tcpdump丢包问题

The kernel has a buffer for packets to be delivered to tcpdump. If
tcpdump doesn't respond quickly enough, the kernel will overwrite old
packets with new ones.

使用tcpdump抓包时，内核分配缓冲区存放向tcpdump传送的数据包，如果tcpdump处理的不够快，新到达的包会覆盖缓冲区中较早的包，即出现dropped丢包的情况。

解决方法：

1. 增大系统缓冲区大小 bpfbufsize

2. 避免tcpdump进行dns解析，使用tcpdump -n参数。尽量减少tcpdump的工作量，其他的参数如-nn等可参考。

Dropped packets

At the end of its run,
TCPdump will inform you if any packets were dropped in the kernel. If
this becomes a problem, it's likely that your host can't keep up with
the network traffic and decode it at the same time. Try using TCPdump's
-w
option to bypass the decoding and write the raw packets to a file, then come back later and decode the file with the -r
switch. You can also try using -s
to reduce the capture snapshot size.