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.
-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