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Re: [libvirt-users] Need information



On 05/22/2016 12:44 AM, Gk Gk wrote:
Hi,

in bridge mode all the guests and hosts should be in the same subnet always ?

(please don't top-post replies)

Your new question is irrelevant both to your original question and to my response. Yes, it is usually the case that the purpose of using a bridge-mode connection is to have the guests on the same subnet as the machines on the physical network, although that isn't necessarily the case - a bridge device doesn't care (or even understand) IP; it just forwards according to learned information about MAC addresses, and if someone wanted, they could put multiple IP subnets on the same bridge.

But you asked why it could be the case that a bridge wouldn't accept packets from a guest that has a public IP. First, as I say above, the IP address is irrelevant when talking about a filter based on MAC address. Second, as I said in my last messages, any managed switch can have each port configured to only accept packets with certain MAC addresses, and many/most hosting providers (and many corporate IT departments) program the ports of their switches to only accept traffic with the source MAC address of a single machine (they do this to prevent hostile hosts spoofing the MAC addresses of other hosts) - if you have a bridge setup between your guest and your physical host, the guest traffic sent to the switch will still have the guest interface's MAC address, which the switch may reject.

If, on the other hand, you use a routed setup, the guest traffic will go through the host's IP routing, and reemerge from the guest with the *host's* MAC address. So it will then at least pass the MAC address filter on the bridge.

But, as I said in my last message, the hosting provider's network would then 1) need to accept traffic from the guest's IP address, and 2) need to have an entry in the routing tables of its routers pointing to your host for the subnet you've defined for your guests. It is *highly* unlikely that any hosting provider would do this for you, since IPv4 address space is at such a premium. It's more likely that they would allow you to register extra  MAC addresses.


If this still doesn't make sense, I suggest you read the following two wikipedia entries:

https://en.wikipedia.org/wiki/Bridging_%28networking%29
https://en.wikipedia.org/wiki/IP_forwarding




Thanks
Kumar

On Sun, May 22, 2016 at 12:41 AM, Laine Stump <laine laine org> wrote:
On 05/20/2016 03:21 AM, Gk Gk wrote:
Hi,

Referring to the link http://wiki.libvirt.org/page/VirtualNetworking,  the scenario  described for the routed mode, explains that 

"Host has public IP and virtual machines have static public IPs. But one can't use bridged networking, since provider accept only packets from the MAC address of the host"

Can someone explain to me why doesn't the switch accept the packets from the guest vms' mac addresses also since they have public IPs  in the bridged mode ?

Because the people who have administrative control over the switch have configured it that way. (Of course, if they're that restrictive, it's doubtful that they would allocate an entire subnet to a customer's machine, and reconfigure their routing tables to deal with it).




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