Centos 7 KVM IP forwarding problem

[suityou01]

I have a Centos 7 server with working wired LAN connection.

Internet access all good.

I have installed KVM, and the Hypervisor has a correctly configured NAT connection, with DHCP.

I have a windows 7 guest, running the VirtIo drivers.

From the guest I can ping the host, and google. However the google pings do not get a response so I am suspecting IP forwarding to be the issue.

According to what I have read from CEntos wiki, ip forwarding is configured correctly.

Iptables and firewall are off. selinux is off.

Linux isn't my strong suit so please help.

[suityou01]

Cheers SWB.

I'm certain it's me being an idiot.

And what smells bad is that I originally set up a bridged adaptor and turned it off. After all I'm using NAT, right?

Then what I read is that I need a bridged adaptor regardless of using bridged or NAT.

To get an answer on that alone would give me a nudge in the right direction.

Trust me, google has been my friend for many nights before scampering on here.

[suityou01]

Back on site this morning. Google has not been my friend on this one as I have found equal amounts of web sites, one saying yes you need a bridged adaptor, others say not.

Of course I have tried with and without but nothing works.

If I could just get a steer on whether KVM NAT requires a bridging adapter to do NAT then it would focus my googling somewhat.

Anyone?

[stek]

did u say the host is natted and uses DHCP? I reckon you set the guest to bridged adapter and let i get its own natted ip from the dhcp server

[suityou01]

did u say the host is natted and uses DHCP? I reckon you set the guest to bridged adapter and let i get its own natted ip from the dhcp server

Yes.

So I managed to configure a NAT virtual network for the hypervisor. The only problem is now that the guests don't get a DHCP address.

DHCP is configured correctly as far as I can see in KVM.

I turned off iptables again in case it was blocking DHCP requests.

So it seems KVM creates it's on virtual bridge when you create the virtual natted network through the KVM front end. And seemingly this configuration is creates looks ok, it has a DHCP section in the config file.

But the client doesn't get an IP address.

One could really go off Linux, but only a bad workman ....

[stek]

Yes.

So I managed to configure a NAT virtual network for the hypervisor. The only problem is now that the guests don't get a DHCP address.

DHCP is configured correctly as far as I can see in KVM.

I turned off iptables again in case it was blocking DHCP requests.

So it seems KVM creates it's on virtual bridge when you create the virtual natted network through the KVM front end. And seemingly this configuration is creates looks ok, it has a DHCP section in the config file.

But the client doesn't get an IP address.

One could really go off Linux, but only a bad workman ....

Whats issuing DHCP ips? The host or another DHCP server in the network?

[suityou01]

Whats issuing DHCP ips? The host or another DHCP server in the network?

The host. AFAIU it uses DNSMasq.

I set up the DHCP range to be 10.0.0.2 - 254.

I have since deleted the Virtual network and started over.

When I tell it the IP address is 10.0.0.1/24 it then defaults the start of the DHCP range to 10.0.0.128. Lawd knows why.

So I left it alone as it knows far better than me. And now it works.

The odd thing is though that the first guest gets .163 as a lease

[suityou01]

And it's busted again

My mistake was going for lunch.

Installed windows updates on the VM and rebooted and now it's getting 169.xxx as the ip address

I think KVM is flaky

[stek]

I think you should set up the Host to get its IP from DHCP on the local network and the Guest to do the same, if you are setting up DCHPD on the host and there's another DHCPD server on the network it can see it's a world of pain with DHCP Scopes, option 66 etc and helpers on routers......

Been there with a replacement xCAT server that gave out DHCP to build Linux HPC nodes, had to get the network guys to help with the 'helper' business...

I'm assuming this a box u have plugged into ClientCos network?

[suityou01]

Have you turned off netmanager? I would reboot the dhcpd service as well.

Yes, spot on. Netmanager is the curse of RHEL and it's derivatives it seems.

It seems to happen like this.

Boot machine. Network service fails to start. However the em2 ethernet interface still comes up.

Then in libvirt, the virbr01 (virtual bridge that does the nat stuff) fails to come up also.

Then start the network service by CLI and it all comes back online. At least that was how it seemed.

Now virbr01 won't come back online at all.

Failed to start the interface. Unspecified error. Interface virbr01 failed to become active. Possible disconnected cable.

So I am now troubleshooting an "unspecified error". The programmer that wrote that error message needs to be taken out the back and shot.