1In order for libpcap to be able to capture packets on a Linux system,
2the "packet" protocol must be supported by your kernel.  If it is not,
3you may get error messages such as
4
5	modprobe: can't locate module net-pf-17
6
7in "/var/adm/messages", or may get messages such as
8
9	socket: Address family not supported by protocol
10
11from applications using libpcap.
12
13You must configure the kernel with the CONFIG_PACKET option for this
14protocol; the following note is from the Linux "Configure.help" file for
15the 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
31and 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
46In addition, there is an option that, in 2.2 and later kernels, will
47allow packet capture filters specified to programs such as tcpdump to be
48executed in the kernel, so that packets that don't pass the filter won't
49be copied from the kernel to the program, rather than having all packets
50copied to the program and libpcap doing the filtering in user mode. 
51
52Copying packets from the kernel to the program consumes a significant
53amount of CPU, so filtering in the kernel can reduce the overhead of
54capturing packets if a filter has been specified that discards a
55significant number of packets.  (If no filter is specified, it makes no
56difference whether the filtering isn't performed in the kernel or isn't
57performed in user mode. :-))
58
59The option for this is the CONFIG_FILTER option; the "Configure.help"
60file 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
73Statistics:
74Statistics reported by pcap are platform specific.  The statistics
75reported by pcap_stats on Linux are as follows:
76
772.2.x
78=====
79ps_recv   Number of packets that were accepted by the pcap filter
80ps_drops  Always 0, this statistic is not gatherd on this platform
81
822.4.x
83=====
84ps_rec    Number of packets that were accepted by the pcap filter
85ps_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