Can I use afpacket fanout?
Background
The Linux kernel has a feature for efficiently capturing packets called afpacket.
A related feature, fanout groups, exists so that you can capture from N processes or threads at the
same time, and each thread will see 1/Nth of the traffic. For stateful
applications that perform stream reassembly, instead of a simple round robin distribution the packets need to
be hashed according to their 5 tuple. To further complicate the issue, the hash function
needs to be symmetrical so that a packet from HostA to HostB is hashed to the
same process as the packet from HostB to HostA. This feature is known as PACKET_FANOUT_HASH
Unfortunately, some versions of the Linux kernel are broken and do not properly implement the symmetric hash. This issue has been fixed, but the buggy code made its way into various distribution kernels.
It's not easy to know just by looking at a kernel version whether or not it will work properly.
Furthermore, in some cases -- like, for example, with MPLS traffic -- afpacket fanout might not do the balancing as you expect.
can-i-use-afpacket-fanout is a tool that runs multiple threads in a fanout group and checks to
see if flows are routed to the appropriate workers. If it sees a flow
on two different workers, or the reverse flow on a different worker, it
will log a Failure.
Install
$ export GOPATH=~/go # If you don't already have this set to something
$ go install github.com/JustinAzoff/can-i-use-afpacket-fanout@latest
RUN
$ sudo ~/go/bin/can-i-use-afpacket-fanout -interface wlan0 -maxflows 500
2016/11/08 03:55:20 Starting worker id 0 on interface wlan0
2016/11/08 03:55:20 Starting worker id 1 on interface wlan0
2016/11/08 03:55:20 Starting worker id 2 on interface wlan0
2016/11/08 03:55:20 Starting worker id 3 on interface wlan0
2016/11/08 03:55:20 Starting worker id 4 on interface wlan0
2016/11/08 03:55:20 Starting worker id 5 on interface wlan0
2016/11/08 03:55:20 Starting worker id 6 on interface wlan0
2016/11/08 03:55:20 Starting worker id 7 on interface wlan0
2016/11/08 03:55:20 Collecting results until 500 flows have been seen..
2016/11/08 03:55:22 Stats: packets=100 flows=31 success=69 reverse_success=83 failures=0 reverse_failures=0
2016/11/08 03:55:23 Stats: packets=200 flows=42 success=158 reverse_success=176 failures=0 reverse_failures=0
2016/11/08 03:55:24 Stats: packets=300 flows=72 success=228 reverse_success=258 failures=0 reverse_failures=0
2016/11/08 03:55:25 Stats: packets=400 flows=87 success=313 reverse_success=348 failures=0 reverse_failures=0
2016/11/08 03:55:27 Stats: packets=500 flows=123 success=377 reverse_success=425 failures=0 reverse_failures=0
2016/11/08 03:55:29 Stats: packets=600 flows=151 success=449 reverse_success=506 failures=0 reverse_failures=0
2016/11/08 03:55:31 Stats: packets=700 flows=179 success=521 reverse_success=588 failures=0 reverse_failures=0
2016/11/08 03:55:33 Stats: packets=800 flows=202 success=598 reverse_success=670 failures=0 reverse_failures=0
2016/11/08 03:55:34 Stats: packets=900 flows=227 success=673 reverse_success=755 failures=0 reverse_failures=0
2016/11/08 03:55:36 Stats: packets=1000 flows=247 success=753 reverse_success=838 failures=0 reverse_failures=0
2016/11/08 03:55:37 Stats: packets=1100 flows=269 success=831 reverse_success=926 failures=0 reverse_failures=0
2016/11/08 03:55:38 Stats: packets=1200 flows=291 success=909 reverse_success=1007 failures=0 reverse_failures=0
2016/11/08 03:55:40 Stats: packets=1300 flows=305 success=995 reverse_success=1094 failures=0 reverse_failures=0
2016/11/08 03:55:41 Stats: packets=1400 flows=325 success=1075 reverse_success=1176 failures=0 reverse_failures=0
2016/11/08 03:55:43 Stats: packets=1500 flows=346 success=1154 reverse_success=1263 failures=0 reverse_failures=0
2016/11/08 03:55:44 Stats: packets=1600 flows=372 success=1228 reverse_success=1345 failures=0 reverse_failures=0
2016/11/08 03:55:45 Stats: packets=1700 flows=386 success=1314 reverse_success=1431 failures=0 reverse_failures=0
2016/11/08 03:55:46 Stats: packets=1800 flows=399 success=1401 reverse_success=1519 failures=0 reverse_failures=0
2016/11/08 03:55:46 Stats: packets=1900 flows=409 success=1491 reverse_success=1608 failures=0 reverse_failures=0
2016/11/08 03:55:48 Stats: packets=2000 flows=431 success=1569 reverse_success=1693 failures=0 reverse_failures=0
2016/11/08 03:55:49 Stats: packets=2100 flows=448 success=1652 reverse_success=1778 failures=0 reverse_failures=0
2016/11/08 03:55:50 Stats: packets=2200 flows=470 success=1730 reverse_success=1864 failures=0 reverse_failures=0
2016/11/08 03:55:51 Stats: packets=2300 flows=493 success=1807 reverse_success=1945 failures=0 reverse_failures=0
2016/11/08 03:55:52 Final Stats: packets=2345 flows=501 success=1844 reverse_success=1983 failures=0 reverse_failures=0
2016/11/08 03:55:52 Worker flow count distribution:
2016/11/08 03:55:52 - worker=0 flows=54
2016/11/08 03:55:52 - worker=1 flows=79
2016/11/08 03:55:52 - worker=2 flows=44
2016/11/08 03:55:52 - worker=3 flows=58
2016/11/08 03:55:52 - worker=4 flows=71
2016/11/08 03:55:52 - worker=5 flows=73
2016/11/08 03:55:52 - worker=6 flows=64
2016/11/08 03:55:52 - worker=7 flows=58
YES!