Re: DDoS attack causing bad effect on conntrack searches

Changli Gao
2010-04-22T13:13:48Z

On Thu, Apr 22, 2010 at 8:58 PM, Jesper Dangaard Brouer <hawk@comx.dk> wrote:
>
> At an unnamed ISP, we experienced a DDoS attack against one of our
> customers.  This attack also caused problems for one of our Linux
> based routers.
>
> The attack was "only" generating 300 kpps (packets per sec), which
> usually isn't a problem for this (fairly old) Linux Router.  But the
> conntracking system chocked and reduced pps processing power to
> 40kpps.
>
> I do extensive RRD/graph monitoring of the machines.  The IP conntrack
> searches in the period exploded, to a stunning 700.000 searches per
> sec.
>
> http://people.netfilter.org/hawk/DDoS/2010-04-12__001/conntrack_searches001.png
>
> First I though it might be caused by bad hashing, but after reading
> the kernel code (func: __nf_conntrack_find()), I think its caused by
> the loop restart (goto begin) of the conntrack search, running under
> local_bh_disable().  These RCU changes to conntrack were introduced in
> ea781f19 by Eric Dumazet.
>
> Code: net/netfilter/nf_conntrack_core.c
> Func: __nf_conntrack_find()
>
> struct nf_conntrack_tuple_hash *
> __nf_conntrack_find(struct net *net, const struct nf_conntrack_tuple *tuple)
> {
>        struct nf_conntrack_tuple_hash *h;
>        struct hlist_nulls_node *n;
>        unsigned int hash = hash_conntrack(tuple);
>
>        /* Disable BHs the entire time since we normally need to disable them
>         * at least once for the stats anyway.
>         */
>        local_bh_disable();
> begin:
>        hlist_nulls_for_each_entry_rcu(h, n, &net->ct.hash[hash], hnnode) {
>                if (nf_ct_tuple_equal(tuple, &h->tuple)) {
>                        NF_CT_STAT_INC(net, found);
>                        local_bh_enable();
>                        return h;
>                }
>                NF_CT_STAT_INC(net, searched);
>        }
>        /*
>         * if the nulls value we got at the end of this lookup is
>         * not the expected one, we must restart lookup.
>         * We probably met an item that was moved to another chain.
>         */
>        if (get_nulls_value(n) != hash)
>                goto begin;
>        local_bh_enable();
>

We should add a retry limit there.

--
Regards，
Changli Gao(xiaosuo@gmail.com)
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