[lvs-users] Strange error (bug?) with source NAT

Michael Schwartzkopff ms at sys4.de
Thu Dec 10 13:01:33 GMT 2015


Hi,

For a test I set up  a LVS that should balance a telnet service inside a LAN.

So the client IP ist 192.168.0.1
the virtual IP is 192.168.0.2

and the real servers IPs are 192.168.0.3 and 4.

I prepared my loadbalancer:
echo "1" > /proc/sys/net/ip4/ip_forward
echo "1" > /proc/sys/net/ipv4/vs/conntrack

and added:

iptables -t nat -I POSTROUTING -p tcp -s 192.168.0.0/24 -d 192.168.0.3 \
  --destination-port=23 -j MASQUERADE
iptables -t nat -I POSTROUTING -p tcp -s 192.168.0.0/24 -d 192.168.0.4 \
  --destination-port=23 -j MASQUERADE

the ipvs config is:

ipvsadm -A -t 192.168.0.2:23 -s rr
ipvsadm -a -t 192.168.0.2:23 -r 192.168.0.3:23 -m -w 1
ipvsadm -a -t 192.168.0.2:23 -r 192.168.0.4:23 -m -w 1

Now a strange thing happens. A telnet fron the client does NOT work

# telnet 192.168.0.2

UNTIL I start tcpdump on the loadbalacer. When I watch the traffic everything 
works. I do not watch, telnet fails. The behavoiur is absolutely reproducable.

Oh, the interface of my loadbalancer ist a bridge br0 over the single 
interface eth0.

Did anybody implement some quantum computing in the ipvs module?
Anybody has a explanation for that behaviour?


Mit freundlichen Grüßen,

Michael Schwartzkopff

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