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Network/libpcap-1.9.0/doc/README.linux.md 4.88 KB
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  In order for libpcap to be able to capture packets on a Linux system,
  the "packet" protocol must be supported by your kernel.  If it is not,
  you may get error messages such as
  
  	modprobe: can't locate module net-pf-17
  
  in "/var/adm/messages", or may get messages such as
  
  	socket: Address family not supported by protocol
  
  from applications using libpcap.
  
  You must configure the kernel with the CONFIG_PACKET option for this
  protocol; the following note is from the Linux "Configure.help" file for
  the 2.0[.x] kernel:
  
  	Packet socket
  	CONFIG_PACKET
  	  The Packet protocol is used by applications which communicate
  	  directly with network devices without an intermediate network
  	  protocol implemented in the kernel, e.g. tcpdump. If you want them
  	  to work, choose Y.
  
  	  This driver is also available as a module called af_packet.o ( =
  	  code which can be inserted in and removed from the running kernel
  	  whenever you want). If you want to compile it as a module, say M
  	  here and read Documentation/modules.txt; if you use modprobe or
  	  kmod, you may also want to add "alias net-pf-17 af_packet" to
  	  /etc/modules.conf.
  
  and the note for the 2.2[.x] kernel says:
  
  	Packet socket
  	CONFIG_PACKET
  	  The Packet protocol is used by applications which communicate
  	  directly with network devices without an intermediate network
  	  protocol implemented in the kernel, e.g. tcpdump. If you want them
  	  to work, choose Y. This driver is also available as a module called
  	  af_packet.o ( = code which can be inserted in and removed from the
  	  running kernel whenever you want). If you want to compile it as a
  	  module, say M here and read Documentation/modules.txt.  You will
  	  need to add 'alias net-pf-17 af_packet' to your /etc/conf.modules
  	  file for the module version to function automatically.  If unsure,
  	  say Y.
  
  In addition, there is an option that, in 2.2 and later kernels, will
  allow packet capture filters specified to programs such as tcpdump to be
  executed in the kernel, so that packets that don't pass the filter won't
  be copied from the kernel to the program, rather than having all packets
  copied to the program and libpcap doing the filtering in user mode.
  
  Copying packets from the kernel to the program consumes a significant
  amount of CPU, so filtering in the kernel can reduce the overhead of
  capturing packets if a filter has been specified that discards a
  significant number of packets.  (If no filter is specified, it makes no
  difference whether the filtering isn't performed in the kernel or isn't
  performed in user mode. :-))
  
  The option for this is the CONFIG_FILTER option; the "Configure.help"
  file says:
  
  	Socket filtering
  	CONFIG_FILTER
  	  The Linux Socket Filter is derived from the Berkeley Packet Filter.
  	  If you say Y here, user-space programs can attach a filter to any
  	  socket and thereby tell the kernel that it should allow or disallow
  	  certain types of data to get through the socket. Linux Socket
  	  Filtering works on all socket types except TCP for now. See the text
  	  file linux/Documentation/networking/filter.txt for more information.
  	  If unsure, say N.
  
  Note that, by default, libpcap will, if libnl is present, build with it;
  it uses libnl to support monitor mode on mac80211 devices.  There is a
  configuration option to disable building with libnl, but, if that option
  is chosen, the monitor-mode APIs (as used by tcpdump's "-I" flag, and as
  will probably be used by other applications in the future) won't work
  properly on mac80211 devices.
  
  Linux's run-time linker allows shared libraries to be linked with other
  shared libraries, which means that if an older version of a shared
  library doesn't require routines from some other shared library, and a
  later version of the shared library does require those routines, the
  later version of the shared library can be linked with that other shared
  library and, if it's otherwise binary-compatible with the older version,
  can replace that older version without breaking applications built with
  the older version, and without breaking configure scripts or the build
  procedure for applications whose configure script doesn't use the
  pcap-config script if they build with the shared library.  (The build
  procedure for applications whose configure scripts use the pcap-config
  script if present will not break even if they build with the static
  library.)
  
  Statistics:
  Statistics reported by pcap are platform specific.  The statistics
  reported by pcap_stats on Linux are as follows:
  
  2.2.x
  =====
  ps_recv   Number of packets that were accepted by the pcap filter
  ps_drop   Always 0, this statistic is not gatherd on this platform
  
  2.4.x
  =====
  ps_recv   Number of packets that were accepted by the pcap filter
  ps_drop   Number of packets that had passed filtering but were not
            passed on to pcap due to things like buffer shortage, etc.
            This is useful because these are packets you are interested in
            but won't be reported by, for example, tcpdump output.