[Bug]: High VBoxHeadless CPU on Intel Mac when guest uses NAT; guest idle, CPU drops after switching to host-only only

[cliffbdf]

Version

7.1.4

Host OS Type

macOS

Host OS name + version

Mac OS 14.6.1

Host Architecture

x86

Guest OS Type

Linux

Guest Architecture

x86

Guest OS name + version

Rocky Linux 9.5 x86_64

Component

Unspecified

What happened?

Summary

On an Intel Mac host, VirtualBox NAT causes VBoxHeadless to consume high CPU even when the Linux guest is mostly idle.

The guest itself is not busy. The problem disappears when NAT is removed and the VM is run with host-only networking only.

This appears to be a host-side VirtualBox NAT issue, possibly related to IPv6/router advertisement behavior on the NAT interface.

Host

• Host OS: macOS on Intel Mac
• Host hardware: Intel MacBook Pro
• VirtualBox version: 7.1.4 r165100
• Frontend: VBoxHeadless

Guest

• Guest OS: Rocky Linux 9.5 x86_64
• Guest type: Red Hat / Linux 64-bit
• Guest Additions: 7.1.4 r165100

Symptoms

With NAT enabled:

• VBoxHeadless on the Mac consumes high CPU while the guest is nearly idle.
• Observed host CPU for VBoxHeadless was roughly in the 70% to 245% range.
• Inside the guest, load was low and top processes were near idle.
• The guest saw IPv6 router advertisements / default route behavior on the NAT NIC.
• Disabling IPv6 inside the guest did not solve the underlying host CPU problem cleanly.

With NAT removed:

• The same VM, same workload, and same host no longer shows high VBoxHeadless CPU.
• CPU usage drops back to normal when the VM uses host-only networking only.

Evidence

When the VM used NAT:

• Guest IPv4 on NAT NIC looked like 10.0.2.15/24
• Guest also saw IPv6 on that NIC, including:
  • link-local address
  • ULA-style address (fd00::...)
  • default IPv6 route via fe80::2
• Guest load remained low while host VBoxHeadless stayed hot

Example guest state before removing NAT:

eth0  10.0.2.15/24  fd00::.../64  fe80::.../64
default via 10.0.2.2
default via fe80::2

Example host-side behavior:

VBoxHeadless high CPU while guest load average remains low

When reconfigured to host-only only:

- NAT removed
- host-only IP used instead
- high host CPU issue disappeared


### How can we reproduce this?

[VBox.log](https://github.com/user-attachments/files/30796554/VBox.log)

## Reproduction

1. Create or boot a Linux guest on an Intel Mac host with VirtualBox NAT enabled.
2. Let the guest sit mostly idle.
3. Observe VBoxHeadless CPU on the host.
4. Compare with guest CPU/load inside the VM.
5. Reconfigure the VM to use host-only networking only, with NAT removed.
6. Boot the guest again and compare host CPU.

## Expected behavior

- An idle or nearly idle guest should not cause sustained high VBoxHeadless CPU on the host merely because NAT is enabled.

## Actual behavior

- With NAT enabled, VBoxHeadless consumes high CPU on the host even though the guest is mostly idle.
- Removing NAT makes the problem disappear.

## Workaround

A reliable workaround was:

- remove NAT entirely
- run the VM with host-only networking only

This eliminated the high host CPU usage.

## Notes

This does not appear to be guest malware or guest workload:

- guest load remained low
- the issue tracked specifically with NAT being present vs absent
- the VM became normal once NAT was removed


### Did you upload *all* of your necessary log files, screenshots, etc.?

- [x] Yes, I've uploaded all pertinent files to this issue.

[galitsyn-oracle]

@cliffbdf,
Issue you are referring to was likely fixed a while ago. Please update to the latest 7.2.14 version.