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1 office 1 As with other systems using BPF, Mac OS X allows users with read access
2 to 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 Mac OS X, the BPF devices live on devfs, but the OS X version of
16 devfs is based on an older (non-default) FreeBSD devfs, and that version
17 of 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 Mac OS X;
23  
24 a launchd daemon for Tiger and later versions of Mac OS X;
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.)