As with other systems using BPF, Mac OS X allows users with read access
to the BPF devices to capture packets with libpcap and allows users with
write access to the BPF devices to send packets with libpcap.

On some systems that use BPF, the BPF devices live on the root file
system, and the permissions and/or ownership on those devices can be
changed to give users other than root permission to read or write those
devices.

On newer versions of FreeBSD, the BPF devices live on devfs, and devfs
can be configured to set the permissions and/or ownership of those
devices to give users other than root permission to read or write those
devices.

On Mac OS X, the BPF devices live on devfs, but the OS X version of
devfs is based on an older (non-default) FreeBSD devfs, and that version
of devfs cannot be configured to set the permissions and/or ownership of
those devices.

Therefore, we supply a "startup item" for OS X that will change the
ownership of the BPF devices so that the "admin" group owns them, and
will change the permission of the BPF devices to rw-rw----, so that all
users in the "admin" group - i.e., all users with "Allow user to
administer this computer" turned on - have both read and write access to
them.

The startup item is in the ChmodBPF directory in the source tree.  A
/Library/StartupItems directory should be created if it doesn't already
exist, and the ChmodBPF directory should be copied to the
/Library/StartupItems directory (copy the entire directory, so that
there's a /Library/StartupItems/ChmodBPF directory, containing all the
files in the source tree's ChmodBPF directory; don't copy the individual
items in that directory to /Library/StartupItems).

If you want to give a particular user permission to access the BPF
devices, rather than giving all administrative users permission to
access them, you can have the ChmodBPF/ChmodBPF script change the
ownership of /dev/bpf* without changing the permissions.  If you want to
give a particular user permission to read and write the BPF devices and
give the administrative users permission to read but not write the BPF
devices, you can have the script change the owner to that user, the
group to "admin", and the permissions to rw-r-----.  Other possibilities
are left as an exercise for the reader.

(NOTE: due to a bug in Snow Leopard, if you change the permissions not
to grant write permission to everybody who should be allowed to capture
traffic, non-root users who cannot open the BPF devices for writing will
not be able to capture outgoing packets.)
