Re: macvlan breaks garp for vrrp

[Lennart Sorensen]

On Thu, Jul 15, 2021 at 11:37:20AM -0400, Lennart Sorensen wrote:
> We are seeing an occational problem where a node in a cluster running
> vrrp stops being able to send packets to the current master due to having
> the wrong arp entry for the virtual IP.  It appears that what happens
> is that if around the time vrrp changes who the master is, a packet is
> sent from the new master to another node, that node will learn the mac
> of the virtual IP as the mac of the physical interface of the master
> (since it always transmits from the physical mac, not the virtual mac).
> In most cases nodes seem to correctly send an arp request for the
> virtual IP and get back the virtual mac (from the maclan interface
> used by keepalived with use_vmac enabled), in which case all is fine.
> Only if the timing is unlucky do you end up learning the wrong mac and
> then you can no longer send packets back to the master since it doesn't
> accept packets on the physical interface mac, only the virtual mac.
> 
> Of course one would have thought that the garp sent by the master would
> take care of the problem, but unfortunately the macvlan code is not
> allowing garp packets with a source mac macthing the macvlan interface to
> reach the physical interface.  The code causing this is below.  keepalived
> uses MACVLAN_MODE_PRIVATE which does not match the condition, so the code
> tries to do its work and consumes the packet, and the physical interface
> hence never gets to see it, and the arp table is hence not corrected.
> I can't actually make sense of what this code is trying to do in this
> case where the source mac of the packet matches the macvlan interface,
> even though it was received from outside (from the current master in
> the vrrp cluster).
> 
>         port = macvlan_port_get_rcu(skb->dev);
>           if (is_multicast_ether_addr(eth->h_dest)) {
>                   unsigned int hash;
> 
>                   skb = ip_check_defrag(dev_net(skb->dev), skb, IP_DEFRAG_MACVLAN);                                                                                           if (!skb)
>                           return RX_HANDLER_CONSUMED;
>                   *pskb = skb;
>                   eth = eth_hdr(skb);
>                   if (macvlan_forward_source(skb, port, eth->h_source))
>                           return RX_HANDLER_CONSUMED;
>                   src = macvlan_hash_lookup(port, eth->h_source);  <- this of course has a match
>                   if (src && src->mode != MACVLAN_MODE_VEPA &&     <- so this is true and eats the packet
>                       src->mode != MACVLAN_MODE_BRIDGE) {
>                           /* forward to original port. */
>                           vlan = src;
>                           ret = macvlan_broadcast_one(skb, vlan, eth, 0) ?:
>                                 netif_rx(skb);
>                           handle_res = RX_HANDLER_CONSUMED;
>                           goto out;
>                   }
> 
>                   hash = mc_hash(NULL, eth->h_dest);
>                   if (test_bit(hash, port->mc_filter))
>                           macvlan_broadcast_enqueue(port, src, skb);
> 
>                   return RX_HANDLER_PASS;
>           }
> 
> If I patch it to do:
> 
>                    if (src && src->mode != MACVLAN_MODE_VEPA &&
> +                      src->mode != MACVLAN_MODE_PRIVATE &&
>                        src->mode != MACVLAN_MODE_BRIDGE) {
> 
> the problem goes away, but since I don't understand what the idea of
> this code was, that doesn't feel safe.
> 
> Can someone that understands the point of this code perhaps give an idea
> how to correctly fix this?  Definitely packets received with the same
> mac address as this macvlan should still be able to make it through to
> the underlying interface, since that is how vrrp is supposed to operate
> per the RFC.  Having the wrong arp entry is bad since each time we
> receive a packet we increase the timeout so it never ages out, even
> though it is the wrong mac address for actually replying (due to how
> macvlan interfaces seem to work).
> 
> So far this doesn't seem to happen very often since the timing has to
> be just "wrong" on the packets, but I have now seen it 4 or 5 times in
> the last year.  The solution each time was to manually delete the arp
> entry for the virtual IP on the affected node, after which it correctly
> arp'd for the right mac and everything worked again.
> 
> To test it I would manually set a temp arp entry using the physical mac of
> the interface on the master as the arp entry, and see if the garp ever
> fixed it, and it did not until I made my small patch, after which it
> does fix the arp table reliably as far as I can tell.

Well I discovered that that my patch breaks other traffic, so clearly that
doesn't work.

Does anyone have any idea how to fix the code so garp packets from other
systems with the sae macvlan mac address can actually be sent to the
physical interface so the arp table can be updated?

-- 
Len Sorensen