thanks julian

What happens in this situation is that if we set the wait of the
realserver to 0 and do NOT remove the weight zero realserver with
sysctl settings (conn_reuse_mode == 0 && expire_nodest_conn == 1), and
the client reuses its source ports, the kernel will constantly
reuse connections and send the traffic to the weight 0 realserver.

you may check the details from
https://github.com/kubernetes/kubernetes/issues/81775

On Tue, Oct 26, 2021 at 2:12 AM Julian Anastasov [off-list ref] wrote:

        Hello,

On Mon, 25 Oct 2021, yangxingwu wrote:

quoted

Since commit dc7b3eb900aa ("ipvs: Fix reuse connection if real server is
dead"), new connections to dead servers are redistributed immediately to
new servers.

Then commit d752c3645717 ("ipvs: allow rescheduling of new connections when
port reuse is detected") disable expire_nodest_conn if conn_reuse_mode is
0. And new connection may be distributed to a real server with weight 0.

        Your change does not look correct to me. At the time
expire_nodest_conn was created, it was not checked when
weight is 0. At different places different terms are used
but in short, we have two independent states for real server:

- inhibited: weight=0 and no new connections should be served,
        packets for existing connections can be routed to server
        if it is still available and packets are not dropped
        by expire_nodest_conn.
        The new feature is that port reuse detection can
        redirect the new TCP connection into a new IPVS conn and
        to expire the existing cp/ct.

- unavailable (!IP_VS_DEST_F_AVAILABLE): server is removed,
        can be temporary, drop traffic for existing connections
        but on expire_nodest_conn we can select different server

        The new conn_reuse_mode flag allows port reuse to
be detected. Only then expire_nodest_conn has the
opportunity with commit dc7b3eb900aa to check weight=0
and to consider the old traffic as finished. If a new
server is selected, any retrans from previous connection
would be considered as part from the new connection. It
is a rapid way to switch server without checking with
is_new_conn_expected() because we can not have many
conns/conntracks to different servers.

quoted

Signed-off-by: yangxingwu <redacted>
---
 Documentation/networking/ipvs-sysctl.rst | 3 +--
 net/netfilter/ipvs/ip_vs_core.c          | 5 +++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/Documentation/networking/ipvs-sysctl.rst b/Documentation/networking/ipvs-sysctl.rst
index 2afccc63856e..1cfbf1add2fc 100644
--- a/Documentation/networking/ipvs-sysctl.rst
+++ b/Documentation/networking/ipvs-sysctl.rst

@@ -37,8 +37,7 @@ conn_reuse_mode - INTEGER

      0: disable any special handling on port reuse. The new
      connection will be delivered to the same real server that was
-     servicing the previous connection. This will effectively
-     disable expire_nodest_conn.
+     servicing the previous connection.

      bit 1: enable rescheduling of new connections when it is safe.
      That is, whenever expire_nodest_conn and for TCP sockets, when

diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
index 128690c512df..9279aed69e23 100644
--- a/net/netfilter/ipvs/ip_vs_core.c
+++ b/net/netfilter/ipvs/ip_vs_core.c

@@ -2042,14 +2042,15 @@ ip_vs_in(struct netns_ipvs *ipvs, unsigned int hooknum, struct sk_buff *skb, int
                           ipvs, af, skb, &iph);

      conn_reuse_mode = sysctl_conn_reuse_mode(ipvs);
-     if (conn_reuse_mode && !iph.fragoffs && is_new_conn(skb, &iph) && cp) {
+     if (!iph.fragoffs && is_new_conn(skb, &iph) && cp) {
              bool old_ct = false, resched = false;

              if (unlikely(sysctl_expire_nodest_conn(ipvs)) && cp->dest &&
                  unlikely(!atomic_read(&cp->dest->weight))) {
                      resched = true;
                      old_ct = ip_vs_conn_uses_old_conntrack(cp, skb);
-             } else if (is_new_conn_expected(cp, conn_reuse_mode)) {
+             } else if (conn_reuse_mode &&
+                        is_new_conn_expected(cp, conn_reuse_mode)) {
                      old_ct = ip_vs_conn_uses_old_conntrack(cp, skb);
                      if (!atomic_read(&cp->n_control)) {
                              resched = true;
--

2.30.2

Regards

--
Julian Anastasov [off-list ref]