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Re: [Qemu-devel] [PATCH] qemu-bridge-helper: force usage of a very high


From: Corey Bryant
Subject: Re: [Qemu-devel] [PATCH] qemu-bridge-helper: force usage of a very high MAC address for the bridge
Date: Fri, 22 Mar 2013 17:37:08 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4



On 03/22/2013 12:57 PM, Paolo Bonzini wrote:
Linux uses the lowest enslaved MAC address as the MAC address of
the bridge.  Set MAC address to a high value so that it does not
affect the MAC address of the bridge.

Changing the MAC address of the bridge could cause a few seconds
of network downtime.

Cc: address@hidden
Signed-off-by: Paolo Bonzini <address@hidden>
---
  qemu-bridge-helper.c | 18 ++++++++++++++++++
  1 file changed, 18 insertions(+)

diff --git a/qemu-bridge-helper.c b/qemu-bridge-helper.c
index 287bfd5..6a0974e 100644
--- a/qemu-bridge-helper.c
+++ b/qemu-bridge-helper.c
@@ -367,6 +367,24 @@ int main(int argc, char **argv)
          goto cleanup;
      }

+    /* Linux uses the lowest enslaved MAC address as the MAC address of
+     * the bridge.  Set MAC address to a high value so that it doesn't
+     * affect the MAC address of the bridge.
+     */
+    if (ioctl(ctlfd, SIOCGIFHWADDR, &ifr) < 0) {
+        fprintf(stderr, "failed to get MAC address of device `%s': %s\n",
+                iface, strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+    ifr.ifr_hwaddr.sa_data[0] = 0xFE;
+    if (ioctl(ctlfd, SIOCSIFHWADDR, &ifr) < 0) {
+        fprintf(stderr, "failed to set MAC address of device `%s': %s\n",
+                iface, strerror(errno));
+        ret = EXIT_FAILURE;
+        goto cleanup;
+    }
+
      /* add the interface to the bridge */
      prep_ifreq(&ifr, bridge);
      ifindex = if_nametoindex(iface);


Is it desirable to change a mac address under the covers? I know you mentioned that libvirt does something like this already.

Also it seems like this might be better if it was optional. qemu-bridge-helper can take command line options like this:

-net tap,helper="/usr/local/libexec/qemu-bridge-helper --br=br0"

Perhaps adding a --macaddr option is a better approach?

--
Regards,
Corey Bryant




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