1 In order for libpcap to be able to capture packets on a Linux system, 2 the "packet" protocol must be supported by your kernel. If it is not, 3 you may get error messages such as 4 5 modprobe: can't locate module net-pf-17 6 7 in "/var/adm/messages", or may get messages such as 8 9 socket: Address family not supported by protocol 10 11 from applications using libpcap. 12 13 You must configure the kernel with the CONFIG_PACKET option for this 14 protocol; the following note is from the Linux "Configure.help" file for 15 the 2.0[.x] kernel: 16 17 Packet socket 18 CONFIG_PACKET 19 The Packet protocol is used by applications which communicate 20 directly with network devices without an intermediate network 21 protocol implemented in the kernel, e.g. tcpdump. If you want them 22 to work, choose Y. 23 24 This driver is also available as a module called af_packet.o ( = 25 code which can be inserted in and removed from the running kernel 26 whenever you want). If you want to compile it as a module, say M 27 here and read Documentation/modules.txt; if you use modprobe or 28 kmod, you may also want to add "alias net-pf-17 af_packet" to 29 /etc/modules.conf. 30 31 and the note for the 2.2[.x] kernel says: 32 33 Packet socket 34 CONFIG_PACKET 35 The Packet protocol is used by applications which communicate 36 directly with network devices without an intermediate network 37 protocol implemented in the kernel, e.g. tcpdump. If you want them 38 to work, choose Y. This driver is also available as a module called 39 af_packet.o ( = code which can be inserted in and removed from the 40 running kernel whenever you want). If you want to compile it as a 41 module, say M here and read Documentation/modules.txt. You will 42 need to add 'alias net-pf-17 af_packet' to your /etc/conf.modules 43 file for the module version to function automatically. If unsure, 44 say Y. 45 46 In addition, there is an option that, in 2.2 and later kernels, will 47 allow packet capture filters specified to programs such as tcpdump to be 48 executed in the kernel, so that packets that don't pass the filter won't 49 be copied from the kernel to the program, rather than having all packets 50 copied to the program and libpcap doing the filtering in user mode. 51 52 Copying packets from the kernel to the program consumes a significant 53 amount of CPU, so filtering in the kernel can reduce the overhead of 54 capturing packets if a filter has been specified that discards a 55 significant number of packets. (If no filter is specified, it makes no 56 difference whether the filtering isn't performed in the kernel or isn't 57 performed in user mode. :-)) 58 59 The option for this is the CONFIG_FILTER option; the "Configure.help" 60 file says: 61 62 Socket filtering 63 CONFIG_FILTER 64 The Linux Socket Filter is derived from the Berkeley Packet Filter. 65 If you say Y here, user-space programs can attach a filter to any 66 socket and thereby tell the kernel that it should allow or disallow 67 certain types of data to get through the socket. Linux Socket 68 Filtering works on all socket types except TCP for now. See the text 69 file linux/Documentation/networking/filter.txt for more information. 70 If unsure, say N. 71 72 Note that, by default, libpcap will, if libnl is present, build with it; 73 it uses libnl to support monitor mode on mac80211 devices. There is a 74 configuration option to disable building with libnl, but, if that option 75 is chosen, the monitor-mode APIs (as used by tcpdump's "-I" flag, and as 76 will probably be used by other applications in the future) won't work 77 properly on mac80211 devices. 78 79 Linux's run-time linker allows shared libraries to be linked with other 80 shared libraries, which means that if an older version of a shared 81 library doesn't require routines from some other shared library, and a 82 later version of the shared library does require those routines, the 83 later version of the shared library can be linked with that other shared 84 library and, if it's otherwise binary-compatible with the older version, 85 can replace that older version without breaking applications built with 86 the older version, and without breaking configure scripts or the build 87 procedure for applications whose configure script doesn't use the 88 pcap-config script if they build with the shared library. (The build 89 procedure for applications whose configure scripts use the pcap-config 90 script if present will not break even if they build with the static 91 library.) 92 93 Statistics: 94 Statistics reported by pcap are platform specific. The statistics 95 reported by pcap_stats on Linux are as follows: 96 97 2.2.x 98 ===== 99 ps_recv Number of packets that were accepted by the pcap filter 100 ps_drop Always 0, this statistic is not gatherd on this platform 101 102 2.4.x 103 ===== 104 ps_recv Number of packets that were accepted by the pcap filter 105 ps_drop Number of packets that had passed filtering but were not 106 passed on to pcap due to things like buffer shortage, etc. 107 This is useful because these are packets you are interested in 108 but won't be reported by, for example, tcpdump output. 109
