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Going through the man page of tcpdump, it seems kernel can drop the packets if the buffer is full. I was wondering if:

  1. that size is configurable and/or
  2. where can I see the size for my distro?

From the man page (for easy reference):

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).

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Tcpdump has the option -B to set the capture buffer size. The value is then passed to libpcap (library used by tcpdump to do the actual packet capturing) via pcap_set_buffer_size() function. Tcpdump manpage does not specify in what units the buffer size is specified with -B, but from the source it seems that it is KiB.

manual page of pcap_set_buffer_size() does not specify default buffer size (which is used if this function is not called), but again, from the libpcap source, this seems to be 2 MiB, at least on linux (but is most likely system dependent).

With regard to packet buffering and dropping, you should also pay attention to setting snaplen (-s) parameter accordingly. man tcpdump:

-s     Snarf  snaplen bytes of data from each packet rather than the
default of 65535 bytes.  Packets truncated because of a limited snapshot
are indicated in the output with ``[|proto]'', where proto is the name of
the protocol level at which the truncation has occurred. Note that  taking
larger  snapshots both increases the amount of time it  takes  to
process packets and, effectively, decreases the amount of packet buffering.
This may cause packets to be lost. You should limit snaplen to the
smallest number that will capture the protocol information you're
interested in. Setting snaplen to 0 sets it to the default of 65535, for
back-wards compatibility with recent older versions of tcpdump.

This means that with fixed buffer size, you can increase the number of packets that fit into the buffer (and thus not being dropped) by decreasing the snaplen size.

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+1 for good reasearch and showing sources – Marcin Aug 30 '11 at 13:32
I know this is an old thread (and I like this answer), but when you cite sources on GitHub, please point to the current commit (as the master branch can change), e.g.: github.com/mcr/tcpdump/blob/…. – bruno nery Jul 30 '12 at 20:25
@brunonery That is not a problem with Stack Exchange sites, where we actually encourage users to improve answers, even if the post itself is quite old. (Unlike forums where it is frowned upon.) – Léo Lam Apr 3 '15 at 20:24
fixed the outdated links – Eli Heady Jan 11 at 17:42

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