Verified bubblewrap sandbox recipes and gotchas

For the BYOA harness sandbox (bubblewrap 0.13.1 at /usr/bin/bwrap, unprivileged userns enabled, uid 1000) these recipes were executed and verified:

Base: bwrap —ro-bind / / —dev /dev —proc /proc —tmpfs /tmp —bind “$WORK""$WORK” —chdir “$WORK"--die-with-parent--new-session--setenvHOME"$WORK” /bin/sh -c ’…’

Gotchas found by testing, not docs:

1. With —ro-bind / /, any bind whose DESTINATION does not exist fails hard: “bwrap: Can’t create file /mnt/extra: Read-only file system” — and —dir does NOT fix it. Fixes that work: put —tmpfs on the parent first (—tmpfs /mnt then —ro-bind SRC /mnt/extra), pre-create the dir on the host, or skip —ro-bind / / and build a skeleton root (/usr /etc /bin /lib /lib64 + —dir /opt).
2. —unshare-all gives a netns where lo is ALREADY up with 127.0.0.1/8, so local bind/connect works and outbound is dead. Do not try “ip link set lo up” — it fails with RTNETLINK Operation not permitted even at uid 0 inside the userns.
3. —unshare-all —share-net exposes the HOST netns (reaches host 127.0.0.1 but also LAN/internet). There is no “host loopback only” flag. To reach exactly one service, keep —unshare-all and bind only that service’s unix socket directory — AF_UNIX works across net isolation.
4. Missing —proc /proc breaks tools that read /proc/self/*. Missing /etc/resolv.conf breaks DNS only when /etc is shadowed; re-bind resolv.conf + hosts to restore.
5. uid mapping: —unshare-all alone still shows uid=1000; —unshare-user —uid 0 —gid 0 gives uid=0 inside.

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