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I have a strange networking problem with Windows 2008 server R2, running as guest under KVM-Qemu host. Host is CentOS 6.3 x86_64. It's network settings:

# cat /etc/sysconfig/network-scripts/ifcfg-br0

# cat /etc/sysconfig/network-scripts/ifcfg-eth0

# cat /etc/sysconfig/network

# cat /etc/sysctl
net.ipv4.ip_forward = 1   # tried to set it to 0 without any changes
net.ipv4.conf.default.rp_filter = 1   # tried to set it to 0 without any changes
net.ipv4.conf.default.accept_source_route = 0   # tried to set it to 1 without any changes
kernel.sysrq = 0
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 68719476736
kernel.shmall = 4294967296

# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
xx.xx.xx.0 U     0      0        0 br0     U     1004   0        0 br0         xx.xx.xx.1         UG    0      0        0 br0

Node IP is xx.xx.xx.4, guest IP is xx.xx.xx.24, both host and guest is in the same network (/26). There are several linux guest running fine on the node (centos, debian, ubuntu, arch), and even Windows 2003 x86 also running fine. But Win2008 does not. I wonder, what's the difference. From Win2008 guest I can ping nothing: neither gateway, nor any other IP, even they are in the same subnet. From outside I also cannot ping guest. Almost. If I ping it from another server in same subnet, it's barely pinging, losing more than 90% packets. Firewall on the guest is completely off. Tried to set up network manually as well as via DHCP without success (BTW, DHCP set up network settings correctly). I suspect that is a kind of routing problem, but I spent whole day and still cannot figure it out. I would be appreciate for any help.

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up vote 1 down vote accepted

Finally, I figured it out myself. The problem was in Proxy ARP setting in sysctl. It's turned off by default. I just added in /etc/sysctl.conf the string as follows:

net.ipv4.conf.all.proxy_arp = 1

and it works perfect now.

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