[lvs-users] DR : real server unable to reach VIP

Anders Henke anders.henke at 1und1.de
Wed May 20 12:32:03 BST 2015

On 20.05.2015, Florent B wrote:
> On 05/20/2015 12:21 PM, Michael Schwartzkopff wrote:
> > Am Mittwoch, 20. Mai 2015, 12:00:01 schrieb Florent B:
> >> Some precisions :
> >>
> >> In fact packets are computed by LVS, the problem is that RS receives a
> >> packets with :
> >> - its proper eth0 IP in src, but with mac address of LVS server
> >
> > "its": which IP in the source field? The source IP address should
> always be the
> > IP address of the client. The source address should never be the IP
> adress of
> > the real server.
> >
> > MAC address of the director: That is corret.
> The source IP address is the IP address of the client, but the client
> here is my real server. It tries to connect to VIP.

Hi Florent,

You've hit a very special corner case.

When you're trying to access your VIP from a real server, your real server does discover the VIP to be a local IP address. As a consequence, the traffic isn't bpassed onto your local network, but routed via loopback and this traffic is not being loadbalanced at all, 100% of this traffic is delivered to the same real server.

If your client application doesn't specify a specific source IP address, the Linux kernel does select an IP address which is (in terms of routing) most close to the destination address. In this special cornercase, it is the very same IP address than the destination address.

You can verify this behaviour by running "ip route get VIP" on your realserver (replace "VIP" by the VIP address). As an example:

$ ip route get
local dev lo  src 
    cache <local> 

This reads as following:
- is locally configured on this host.
- the packet is being routed via the loopback device ("dev lo").
- source IP address will be


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