Question

Going through the man page of tcpdump here
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).

Answer 1

There are several areas you might check to mitigate packets dropped by kernel:

• Look at configuring /proc/sys/net/core/netdev_max_backlog and /proc/sys/net/core/netdev_budget. The default is probably pretty low; try setting each to something like 2000.
• Writing to the output device screen may be blocking/slowing the tcpdump process long enough to fill the recv buffer
  • Use -nn to turn off DNS lookups and port naming
  • Write to file instead of the screen
  • Try a tool such as gulp
• If you have a multi-processor machine look at using taskset
• Use nice to set the priority of the process

Even with those settings, it may just be that you can not keep up with the speed of the traffic you are trying to capture. Look at the details of your NIC and machine and ensure that what you expect is even possible.