network isolation

[jeckersb]

• feat: Add --network-isolation CLI flag (no-op)
  Add the --network-isolation flag to both the ephemeral (CommonVmOpts)
  and libvirt (LibvirtRunOpts) CLI option structs. The flag is accepted
  by the CLI parser but has no effect yet; the actual QEMU restrict=on
  wiring follows in subsequent commits.

  This allows the integration test to be written against the flag
  immediately, following TDD practice.

  Closes: Add network isolation support #304

  Assisted-by: AI
  Signed-off-by: John Eckersberg jeckersb@redhat.com

• test: Add integration tests for network isolation
  Add two tests for --network-isolation support:

  • test_run_ephemeral_network_reachable: positive control that boots a
    normal VM and verifies it can ping 8.8.8.8 from inside the guest.
    This proves run-ssh, guest networking, and external reachability
    all work. Without this, the isolation test could pass vacuously
    when SSH or networking is broken for unrelated reasons.

  • test_run_ephemeral_network_isolation: boots a VM with
    --network-isolation and verifies that pinging 8.8.8.8 from inside
    the guest fails. Includes the same positive control as a first step
    to guard against false passes. The fact that run-ssh itself succeeds
    proves SSH (host-to-guest via hostfwd) is preserved.

  Both tests skip gracefully with a loud warning when the host cannot
  reach 8.8.8.8 (e.g. offline environments).

  The isolation test is expected to fail against the current no-op flag
  (the positive control succeeds, then the isolation check fails because
  the guest can still reach the internet). This establishes the TDD
  baseline that the implementation commits will fix.

  Assisted-by: AI
  Signed-off-by: John Eckersberg jeckersb@redhat.com

• feat(qemu): Add restrict support to NetworkMode::User
  Add a restrict field to NetworkMode::User that maps to QEMU's
  slirp restrict=on option. When enabled, the guest cannot initiate
  outbound connections except through explicit hostfwd rules (e.g. SSH
  port forwarding). This is the mechanism that --network-isolation will
  use.

  • Add restrict: bool to NetworkMode::User, defaulting to false
  • Add set_network_restrict() builder method on QemuConfig
  • Update enable_ssh_access() to preserve the restrict setting
  • Emit restrict=on in the -netdev argument when enabled
  • Add unit tests for the new functionality

  Assisted-by: AI
  Signed-off-by: John Eckersberg jeckersb@redhat.com

• feat: Wire --network-isolation to QEMU restrict for ephemeral VMs
  When --network-isolation is passed to bcvk ephemeral run or
  bcvk ephemeral run-ssh, set restrict=on on the QEMU slirp netdev.
  This blocks all guest-initiated outbound connections while preserving
  SSH access via hostfwd.

  DNS server injection is also skipped under isolation since external
  DNS servers are unreachable and the entries would only cause timeouts.

  Assisted-by: AI
  Signed-off-by: John Eckersberg jeckersb@redhat.com

• feat: Wire --network-isolation to QEMU restrict for libvirt VMs
  When --network-isolation is passed to bcvk libvirt run, add
  restrict=on to the QEMU user-mode netdev arguments injected via
  qemu:commandline. This blocks all guest-initiated outbound
  connections while preserving SSH access via hostfwd, matching the
  ephemeral path behavior.

  Assisted-by: AI
  Signed-off-by: John Eckersberg jeckersb@redhat.com

• docs: Sync manpages with --network-isolation option
  Run cargo xtask sync-manpages to regenerate the OPTIONS sections,
  picking up the new --network-isolation flag in:

  • bcvk-ephemeral-run(8)
  • bcvk-ephemeral-run-ssh(8)
  • bcvk-libvirt-run(8)
  • bcvk-to-disk(8)

  Also picks up a previously missing --virtiofsd option in
  bcvk-libvirt-run(8).

  Assisted-by: AI
  Signed-off-by: John Eckersberg jeckersb@redhat.com

• docs: Add network isolation examples and prose
  Add hand-written documentation for --network-isolation to:

  Manpages (EXAMPLES sections, outside auto-generated markers):

  • bcvk-ephemeral-run(8): network isolation example with bind-storage-ro
  • bcvk-ephemeral-run-ssh(8): quick CI testing example
  • bcvk-libvirt-run(8): hermetic testing example

  mdBook conceptual docs:

  • ephemeral-run.md: new Network Isolation section with example
  • libvirt-advanced.md: network isolation subsection under Network
    Configuration

  All hand-written sections are outside the markers and will be preserved by future sync-manpages
  runs.

  Assisted-by: AI
  Signed-off-by: John Eckersberg jeckersb@redhat.com

[jeckersb]

Throwing this up as draft before I disappear for a week. Few quick thoughts:

• I don't really love the way the tests do probing of 8.8.8.8 and then try to "fail loudly", some cursory AI probing suggests the test suite would be able to generate skips if the prereqs aren't met, that would be better but more involved (probably better as a prep PR)
• I did intentionally order the commits though so we could run the commit that adds the tests and see them fail before fast-forwarding to the rest of it.