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Asked: 2012-04-07 06:56:00 +0800 CST

Routing table with two NIC adapters in libvirt/KVM

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I created a virtual NAT network (192.168.100.0/24 network) in my libvirt and new guest with two interfaces - one in this network, one as bridged (10.34.1.0/24 network) to the local LAN. The reason for that is I need to have my own virtual network for my DHCP/TFTP/DNS testing and still want to access my guest externally from my LAN. On both networks I have working DHCP, both giving them IP addresses.

When I setup NAT port forwarding (e.g. for ssh), I can connect to the eth0 (virtual network), everything is fine.

But when I try to access the eth1 via bridged interface, I have no response. I guess I have problem with my routing table - outgoing packets are routed to the virtual NAT network (which has access to the machine I am connecting from - I can ping it). But I am not sure if this setup is correct. I think I need to add something to my routing table.

# ifconfig 
eth0      Link encap:Ethernet  HWaddr 52:54:00:B4:A7:5F  
          inet addr:192.168.100.14  Bcast:192.168.100.255  Mask:255.255.255.0
          inet6 addr: fe80::5054:ff:feb4:a75f/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:16468 errors:0 dropped:27 overruns:0 frame:0
          TX packets:6081 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:22066140 (21.0 MiB)  TX bytes:483249 (471.9 KiB)
          Interrupt:11 Base address:0x2000 

eth1      Link encap:Ethernet  HWaddr 52:54:00:DE:16:21  
          inet addr:10.34.1.111  Bcast:10.34.1.255  Mask:255.255.255.0
          inet6 addr: fe80::5054:ff:fede:1621/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:34 errors:0 dropped:0 overruns:0 frame:0
          TX packets:189 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:4911 (4.7 KiB)  TX bytes:9

# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.100.0   0.0.0.0         255.255.255.0   U     0      0        0 eth0
10.34.1.0       0.0.0.0         255.255.255.0   U     0      0        0 eth1
169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1003   0        0 eth1
0.0.0.0         192.168.100.1   0.0.0.0         UG    0      0        0 eth0

Network I am trying to connect from is different than network the hypervisor is connected to: 10.36.0.0. But it is accessible from that network. So I tried to add new route rule:

route add -net 10.36.0.0 netmask 255.255.0.0 dev eth1

And it is not working. I thought setting correct interface would be sufficient. What is needed to get my packets coming through?

linux redhat kvm-virtualization libvirt qemu

1 Answers

  1. Best Answer

    So the issue was really in the NAT. I changed my default route to be on the second (bridged) interface and it worked. Btw if you really need two default routes, this is possible:

    http://kindlund.wordpress.com/2007/11/19/configuring-multiple-default-routes-in-linux/

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