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.)