Starting tcpdump causes bluetooth drivers to be loaded .. on a virtual machine / 2019-02-05

I noticed something really weird in the kernel log of a virtual machine:

Feb  5 11:46:54 server kernel: [2936066.990621] Bluetooth: Core ver 2.22
Feb  5 11:46:54 server kernel: [2936067.005355] NET: Registered protocol family 31
Feb  5 11:46:54 server kernel: [2936067.005901] Bluetooth: HCI device and connection manager initialized
Feb  5 11:46:54 server kernel: [2936067.006404] Bluetooth: HCI socket layer initialized
Feb  5 11:46:54 server kernel: [2936067.006838] Bluetooth: L2CAP socket layer initialized
Feb  5 11:46:54 server kernel: [2936067.007280] Bluetooth: SCO socket layer initialized
Feb  5 11:46:54 server kernel: [2936067.009650] Netfilter messages via NETLINK v0.30.
Feb  5 11:46:54 server kernel: [2936067.056017] device eth0 entered promiscuous mode

The last two are the giveaway about what really happened: I started tcpdump to debug a problem. But I did not expect (and do not need) bluetooth drivers on a virtual machine, it will never have access to a bluetooth dongle.

After setting up /etc/modprobe.d/local-config.conf with

blacklist bluetooth

tcpdump still works fine and no bluetooth drivers are loaded.

Update: Most recommendations are to disable the bluetooth network family:

alias net-pf-31 off