AF_Packet hash in 4.4 is not symmetric anymore, needs a different tactics

Description

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
https://patchwork.ozlabs.org/patch/467861/

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).

https://lwn.net/Articles/655295/
https://www.kernel.org/doc/Documentation/networking/filter.txt

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

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Reporter

Michał Purzyński

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