Re: [bug report, linux 6.15-rc4] A large number of connections in the SYN_SENT state caused the nf_conntrack table to be full.

[ying chen <yc1082463@xxxxxxxxx>]

On Wed, May 28, 2025 at 9:41 PM Jozsef Kadlecsik
<kadlec@xxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 28 May 2025, ying chen wrote:
>
> > On Wed, May 28, 2025 at 9:10 PM Florian Westphal <fw@xxxxxxxxx> wrote:
> >>
> >> ying chen <yc1082463@xxxxxxxxx> wrote:
> >>> Hello all,
> >>>
> >>> I encountered an "nf_conntrack: table full" warning on Linux 6.15-rc4.
> >>> Running cat /proc/net/nf_conntrack showed a large number of
> >>> connections in the SYN_SENT state.
> >>> As is well known, if we attempt to connect to a non-existent port, the
> >>> system will respond with an RST and then delete the conntrack entry.
> >>> However, when we frequently connect to non-existent ports, the
> >>> conntrack entries are not deleted, eventually causing the nf_conntrack
> >>> table to fill up.
> >>
> >> Yes, what do you expect to happen?
> > I understand that the conntrack entry should be deleted immediately
> > after receiving the RST reply.
>
> No, the conntrack entry will be in the CLOSE state with the timeout value
> of /proc/sys/net/netfilter/nf_conntrack_tcp_timeout_close
>
> Best regards,
> Jozsef
The conntrack entry does not transition to the CLOSE state and remains
in the SYN_SENT state until the nf_conntrack_tcp_timeout_syn_sent
timeout is reached.
According to the code, the conntrack  entry should be deleted
immediately after the RST reply.
int nf_conntrack_tcp_packet(struct nf_conn *ct,
                            struct sk_buff *skb,
                            unsigned int dataoff,
                            enum ip_conntrack_info ctinfo,
                            const struct nf_hook_state *state)
{
        ......
        if (!test_bit(IPS_SEEN_REPLY_BIT, &ct->status)) {
                /* If only reply is a RST, we can consider ourselves not to
                   have an established connection: this is a fairly common
                   problem case, so we can delete the conntrack
                   immediately.  --RR */
                if (th->rst) {
                        nf_ct_kill_acct(ct, ctinfo, skb);
                        return NF_ACCEPT;
                }