I’ve got a Dell R320 running CentOS 7, and a 10G NIC. I’m running a DNS
server on it, for testing. As part of my testing, I’m attempting to capture all the DNS queries arriving on the server, using tcpdump. However, tcpdump’s performance is abysmal, and it loses lots of the packets. Here’s the output when I stop the capture:
# tcpdump -nn -i p1p1 -w /dev/shm/queries.pcap -B 65536 dst host
tcpdump: listening on p1p1, link-type EN10MB (Ethernet), capture size
^C4096558 packets captured
6882162 packets received by filter
2785604 packets dropped by kernel
The number 6882162 is exactly the number of DNS queries I am sending from another server (the source). The filter is seeing them. However, not all of them make it into the pcap file. First I was writing to the disk, and thought it might be a bottleneck, so I tried to write to
/dev/shm. You’ll see I’ve also increased the buffer size with -B, but none of this helps.
The packet rate is also not that high. From the sending side, this is what I have:
# tcpreplay -i qtx:p1p1 5min.pcap Actual: 6882162 packets (620533334 bytes) sent in 317.08 seconds. Rated: 1951900.0 Bps, 15.61 Mbps, 21648.97 pps Flows: 4254416 flows, 13382.96 fps, 6882162 flow packets, 0 non-flow
This is a very modest rate.
Any ideas why tcpdump loses so many packets? And how to make it do better?