When libvirt is managing the MAC table of a Linux host bridge, it must
turn off learning and unicast_flood for each tap device attached to
that bridge, then add a Forwarding Database (fdb) entry for the tap
device using the MAC address from the domain interface config.
Once we have disabled learning and flooding, any packet that has a
destination MAC address not present in the fdb will be dropped by the
bridge. This, along with the opportunistic disabling of promiscuous
mode[*], can result in enhanced network performance. and a potential
slight security improvement.
[*] If there is only one device on the bridge with learning/unicast_flood
enabled, then that device will automatically have promiscuous mode
disabled. If there are *no* devices with learning/unicast_flood
enabled (e.g. for a libvirt "route", "nat", or isolated network that
has no physical device attached), then all non-tap devices will have
promiscuous mode disabled (tap devices always have promiscuous mode
enabled, which may be a bug in the kernel, but in practice has 0
effect).
None of this has any effect for kernels prior to 3.15 (upstream kernel
commit 2796d0c648c940b4796f84384fbcfb0a2399db84 "bridge: Automatically
manage port promiscuous mode"). Even after that, until kernel 3.17
(upstream commit 5be5a2df40f005ea7fb7e280e87bbbcfcf1c2fc0 "bridge: Add
filtering support for default_pvid") traffic will not be properly
forwarded without manually adding vlan table entries. Unfortunately,
although the presence of the first patch is signalled by existence of
the "learning" and "unicast_flood" options in sysfs, there is no
reliable way to query whether or not the system's kernel has the
second of those patches installed, the only thing that can be done is
to try the setting and see if traffic continues to pass.
---
Change from V2: attribute name and commit log explanation.
src/qemu/qemu_command.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 1831323..11ed58b 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -35,6 +35,7 @@
#include "virerror.h"
#include "virfile.h"
#include "virnetdev.h"
+#include "virnetdevbridge.h"
#include "virstring.h"
#include "virtime.h"
#include "viruuid.h"
@@ -344,6 +345,24 @@ qemuNetworkIfaceConnect(virDomainDefPtr def,
virDomainAuditNetDevice(def, net, tunpath, false);
goto cleanup;
}
+ if (virDomainNetGetActualBridgeMACTableManager(net)
+ == VIR_NETWORK_BRIDGE_MAC_TABLE_MANAGER_LIBVIRT) {
+ /* libvirt is managing the FDB of the bridge this device
+ * is attaching to, so we need to turn off learning and
+ * unicast_flood on the device to prevent the kernel from
+ * adding any FDB entries for it, then add an fdb entry
+ * outselves, using the MAC address from the interface
+ * config.
+ */
+ if (virNetDevBridgePortSetLearning(brname, net->ifname, false) < 0)
+ goto cleanup;
+ if (virNetDevBridgePortSetUnicastFlood(brname, net->ifname, false) < 0)
+ goto cleanup;
+ if (virNetDevBridgeFDBAdd(&net->mac, net->ifname,
+ VIR_NETDEVBRIDGE_FDB_FLAG_MASTER |
+ VIR_NETDEVBRIDGE_FDB_FLAG_TEMP) < 0)
+ goto cleanup;
+ }
} else {
if (qemuCreateInBridgePortWithHelper(cfg, brname,
&net->ifname,
--
1.9.3
--
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