linux网桥设置MAC地址时的行为

brctl addbr br0
ifconfig br0
br0 MAC is 00:00:00:00:00:00
brctl addif br0 eth1  (eth1 is xx:xx:xx:xx:xx:33)
ifconfig br0
br0 MAC is xx:xx:xx:xx:xx:33  same as eth1, auto change
brctl addif br0 eth2  (eth2 is xx:xx:xx:xx:xx:30)
ifconfig br0
br0 MAC is xx:xx:xx:xx:xx:30  same as eth2, For eth2 less than eth1, auto change
ifconfig eth2 hw ether xx:xx:xx:xx:xx:50
ifconfig br0
br0 MAC is xx:xx:xx:xx:xx:33  same as eth1, auto change
ifconfig br0 hw ether xx:xx:xx:xx:xx:99
ifconfig br0
br0 MAC is xx:xx:xx:xx:xx:33  same as eth1, NOT change
ifconfig br0 hw ether xx:xx:xx:xx:xx:33  ;same as eth1
ifconfig eth2 hw ether xx:xx:xx:xx:xx:20  ;less than eth1
ifconfig br0
br0 MAC is xx:xx:xx:xx:xx:33  same as eth1, `ifconfig br0 hw` NOT effective
ifconfig eth1 hw ether xx:xx:xx:xx:xx:50  ;upper op,we set br0 = eth1's MAC,now we change eth1 MAC
ifconfig br0
br0 MAC is xx:xx:xx:xx:xx:20  same as eth2, auto change
结论：
  br0如果没有指定hw MAC, br0的MAC地址会根据bridge中port的变化，自动选择port中最小的一个MAC地址作为br0的MAC地址。
  br0只能指定port中有的interface的MAC作为br0的MAC地址。
 
源代码分析：
  source code dir is: linux-2.4.x/net/bridge
 
  - br_device.c
      br_dev_setup() 注册了一些函数，其中  dev->set_mac_address = br_set_mac_address;  //这个就是ifconfig br0 hw ether调用的函数了
     
      static int  br_set_mac_address(struct net_device *dev, void *addr)
          {
            struct net_bridge *br = dev->priv;
            struct sockaddr *sa = (struct sockaddr *) addr;
           
          。。。。