Once BIOCLOCK is executed, the device becomes locked which prevents the
execution of ioctl(2) commands which can change the underlying parameters
of the bpf(4) device. An example might be the setting of bpf(4) filter
programs or attaching to different network interfaces.
BIOCSETWF can be used to set write filters for outgoing packets.
Currently if a bpf(4) consumer is compromised, the bpf(4) descriptor can
essentially be used as a raw socket, regardless of consumer's UID.
Write filters give users the ability to constrain which packets can be sent
through the bpf(4) descriptor.
Taken from OpenBSD.
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