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 older versions of Mac OS X; a launchd daemon for Tiger and later versions of Mac OS X; 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.)