109 lines
4.9 KiB
Plaintext
109 lines
4.9 KiB
Plaintext
In order for libpcap to be able to capture packets on a Linux system,
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the "packet" protocol must be supported by your kernel. If it is not,
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you may get error messages such as
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modprobe: can't locate module net-pf-17
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in "/var/adm/messages", or may get messages such as
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socket: Address family not supported by protocol
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from applications using libpcap.
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You must configure the kernel with the CONFIG_PACKET option for this
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protocol; the following note is from the Linux "Configure.help" file for
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the 2.0[.x] kernel:
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Packet socket
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CONFIG_PACKET
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The Packet protocol is used by applications which communicate
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directly with network devices without an intermediate network
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protocol implemented in the kernel, e.g. tcpdump. If you want them
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to work, choose Y.
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This driver is also available as a module called af_packet.o ( =
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code which can be inserted in and removed from the running kernel
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whenever you want). If you want to compile it as a module, say M
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here and read Documentation/modules.txt; if you use modprobe or
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kmod, you may also want to add "alias net-pf-17 af_packet" to
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/etc/modules.conf.
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and the note for the 2.2[.x] kernel says:
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Packet socket
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CONFIG_PACKET
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The Packet protocol is used by applications which communicate
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directly with network devices without an intermediate network
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protocol implemented in the kernel, e.g. tcpdump. If you want them
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to work, choose Y. This driver is also available as a module called
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af_packet.o ( = code which can be inserted in and removed from the
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running kernel whenever you want). If you want to compile it as a
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module, say M here and read Documentation/modules.txt. You will
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need to add 'alias net-pf-17 af_packet' to your /etc/conf.modules
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file for the module version to function automatically. If unsure,
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say Y.
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In addition, there is an option that, in 2.2 and later kernels, will
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allow packet capture filters specified to programs such as tcpdump to be
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executed in the kernel, so that packets that don't pass the filter won't
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be copied from the kernel to the program, rather than having all packets
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copied to the program and libpcap doing the filtering in user mode.
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Copying packets from the kernel to the program consumes a significant
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amount of CPU, so filtering in the kernel can reduce the overhead of
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capturing packets if a filter has been specified that discards a
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significant number of packets. (If no filter is specified, it makes no
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difference whether the filtering isn't performed in the kernel or isn't
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performed in user mode. :-))
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The option for this is the CONFIG_FILTER option; the "Configure.help"
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file says:
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Socket filtering
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CONFIG_FILTER
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The Linux Socket Filter is derived from the Berkeley Packet Filter.
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If you say Y here, user-space programs can attach a filter to any
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socket and thereby tell the kernel that it should allow or disallow
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certain types of data to get through the socket. Linux Socket
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Filtering works on all socket types except TCP for now. See the text
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file linux/Documentation/networking/filter.txt for more information.
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If unsure, say N.
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Note that, by default, libpcap will, if libnl is present, build with it;
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it uses libnl to support monitor mode on mac80211 devices. There is a
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configuration option to disable building with libnl, but, if that option
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is chosen, the monitor-mode APIs (as used by tcpdump's "-I" flag, and as
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will probably be used by other applications in the future) won't work
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properly on mac80211 devices.
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Linux's run-time linker allows shared libraries to be linked with other
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shared libraries, which means that if an older version of a shared
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library doesn't require routines from some other shared library, and a
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later version of the shared library does require those routines, the
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later version of the shared library can be linked with that other shared
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library and, if it's otherwise binary-compatible with the older version,
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can replace that older version without breaking applications built with
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the older version, and without breaking configure scripts or the build
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procedure for applications whose configure script doesn't use the
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pcap-config script if they build with the shared library. (The build
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procedure for applications whose configure scripts use the pcap-config
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script if present will not break even if they build with the static
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library.)
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Statistics:
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Statistics reported by pcap are platform specific. The statistics
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reported by pcap_stats on Linux are as follows:
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2.2.x
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=====
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ps_recv Number of packets that were accepted by the pcap filter
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ps_drops Always 0, this statistic is not gatherd on this platform
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2.4.x
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=====
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ps_rec Number of packets that were accepted by the pcap filter
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ps_drops Number of packets that had passed filtering but were not
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passed on to pcap due to things like buffer shortage, etc.
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This is useful because these are packets you are interested in
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but won't be reported by, for example, tcpdump output.
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