Linux Ethernet Bridging - May 2009 - [Bridge] 2.6.27: persistent MAC assignment to bridge not working

Hi All,

I am trying to figure out how to get a persistent, non-changing MAC
address assigned to a bridge. It looks like there is new functionality
in 2.6.27 to allow this to happen, specifically, in:

void br_stp_recalculate_bridge_id(struct net_bridge *br)
[...]
       /* user has chosen a value so keep it */
        if (br->flags & BR_SET_MAC_ADDR)
                return;

and in..

static int br_set_mac_address(struct net_device *dev, void *p)
[...]
        br->flags |= BR_SET_MAC_ADDR;

However, I cannot get this functionality to work as it appears it
should. If I manually set a bridge's MAC address to an arbitrary
value, as follows:

# ifconfig br0 hw ether "xx:xx:xx:xx:xx:xx"

Then, subsequent calls such as the following seem to still change the
bridge's MAC address:

# brctl addif br0 veth100.0
# brctl delif br0 veth100.0

Am I doing something wrong? What is the proper way to take advantage
of the new code in 2.6.27?

The only mechanism that I have found that appears to achieve my
desired result is application of Deitmar Maurer's 2.6.24 patch which,
after being applied, seems to disable the dynamic nature of the
bridge's MAC, so that br0 persistently uses the MAC of the first
interface that was added to the bridge. Here is this patch that seems
to be working for me:

--- linux-2.6.24-openvz.org/net/bridge/br_stp_if.c.org  2008-06-11
09:15:16.000000000 +0200
+++ linux-2.6.24-openvz.org/net/bridge/br_stp_if.c      2008-06-11
09:44:53.000000000 +0200
@@ -217,10 +217,7 @@
       struct net_bridge_port *p;

       list_for_each_entry(p, &br->port_list, list) {
-               if (addr == br_mac_zero ||
-                   memcmp(p->dev->dev_addr, addr, ETH_ALEN) < 0)
-                       addr = p->dev->dev_addr;
-
+               addr = p->dev->dev_addr;
       }

       if (compare_ether_addr(br->bridge_id.addr, addr))
--------------------------

However, when I look at the 2.6.27 source code, it seems like this
patch should no longer be needed and an "ifconfig br0 hw ether
"xx:xx:xx:xx:xx:xx" should be sufficient. Am I doing something wrong?

Thanks for your time,

Daniel Robbins

On Fri, 8 May 2009 15:31:43 -0600
Daniel Robbins <drobbins at funtoo.org> wrote:
> Hi All,
> 
> I am trying to figure out how to get a persistent, non-changing MAC
> address assigned to a bridge. It looks like there is new functionality
> in 2.6.27 to allow this to happen, specifically, in:
> 
> void br_stp_recalculate_bridge_id(struct net_bridge *br)
> [...]
>        /* user has chosen a value so keep it */
>         if (br->flags & BR_SET_MAC_ADDR)
>                 return;
> 
> and in..
> 
> static int br_set_mac_address(struct net_device *dev, void *p)
> [...]
>         br->flags |= BR_SET_MAC_ADDR;
> 
> However, I cannot get this functionality to work as it appears it
> should. If I manually set a bridge's MAC address to an arbitrary
> value, as follows:
> 
> # ifconfig br0 hw ether "xx:xx:xx:xx:xx:xx"
> 
> Then, subsequent calls such as the following seem to still change the
> bridge's MAC address:
> 
> # brctl addif br0 veth100.0
> # brctl delif br0 veth100.0
> 
> Am I doing something wrong? What is the proper way to take advantage
> of the new code in 2.6.27?
> 
> The only mechanism that I have found that appears to achieve my
> desired result is application of Deitmar Maurer's 2.6.24 patch which,
> after being applied, seems to disable the dynamic nature of the
> bridge's MAC, so that br0 persistently uses the MAC of the first
> interface that was added to the bridge. Here is this patch that seems
> to be working for me:
> 
> --- linux-2.6.24-openvz.org/net/bridge/br_stp_if.c.org  2008-06-11
> 09:15:16.000000000 +0200
> +++ linux-2.6.24-openvz.org/net/bridge/br_stp_if.c      2008-06-11
> 09:44:53.000000000 +0200
> @@ -217,10 +217,7 @@
>        struct net_bridge_port *p;
> 
>        list_for_each_entry(p, &br->port_list, list) {
> -               if (addr == br_mac_zero ||
> -                   memcmp(p->dev->dev_addr, addr, ETH_ALEN) < 0)
> -                       addr = p->dev->dev_addr;
> -
> +               addr = p->dev->dev_addr;
>        }
> 
>        if (compare_ether_addr(br->bridge_id.addr, addr))
> --------------------------
> 
> However, when I look at the 2.6.27 source code, it seems like this
> patch should no longer be needed and an "ifconfig br0 hw ether
> "xx:xx:xx:xx:xx:xx" should be sufficient. Am I doing something
wrong?
> 
> Thanks for your time,
> 
> Daniel Robbins
It was fixed by:

Author: Stephen Hemminger <shemminger at vyatta.com>  2008-06-17 16:10:06
Committer: David S. Miller <davem at davemloft.net>  2008-06-17 16:10:06
Parent: 0b040829952d84bf2a62526f0e24b624e0699447 (net: remove CVS keywords)
Branches: master, remotes/origin/master
Follows: v2.6.26-rc6
Precedes: v2.6.27-rc1

    bridge: make bridge address settings sticky
    
    Normally, the bridge just chooses the smallest mac address as the
    bridge id and mac address of bridge device. But if the administrator
    has explictly set the interface address then don't change it.
    
    Signed-off-by: Stephen Hemminger <shemminger at vyatta.com>
    Signed-off-by: David S. Miller <davem at davemloft.net>