сбрасывать трафик в сети (dump traffic on a network)
Описание (Description)
Tcpdump prints out a description of the contents of packets on a
network interface that match the Boolean expression; the
description is preceded by a time stamp, printed, by default, as
hours, minutes, seconds, and fractions of a second since
midnight. It can also be run with the -w flag, which causes it
to save the packet data to a file for later analysis, and/or with
the -r flag, which causes it to read from a saved packet file
rather than to read packets from a network interface. It can
also be run with the -V flag, which causes it to read a list of
saved packet files. In all cases, only packets that match
expression will be processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue capturing
packets until it is interrupted by a SIGINT signal (generated,
for example, by typing your interrupt character, typically
control-C) or a SIGTERM signal (typically generated with the
kill(1) command); if run with the -c flag, it will capture
packets until it is interrupted by a SIGINT or SIGTERM signal or
the specified number of packets have been processed.
When tcpdump finishes capturing packets, it will report counts
of:
packets ``captured'' (this is the number of packets that
tcpdump has received and processed);
packets ``received by filter'' (the meaning of this
depends on the OS on which you're running tcpdump, and
possibly on the way the OS was configured - if a filter
was specified on the command line, on some OSes it counts
packets regardless of whether they were matched by the
filter expression and, even if they were matched by the
filter expression, regardless of whether tcpdump has read
and processed them yet, on other OSes it counts only
packets that were matched by the filter expression
regardless of whether tcpdump has read and processed them
yet, and on other OSes it counts only packets that were
matched by the filter expression and were processed by
tcpdump);
packets ``dropped by kernel'' (this is the number of
packets that were dropped, due to a lack of buffer space,
by the packet capture mechanism in the OS on which tcpdump
is running, if the OS reports that information to
applications; if not, it will be reported as 0).
On platforms that support the SIGINFO signal, such as most BSDs
(including macOS) and Digital/Tru64 UNIX, it will report those
counts when it receives a SIGINFO signal (generated, for example,
by typing your ``status'' character, typically control-T,
although on some platforms, such as macOS, the ``status''
character is not set by default, so you must set it with stty(1)
in order to use it) and will continue capturing packets. On
platforms that do not support the SIGINFO signal, the same can be
achieved by using the SIGUSR1 signal.
Using the SIGUSR2 signal along with the -w flag will forcibly
flush the packet buffer into the output file.
Reading packets from a network interface may require that you
have special privileges; see the pcap(3PCAP) man page for
details. Reading a saved packet file doesn't require special
privileges.
