Ross Boylan wrote:
On Thu, 2009-05-07 at 11:13 -0600, Cam Macdonell wrote:
Ross Boylan wrote:
I'm trying to understand bridging with KVM, but am still puzzled.
I think that the recommended bridging with TAP means that packets
from
the VM will end up going out the host card attached to the default
gateway. But it looks to me as if their IP address is unchanged,
which
means replies will never reach me. Is that correct? Do I need to
NAT
the packets, or is something already doing that?
Hi Ross,
This is the place to start
http://www.linux-kvm.org/page/Networking.
I saw that; it gives some recipes but I wasn't sure what their effect
was.
You want a public bridge.
I'm not sure what "their" and "me" mean in your email. In short,
with
bridging each VM has its own IP and that VM can be accessed directly
from the network.
"their" = the VM.
"me" = my host machine.
So if the VM's are running on their own subnet,
VMs do not run on their own subnet with bridged networking.
e.g., 10.0.2.* (I've
been assuming the subnet with TAP is like the one with the User Mode
Network stack in 3.7.3 of http://www.nongnu.org/qemu/qemu-doc.html) and
my host machine is on another net, e.g., 10.0.8.* then I think the
packet will go out with an IP of 10.0.2.2 (say). When some other
machine tries to reply to 10.0.2.2, the packet gets lost because the
outside network thinks 10.0.2.* is not for it. At least that's my
concern. If the return IP address on the packet were 10.0.8.44
(supposing that's the IP of my host machine) then the packets could find
their way back.
Using bridged networking is very different from the user stack. The
user stack is extremely limited and slow.
My host machine is on an internal network with a 10.* IP. The example
might be clearer if one supposed that the VM's were on a 192.168.*
network.
I am perhaps being influenced by the fact that I don't want to ask for
more IP's, so I don't want to configure the VM's to use an IP on our
10.0.8 network.
Then you probably want to use a NAT network. A NAT setup puts all the
VMs on their own network within the host machine. iptables is necessary
to forward the subnet packets out to the world and back.
Here is some older documentation, but not much has changed. Look at the
first entry under "Advanced Networking".
https://help.ubuntu.com/community/KVMFeisty
Does the TAP networking setup a whole subnet like the user mode network
stack (e.g., running a DHCP server), or is the idea that I would just
give the VM an IP on my subnet (10.0.8.*) in this example?
No, bridge networking using taps (one tap per VM) and effectively sits
all the VMs on the same network your host is on. You would need to get
IPs from sysadmin for each VM.
If the latter is the case (I'm now suspecting it is) then I think the
solution is clear. I just stick the VM's on a private (to my machine)
subnet, like 192.168.*, and I do NAT on the packets as they go out.
NAT is a very common solution. Use VDE (vde.sourceforget.net) to create
a virtual switch on your host for the VMs. dnsmasq can serve dynamic
IPs to the VMs on their own subnet that doesn't bother your sysadmin at
all. Use iptables to forward and receive packets through your host's
NIC.
Cam
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
