[aremily]

Summary

Adds an opt-in alwaysUseSocketProxy option (default false) that always uses the Feature's
existing socat proxy instead of changing the GID of the container's docker group with
groupmod. Behavior is unchanged when the option is off. Version bump 1.10.1 -> 1.11.0.

Problem

When the host socket's GID is nonzero and not used by any group in the container, docker-init.sh
runs groupmod --gid <socket gid> docker. That updates /etc/group but not the supplementary
groups of processes that are already running, so they keep getting EACCES on the socket.

The dev container CLI (checked in 0.90.0) starts the container as:

/bin/sh -c 'echo Container started
trap "exit 0" 15
/usr/local/share/docker-init.sh      # Feature entrypoints, called with no arguments
exec "$@"                            # the image's entrypoint/cmd when overrideCommand is false
while sleep 1 & wait $!; do :; done' -

That wrapper shell starts before groupmod and keeps its old groups, as does anything it execs.
Processes started later (docker exec, terminals, lifecycle commands) get fresh groups and are not
affected. That is why tests that run in a new process do not catch this.

Observed here (host socket GID 997, container docker group 999):

• wrapper shell (PID 1) groups: 999 1000
• docker ps as a fresh process: works
• docker ps with exactly the wrapper's groups: permission denied while trying to connect to the docker API at unix:///var/run/docker.sock

Change

• devcontainer-feature.json: new alwaysUseSocketProxy boolean, version 1.11.0.
• install.sh: reads the option and adds it to the condition that selects the groupmod branch.
  With it set, the existing proxy branch runs. The proxy socket is owned by the user, so no group
  membership is needed.
• README.md / NOTES.md: option documented, including the docker-outside-of-docker: socat breaks interactive support #483 limitation (see below).
• Test scenario socket_proxy_always (see below).

Why opt-in

The default (auto) stays as it is: where groupmod works, /var/run/docker.sock is the host's own
socket, with no extra process or hop. The proxy replaces it with a separate socket owned by the user
(mode 660), adds a long-lived socat process, and has its own failure modes, so existing users should
not get it unless they ask. This option changes nothing for anyone who does not set it.

The proxy also has a known defect today: socat drops output that arrives after the client stops
sending (#483; reproduced: echo "sleep 2; uname; exit 2" | docker run -i ubuntu bash prints nothing
through the proxy). That affects the existing fallback too, and it is fixed in the related PR below.

Related PR

#1762, "[docker-outside-of-docker] Fix socat proxy dropping delayed output" (fixes #483), is stacked on this
branch. The two address different problems and either is useful without the other. #1762 is stacked
because its regression test needs the proxy to be in use, and this option's scenario
(socket_proxy_always) is the only deterministic way to do that. Until #1762 lands, NOTES.md here
mentions #483; #1762 removes that note. Both bump the Feature version (1.11.0 here, 1.11.1 in #1762), so
expect a small rebase if one merges first.

Testing

devcontainer features test -f docker-outside-of-docker locally (arm64 host, Docker 29.9.2, CLI 0.90.0):
all autogenerated tests and all scenarios pass except docker_dash_compose_v1 and
docker_dash_compose_v1_moby, which fail with error: externally-managed-environment (pip) and fail
identically on unmodified main.

New scenario socket_proxy_always (mcr.microsoft.com/devcontainers/base:ubuntu-24.04,
containerUser: vscode, alwaysUseSocketProxy: true). It has two parts.

1. Stale groups. A fresh process hides the bug, so the script takes the groups of the container's own
   command (/proc/<pid>/status) and runs docker ps through sudo setpriv with exactly those groups.
   It also checks that the socket is a real socket, socat runs with the expected arguments, the log says
   "Enabling socket proxy", and a plain docker ps works.
2. Proxy selection. Whether the host socket's GID would pick groupmod depends on the host (GID 0 or a GID
   already in the image picks the proxy anyway), so the script builds a fixture: a socket owned by an unused
   GID (4242) that forwards to the real socket. The host socket is not touched. It runs a path-rewritten
   copy of the generated docker-init.sh against it. With the option on, the proxy starts, the docker
   group's GID is unchanged, the proxy socket is owned by the user, and docker ps works through it. A
   control run with the condition flipped to false takes the groupmod path (the group's GID becomes 4242)
   and does not start the proxy.

I checked that the scenario fails when the option's condition is removed from install.sh, including with
the checks from part 1 disabled, so part 2 catches it on its own. The test matches the literal
[ "true" != "true" ] in the generated script; a change to that line makes the
control-condition-present-in-generated-script check fail rather than pass silently.

Not tested: other base images in test-pr.yaml, x86_64 hosts, and an end-to-end overrideCommand: false
(Compose) container. The latter is expected to hit the issue from reading the CLI source.

Related

• docker-outside-of-docker: processes started before the entrypoint's groupmod keep stale groups and cannot use the socket #1760: the stale-groups problem this addresses, and the questions about the best fix.
• docker-outside-of-docker: socat breaks interactive support #483 socat breaks interactive support (open; fixed by the related PR above).
• fix(docker-outside-of-docker): check DOCKER_GID on container start #1749 (open) changes the same branch logic (reads DOCKER_GID at runtime, uses getent) and bumps
  the version to 1.10.2, so a rebase and a version change are possible.
• Docker-outside-of-docker does not work when running docker as a non-root user. #1005 (open) reports docker permission denied when host and container docker GIDs differ; it may be
  related, but I did not verify the mechanism there.
• [docker-outside-of-docker] Change owner of source socket if non-root access is enabled #402 (closed) discussion of why the proxy exists.

[aremily]

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[Copilot]

🟡 Changes recommended

The new scenario can pass even if the proxy-option condition is removed, so a deterministic unused, nonzero socket GID fixture is needed to verify the intended behavior.

2 open issues

• Use an unused socket GID to verify proxy selection · New
• Document delayed output after client write-side close · New

What changed in this PR

Adds an opt-in alwaysUseSocketProxy option to avoid Docker socket permission failures in processes that retain their original supplementary groups. Default behavior remains unchanged.

Changes:

• Adds the boolean option and bumps the Feature version to 1.11.0.
• Routes enabled configurations through the existing socat proxy and documents its limitations.
• Adds a scenario checking proxy operation and Docker access with the container command’s group list.

File	Description
test/​docker-outside-of-docker/​socket_proxy_always.sh	Checks proxy setup and socket access with startup groups.
test/​docker-outside-of-docker/​scenarios.json	Adds a scenario enabling the proxy option.
src/​docker-outside-of-docker/​README.md	Lists the new option and its default.
src/​docker-outside-of-docker/​NOTES.md	Explains stale-group permissions and the proxy limitation.
src/​docker-outside-of-docker/​install.sh	Bypasses groupmod when the option is enabled.
src/​docker-outside-of-docker/​devcontainer-feature.json	Declares the option and updates the version.

_{Review effort: Balanced}

[aremily]

Agreed, thanks. Fixed in cc68071.

socket_proxy_always now builds a fixture instead of depending on the host socket's GID: a socket owned by
an unused, nonzero GID (4242) that forwards to the real socket. The host socket is not touched. The script
runs a path-rewritten copy of the generated docker-init.sh against it. A fixture-socket-created check
makes the test fail with a clear message if the fixture socket does not appear.

• Option on: the proxy starts ("Enabling socket proxy" in the log), the docker group's GID is unchanged,
  the proxy socket is owned by the user, and docker ps works through it.
• Control, condition flipped to false: the same fixture takes the groupmod path (the group's GID becomes
  4242) and the proxy does not start. This shows the fixture really would select groupmod.

I also checked that the scenario fails when the option's condition is removed from install.sh, including
with the older checks disabled, so the new checks catch it on their own.

One caveat: the test matches the literal [ "true" != "true" ] in the generated script to flip the
condition. If that line changes (for example #1749 rewrites the if), the
control-condition-present-in-generated-script check fails instead of passing silently, and the test needs
updating.

[aremily]

Agreed, you are right that it is not about pauses in input. Fixed in cc68071.

The note now says that the Docker CLI closes its write side right after sending a request, that socat
then waits only 0.5 s before dropping the connection, and that this affects docker run and docker exec
with or without --interactive (with the sleep 5; date example). It also says the same applies when the
Feature falls back to the proxy on its own, so users can weigh it before enabling the option.

While in that section I also added one sentence saying the option has no effect when the remote user is
root, because root can use the socket directly and only a stub entrypoint is installed.

#1762 fixes the cause with socat -t 31536000 and removes this note, so the wording here only needs to be
correct until that lands.