run_wslc_host_loopback_test.ps1 (new): passed. A bridged container gets a per-run token from a listener bound to Windows 127.0.0.1; an isolated container doesn't get it through host.wslc.internal or 169.254.73.254.
tests/scripts/run_wslc_state_aware_tests.ps1: passed (142/142), including new checks that, with both running in one verified session, a bridged sandbox reaches the listener and an isolated peer doesn't.

Not run: x64, the full run_wslc_all_tests.ps1 after the final test changes, and WSL older than 3.0.5.

✅ Checklist

Signed the Contributor License Agreement
Linked to an issue
For features or changes requiring a spec: linked the issue with maintainer-applied feature-accepted
For implementation: linked the required approved and merged design/API spec, or explained why no spec is required
Feature issue remains open unless this PR completes all agreed implementation, validation, and documentation
Updated documentation (if applicable)
Updated Copilot instructions (if build, architecture, or conventions changed)
If this PR changes Cargo.lock, the dependency-feed-check check passes (see pull request builds)

📋 Issue Type

Bug fix
Feature
Task

azure-pipelines · 2026-11-09T23:02:34Z

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Copilot

🟡 Changes recommended

The tests can miss shared-session relay exposure, incorrectly fail supported older WSL runtimes, and lack passing regression coverage for the non-loopback retry.

3 open issues

What changed in this PR

This PR adds Windows host-loopback access to WSLC workload sessions, advancing the built-in proxy stack. Bridged containers can reach Windows loopback services through host.wslc.internal on WSL 3.0.5+.

Changes:

Shares session-creation logic across create-and-run execution and the lifecycle daemon, with a fallback for older runtimes.
Adds token-based loopback reachability and isolation tests.
Documents runtime requirements and the expanded access granted to existing all-allow configurations.

File	Description
`tests/scripts/wslc_loopback_token_server.ps1`	Provides a Windows loopback listener with a per-run token.
`tests/scripts/run_wslc_state_aware_tests.ps1`	Adds lifecycle loopback reachability and isolation checks.
`tests/scripts/run_wslc_host_loopback_test.ps1`	Drives create-and-run loopback tests.
`tests/scripts/run_wslc_all_tests.ps1`	Includes loopback tests in the full WSLC suite.
`tests/configs/wslc_network_host_loopback.json`	Defines the bridged loopback probe.
`tests/configs/wslc_network_host_loopback_isolated.json`	Defines isolated probes by hostname and relay address.
`src/mxc-sdk/src/backends/wslc/common/wslc_bindings.rs`	Loads the loopback-setting API and defines the runtime-update error.
`src/mxc-sdk/src/backends/wslc/common/wsl_container_runner.rs`	Uses shared session creation for create-and-run execution.
`src/mxc-sdk/src/backends/wslc/common/container_steps.rs`	Enables session loopback and retries without it on older runtimes.
`docs/backends/wslc/wslc-state-aware.md`	Documents loopback access through daemon sessions.
`docs/backends/wslc/wsl-container-getting-started.md`	Explains prerequisites, usage, and networking behavior changes.

_{Review effort: Balanced}

Drop the pre-3.0.5 fallback; an older runtime now fails as WSLC runtime unavailable. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: c3f64c2c-9005-43eb-a803-a91fb7265c5b

Copilot

🔵 Needs a closer look

The isolation test can pass without checking containment alongside a live bridged peer in the same session because it tears down the bridged sandbox before probing its saved relay address.

1 open issue

Keep the bridged sandbox alive during shared-session probes

2 resolved since last review

_{Review effort: Balanced}

Run the host loopback checks in one shared session under one listener, so the negative probe cannot pass against a reaped session or a dead listener. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: c3f64c2c-9005-43eb-a803-a91fb7265c5b

Copilot

🔵 Needs a closer look

Maintainer confirmation is still needed for expanded Windows loopback access, with the design PR unmerged and x64 and older-runtime error-path validation outstanding.

No open issues 🎉

1 resolved since last review

Keep the bridged sandbox alive during shared-session probes

_{Review effort: Balanced}

Soham Das (SohamDas2021) · 2026-11-11T23:26:48Z

-            let msg = err_msg.to_string_lossy();
-            return Err(sdk_error("WslcCreateSession failed", hr, &msg));
-        }
+        let session = container_steps::create_session(sdk, &mut settings)?;


On x64/WSL 3.0.5 a bridged create-and-run sandbox fetched another sandbox's port mapping via host.wslc.internal:<windowsPort>: MXC passes a null windowsAddress, so WSL publishes every mapping on Windows 127.0.0.1, and this relay opens all of them to every bridged sandbox.

Don't enable session host loopback until per-container/per-port enforcement exists, and add a two-sandbox negative test to run_wslc_host_loopback_test.ps1.

Wow. Investigating.

Soham Das (SohamDas2021) · 2026-11-11T23:30:10Z

-    let mut err_msg = CoTaskMemPWSTR::null();
    let started = Instant::now();
-    let hr = sdk.WslcCreateSession(&mut settings, &mut session, err_msg.as_mut_ptr());
+    let session = create_session(sdk, &mut settings)?;


The daemon now enables host loopback, but PROTOCOL_VERSION stays 9 and ready_daemon() reuses any ready v9 daemon, so after an upgrade a still-running pre-PR daemon (alive while it holds sandboxes) keeps creating sessions without host.wslc.internal, and a new daemon gives old clients the new access.

Shall we bump the protocol version?

Soham Das (SohamDas2021) · 2026-11-11T23:31:25Z

+/// Create a session with host loopback enabled. A WSL runtime older than 3.0.5
+/// rejects the setting.


Isolated-only sessions (every deny-all create-and-run, where network_is_isolated(request) is already known) gain nothing from host loopback, yet setting it forces WSL's new session path, so they now fail below WSL 3.0.5 while available_backends() still reports WSLC available; #1625 README.md:80-81 and 407-410 say MXC sets it only on 3.0.5+ and NONE/BRIDGED keep working on older runtimes.

Set the hostname only for sessions that can hold a bridged container (pass !network_is_isolated(request) from the runner), or update #1625 and make discovery report the 3.0.5 requirement.

3.0.5 is now the minimum version. I will see if I can enforce that.

Soham Das (SohamDas2021) · 2026-11-11T23:32:04Z

+    let mut err_msg = CoTaskMemPWSTR::null();
+    let hr = sdk.WslcCreateSession(settings, &mut session, err_msg.as_mut_ptr());
+    if hr == WSLC_E_RUNTIME_UPDATE_NEEDED {
+        return Err(WslcError::Unavailable(sdk_init::prerequisite_error(


In the daemon this Unavailable surfaces on the boot thread, and finish_session rewraps it as WorkerError::Backend for every provision parked on the implicit session, so a lifecycle provision on WSL < 3.0.5 gets backend_error instead of the backend_unavailable documented at wslc-state-aware.md:701.
Forward Unavailable/Rejected unchanged to parked replies and add a session_manager test for a boot that fails with Unavailable.

3.0.5 is now the minimum version. I will see if I can enforce that.

Soham Das (SohamDas2021) · 2026-11-11T23:32:42Z

+# HTTP listener bound only to Windows 127.0.0.1 that answers every request with
+# a per-run token. Dot-source this file, then pair Start- with Stop-.
+
+function Start-LoopbackTokenServer {


This duplicates tests/scripts/lib/LoopbackAnchor.ps1, whose Start-LoopbackAnchor -Body already binds an ephemeral 127.0.0.1 port and serves a fixed body from a background runspace.
Use Start-LoopbackAnchor -Body ("HOST_LOOPBACK_" + [guid]::NewGuid().ToString("N")) / Stop-LoopbackAnchor and drop this file?

Elliot (theelliotm) added this pull request to stack #1715 November 9, 2026 23:02

Elliot (theelliotm) changed the title ~~enable hostLoopback on WSL 3.0.5~~ [WSLC] Proxy Pt. 2: Enable session host loopback Nov 9, 2026

Elliot (theelliotm) mentioned this pull request Nov 10, 2026

[WSLC] Proxy Pt. 3: Add runtimeConfig to WSLC provision contract #1717

Open

5 of 11 tasks

Elliot (theelliotm) force-pushed the user/emichlin/wslc-proxy-2-host-loopback branch from 7cbd0d1 to 30303eb Compare November 10, 2026 22:17

Copilot AI balanced review requested due to automatic review settings November 10, 2026 22:17

Copilot started reviewing on behalf of Elliot (theelliotm) November 10, 2026 22:18

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Comment thread src/mxc-sdk/src/backends/wslc/common/container_steps.rs Resolved

Comment thread tests/scripts/run_wslc_host_loopback_test.ps1 Resolved

Comment thread tests/scripts/run_wslc_state_aware_tests.ps1 Resolved

Elliot (theelliotm) and others added 2 commits November 10, 2026 16:53

enable hostLoopback on WSL 3.0.5

86227d2

WSLC: require WSL 3.0.5 for host loopback

727d749

Drop the pre-3.0.5 fallback; an older runtime now fails as WSLC runtime unavailable. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: c3f64c2c-9005-43eb-a803-a91fb7265c5b

Elliot (theelliotm) force-pushed the user/emichlin/wslc-proxy-2-host-loopback branch from 30303eb to 727d749 Compare November 11, 2026 20:14

Copilot AI balanced review requested due to automatic review settings November 11, 2026 20:14

Copilot started reviewing on behalf of Elliot (theelliotm) November 11, 2026 20:15

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Copilot AI reviewed Nov 11, 2026

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Copilot AI balanced review requested due to automatic review settings November 11, 2026 22:12

Copilot started reviewing on behalf of Elliot (theelliotm) November 11, 2026 22:12

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Elliot (theelliotm) marked this pull request as ready for review November 11, 2026 22:16

Elliot (theelliotm) requested a review from a team as a code owner November 11, 2026 22:16

Elliot (theelliotm) requested a review from Soham Das (SohamDas2021) November 11, 2026 22:17

Soham Das (SohamDas2021) requested changes Nov 11, 2026

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microsoft-github-policy-service Bot assigned Elliot (theelliotm) Nov 11, 2026

		/// Create a session with host loopback enabled. A WSL runtime older than 3.0.5
		/// rejects the setting.

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Elliot (theelliotm) commented Nov 9, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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📖 Description

🔗 References

🔍 Validation

✅ Checklist

📋 Issue Type

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azure-pipelines Bot commented Nov 9, 2026

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🟡 Changes recommended

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Elliot (theelliotm) commented Nov 9, 2026 •

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