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 73 Statistics: 74 Statistics reported by pcap are platform specific. The statistics 75 reported by pcap_stats on Linux are as follows: 76 77 2.2.x 78 ===== 79 ps_recv Number of packets that were accepted by the pcap filter 80 ps_drops Always 0, this statistic is not gatherd on this platform 81 82 2.4.x 83 ===== 84 ps_rec Number of packets that were accepted by the pcap filter 85 ps_drops Number of packets that had passed filtering but were not 86 passed on to pcap due to things like buffer shortage, etc. 87 This is useful because these are packets you are interested in 88 but won't be reported by, for example, tcpdump output. 89
