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The routing table with two nics may be messed up, due to we sent same router(gateway) information from different DHCP server, in order to specify default gateway. E.g. Network A: 192.168.1.0/24, gw 192.168.1.1 Network B: 192.168.2.0/24, gw 192.168.2.1 User VM: Nic 1 connect to network A, get ip 192.168.1.10; nic 2 connect to network B, get ip 192.168.2.10. Set network A as the default network of user VM. Currently we would send this information to user VM through DHCP offer: In network A: dhcp-option:router 192.168.1.1 In network B: dhcp-option:router 192.168.1.1 So both NIC in the guest VM would receive 192.168.1.1 as router(gateway). But, in CentOS 5.6, dhclient-scripts try to tell if the gateway is reachable for current subnet. So when we try to enable nic 2(eth1) of user VM, dhclient would receive: IP: 192.168.2.10 Mask: 255.255.255.0 Router: 192.168.1.1 Then it would found that the specified gateway(router) is not within its own subnet(192.168.2.0/24). But since we send out this ip(192.168.1.1) as the gateway for it, dhclient thought that it should got someway to access the network through this IP. So it would execute: ip route add 192.168.1.1 dev eth1 ip route replace default via 192.168.1.1 dev eth1 But it can never reach 192.168.1.1(which is in the eth0's subnet and the gateway of eth0) by go through eth1 interface. So it is messed up. We've tested Windows 2008 R2, CentOS 5.3, CentOS 5.6 and Ubuntu 10.04. Windows and Ubuntu are fine with above policy. To solve this, we send different dhcp:router option according to the guest OS type now. We may need expand this list later, but for now we only know that CentOS and RHEL would behavior in this way. status 14042: resolved fixed