1 As with other systems using BPF, macOS allows users with read access to 2 the BPF devices to capture packets with libpcap and allows users with 3 write access to the BPF devices to send packets with libpcap. 4 5 On some systems that use BPF, the BPF devices live on the root file 6 system, and the permissions and/or ownership on those devices can be 7 changed to give users other than root permission to read or write those 8 devices. 9 10 On newer versions of FreeBSD, the BPF devices live on devfs, and devfs 11 can be configured to set the permissions and/or ownership of those 12 devices to give users other than root permission to read or write those 13 devices. 14 15 On macOS, the BPF devices live on devfs, but the macOS version of devfs 16 is based on an older (non-default) FreeBSD devfs, and that version of 17 devfs cannot be configured to set the permissions and/or ownership of 18 those devices. 19 20 Therefore, we supply: 21 22 a "startup item" for older versions of macOS; 23 24 a launchd daemon for Tiger and later versions of macOS; 25 26 Both of them will change the ownership of the BPF devices so that the 27 "admin" group owns them, and will change the permission of the BPF 28 devices to rw-rw----, so that all users in the "admin" group - i.e., all 29 users with "Allow user to administer this computer" turned on - have 30 both read and write access to them. 31 32 The startup item is in the ChmodBPF directory in the source tree. A 33 /Library/StartupItems directory should be created if it doesn't already 34 exist, and the ChmodBPF directory should be copied to the 35 /Library/StartupItems directory (copy the entire directory, so that 36 there's a /Library/StartupItems/ChmodBPF directory, containing all the 37 files in the source tree's ChmodBPF directory; don't copy the individual 38 items in that directory to /Library/StartupItems). The ChmodBPF 39 directory, and all files under it, must be owned by root. Installing 40 the files won't immediately cause the startup item to be executed; it 41 will be executed on the next reboot. To change the permissions before 42 the reboot, run 43 44 sudo SystemStarter start ChmodBPF 45 46 The launchd daemon is the chmod_bpf script, plus the 47 org.tcpdump.chmod_bpf.plist launchd plist file. chmod_bpf should be 48 installed in /usr/local/bin/chmod_bpf, and org.tcpdump.chmod_bpf.plist 49 should be installed in /Library/LaunchDaemons. chmod_bpf, and 50 org.tcpdump.chmod_bpf.plist, must be owned by root. Installing the 51 script and plist file won't immediately cause the script to be executed; 52 it will be executed on the next reboot. To change the permissions 53 before the reboot, run 54 55 sudo /usr/local/bin/chmod_bpf 56 57 or 58 59 sudo launchctl load /Library/LaunchDaemons/org.tcpdump.chmod_bpf.plist 60 61 If you want to give a particular user permission to access the BPF 62 devices, rather than giving all administrative users permission to 63 access them, you can have the ChmodBPF/ChmodBPF script change the 64 ownership of /dev/bpf* without changing the permissions. If you want to 65 give a particular user permission to read and write the BPF devices and 66 give the administrative users permission to read but not write the BPF 67 devices, you can have the script change the owner to that user, the 68 group to "admin", and the permissions to rw-r-----. Other possibilities 69 are left as an exercise for the reader. 70 71 (NOTE: due to a bug in Snow Leopard, if you change the permissions not 72 to grant write permission to everybody who should be allowed to capture 73 traffic, non-root users who cannot open the BPF devices for writing will 74 not be able to capture outgoing packets.) 75
