[Bro-Dev] [JIRA] (BIT-1575) AF_Packet hash in 4.4 is not symmetric anymore, needs a different tactics

Michal Purzynski (JIRA) jira at bro-tracker.atlassian.net
Tue Apr 26 15:17:00 PDT 2016

Michal Purzynski created BIT-1575:

             Summary: AF_Packet hash in 4.4 is not symmetric anymore, needs a different tactics
                 Key: BIT-1575
                 URL: https://bro-tracker.atlassian.net/browse/BIT-1575
             Project: Bro Issue Tracker
          Issue Type: Problem
          Components: Bro
    Affects Versions: git/master
            Reporter: Michal Purzynski

The AF_Packet in 4.4 and onward has undergone a major redesign and rewrite.

In order to make it more generic and correctly support Vlans, GRE tunnels, IPv6, and so on, the hash function has been generalized and is not symmetric anymore.

This affacts the af_packet capture plugin.

For kernel version 4.2, the following function was used

static inline u32 __flow_hash_from_keys(struct flow_keys *keys)
     /* get a *consistent hash* (*same value on both flow directions*) */

In 4.4 it's jhash2, which is *not* symmetric. This results in splitted connections.

static __always_inline u32 __flow_hash_words(const u32 *words, u32 length, u32 keyval)
     return jhash2(words, length, keyval);

I have tested this on 4.2, then upgraded to 4.4, observed lots of SAD connections, went back to 4.2.

This seems to clarify this design decision

After consulting Suricata developers (thank you, Regit!!) seems like there's a new way to achieve consistent hashing. This method must be implemented for kernels >= 4.4 (maybe others, too).


eBPF fanout mode, so you write a filter and it af_packet respects hashing from it.

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