[Bug 684424] Re: Bridges are not transparent for packets

[Thomas Schweikle]

Setting promiscuous mode for all devices connected to the bridge does
not help. Trying to ping any system from the guest only shows the system
not reachable.

This error is, in my opinion, not only "an annoying" bug! It renders
kvm, qemu, xen, ... useless. It is something that has to be fixed!

The idea
"route add default gw 192.168.1.4"

and then trying
"ping google.com" succeeds.

But
"ping 192.168.1.1" does not.

Quite clear: 192.168.1.1 lies within network 192.168.1.0/26! The host
*can* reach the gateway --- no routing necessary (for "google.com" it
was necessary: the answering host does not match the netmask. Packets go
out to 192.168.1.4, which in tune routes them to 192.168.1.1, which in
tune routes them to google.com --- but we're just avoiding the bridge
because it does not work as expected). Because icmp-messages "Who has"
do not make their way trough the bridge there will never be an answer.
Quite correct from the clients standpoint to tell "host not available"!

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https://bugs.launchpad.net/bugs/684424

Title:
  Bridges are not transparent for packets