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1 office 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.