Subject: Re: [PATCH net-next v15 4/7] sch_cake: Add NAT awareness to packet classifier

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 22 May 2018 16:11:06 +0200

Pablo Neira Ayuso <[email-masked]> writes:

> Hi Toke,
>
> On Tue, May 22, 2018 at 03:57:38PM +0200, Toke Høiland-Jørgensen wrote:
>> When CAKE is deployed on a gateway that also performs NAT (which is a
>> common deployment mode), the host fairness mechanism cannot distinguish
>> internal hosts from each other, and so fails to work correctly.
>> 
>> To fix this, we add an optional NAT awareness mode, which will query the
>> kernel conntrack mechanism to obtain the pre-NAT addresses for each packet
>> and use that in the flow and host hashing.
>> 
>> When the shaper is enabled and the host is already performing NAT, the cost
>> of this lookup is negligible. However, in unlimited mode with no NAT being
>> performed, there is a significant CPU cost at higher bandwidths. For this
>> reason, the feature is turned off by default.
>> 
>> Cc: [email-masked]
>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> ---
>>  net/sched/sch_cake.c |   79 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 79 insertions(+)
>> 
>> diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
>> index 68ac908470f1..6f7cae705c84 100644
>> --- a/net/sched/sch_cake.c
>> +++ b/net/sched/sch_cake.c
>> @@ -71,6 +71,12 @@
>>  #include <net/tcp.h>
>>  #include <net/flow_dissector.h>
>>  
>> +#if IS_REACHABLE(CONFIG_NF_CONNTRACK)
>> +#include <net/netfilter/nf_conntrack_core.h>
>> +#include <net/netfilter/nf_conntrack_zones.h>
>> +#include <net/netfilter/nf_conntrack.h>
>> +#endif
>> +
>>  #define CAKE_SET_WAYS (8)
>>  #define CAKE_MAX_TINS (8)
>>  #define CAKE_QUEUES (1024)
>> @@ -516,6 +522,60 @@ static bool cobalt_should_drop(struct cobalt_vars *vars,
>>  	return drop;
>>  }
>>  
>> +#if IS_REACHABLE(CONFIG_NF_CONNTRACK)
>> +
>> +static void cake_update_flowkeys(struct flow_keys *keys,
>> +				 const struct sk_buff *skb)
>> +{
>> +	const struct nf_conntrack_tuple *tuple;
>> +	enum ip_conntrack_info ctinfo;
>> +	struct nf_conn *ct;
>> +	bool rev = false;
>> +
>> +	if (tc_skb_protocol(skb) != htons(ETH_P_IP))
>> +		return;
>> +
>> +	ct = nf_ct_get(skb, &ctinfo);
>> +	if (ct) {
>> +		tuple = nf_ct_tuple(ct, CTINFO2DIR(ctinfo));
>> +	} else {
>> +		const struct nf_conntrack_tuple_hash *hash;
>> +		struct nf_conntrack_tuple srctuple;
>> +
>> +		if (!nf_ct_get_tuplepr(skb, skb_network_offset(skb),
>> +				       NFPROTO_IPV4, dev_net(skb->dev),
>> +				       &srctuple))
>> +			return;
>> +
>> +		hash = nf_conntrack_find_get(dev_net(skb->dev),
>> +					     &nf_ct_zone_dflt,
>> +					     &srctuple);
>> +		if (!hash)
>> +			return;
>> +
>> +		rev = true;
>> +		ct = nf_ct_tuplehash_to_ctrack(hash);
>> +		tuple = nf_ct_tuple(ct, !hash->tuple.dst.dir);
>> +	}
>> +
>> +	keys->addrs.v4addrs.src = rev ? tuple->dst.u3.ip : tuple->src.u3.ip;
>> +	keys->addrs.v4addrs.dst = rev ? tuple->src.u3.ip : tuple->dst.u3.ip;
>> +
>> +	if (keys->ports.ports) {
>> +		keys->ports.src = rev ? tuple->dst.u.all : tuple->src.u.all;
>> +		keys->ports.dst = rev ? tuple->src.u.all : tuple->dst.u.all;
>> +	}
>> +	if (rev)
>> +		nf_ct_put(ct);
>> +}
>
> This is going to pull in the nf_conntrack module, even if you may not
> want it, as soon as cake is in place.

Yeah, we are aware of that; we get a moddep on nf_conntrack. Our main
deployment scenario has been home routers where conntrack is used
anyway, so this has not been much of an issue. However, if there is a
way to avoid this, and instead detect at runtime if conntrack is
available, that would certainly be useful. Is there? :)

-Toke