[libvirt] [PATCH 7/9] qemu: setup tap devices for promiscLinks='no'

Laine Stump laine at laine.org
Mon Nov 24 17:48:35 UTC 2014

In order for the kernel to be able to promiscuous mode on as many
ports of a bridge as possible, at most one attached device can have
learning and unicast_flood enabled (in practice, this one device is
always the physical device that connects the bridge to the real
world). If more than one device has those settings enabled, the kernel
cannot enable promiscuous mode. Since both settings default to
enabled, we need to turn them both off (using
virNetDevBridgeSetLearning() and virNetDevBridgeSetUnicastFlood()) for
every tap device plugged into a bridge that has promiscLinks='no'.

If there is only one device with learning/unicast_flood enabled, then
that device has 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 devices will have promiscuous mode disabled.

Once we have disabled learning and flooding, any packet that has a
destination MAC address not present in the forwarding database (fdb)
will be dropped by the bridge, and libvirt is responsible for adding
entries to the bridge fdb for the MAC address of each guest
interface. That is done with virNetDevBridgeFDBAdd().

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") the following workaround is
required in order for untagged traffic to pass (vlan tagged traffic
will in any case currently not pass with promiscLinks='no')
 src/qemu/qemu_command.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index cbdef9c..d3d129a 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"
@@ -350,6 +351,21 @@ qemuNetworkIfaceConnect(virDomainDefPtr def,
             virDomainAuditNetDevice(def, net, tunpath, false);
             goto cleanup;
+        if (virDomainNetGetActualPromiscLinks(net) == VIR_TRISTATE_BOOL_NO) {
+            /* In order to make as many as possible of the links to a
+             * bridge promiscuous/no-flood, we need to turn off
+             * learning and unicast_flood, and add an fdb entry to the
+             * bridge for this new device.
+             */
+            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,

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