As with other systems using BPF, macOS 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 macOS, the BPF devices live on devfs, but the macOS 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 older versions of macOS;

	a launchd daemon for Tiger and later versions of macOS;

Both of them 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).  The ChmodBPF

directory, and all files under it, must be owned by root.  Installing

the files won't immediately cause the startup item to be executed; it

will be executed on the next reboot.  To change the permissions before

the reboot, run

	sudo SystemStarter start ChmodBPF

The launchd daemon is the chmod_bpf script, plus the

org.tcpdump.chmod_bpf.plist launchd plist file.  chmod_bpf should be

installed in /usr/local/bin/chmod_bpf, and org.tcpdump.chmod_bpf.plist

should be installed in /Library/LaunchDaemons.  chmod_bpf, and

org.tcpdump.chmod_bpf.plist, must be owned by root.  Installing the

script and plist file won't immediately cause the script to be executed;

it will be executed on the next reboot.  To change the permissions

before the reboot, run

	sudo /usr/local/bin/chmod_bpf

or

	sudo launchctl load /Library/LaunchDaemons/org.tcpdump.chmod_bpf.plist

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.)