br0和tap0的互相影响

转载
http://www.cnblogs.com/wlei/archive/2012/12/15/2819429.html

在实际工作中碰到一个问题：设备有两个网口，当把这两个网口桥接到br0上时，之后如果设置了mac地址，就会发现数据包不能正常收发。

如下是相关解释

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转自（http://blog.csdn.net/fanwenbo/article/details/2131193）

先说现象

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;
           
          。。。。。 
          //here ! copy MAC addr to br->preferred_id.addr
            memcpy(br->preferred_id.addr, sa->sa_data, ETH_ALEN);
         
            br_stp_recalculate_bridge_id(br);
         
          。。。。。
          }

 
  - br_stp_if.c
      static unsigned char br_mac_zero[6] = {0,0,0,0,0,0};
     
      /* called under bridge lock */
      void br_stp_recalculate_bridge_id(struct net_bridge *br)
      {
        unsigned char *addr;
        struct net_bridge_port *p;
     
        //初始br0的MAC为00:00:00:00:00:00
        addr = br_mac_zero;
     
        p = br->port_list;
        while (p != NULL) {
          /* match against preferred address first */
          if (memcmp(p->dev->dev_addr, br->preferred_id.addr, ETH_ALEN) == 0) {
            addr = p->dev->dev_addr;
            //匹配port的MAC地址与首选MAC是否相符
            break;
          }
         
          if (addr == br_mac_zero ||
              memcmp(p->dev->dev_addr, addr, ETH_ALEN) < 0)
          //寻找MAC最小的那一个
            addr = p->dev->dev_addr;
     
          p = p->next;
        }
     
        //如果没有Port的话，就为0；
        //如果没有设置首选MAC，就等于最小的那一个。
        //如果设置了首选MAC，则首选MAC必须同其中一个port的MAC匹配，才等于首选MAC
        if (memcmp(br->bridge_id.addr, addr, ETH_ALEN))
          br_stp_change_bridge_id(br, addr);
      }
 
  - 什么时候会执行br_stp_recalculate_bridge_id呢
      - in br_device.c 中 br_set_mac_address()
      - in br_if.c 中 br_del_if() & br_add_if()
      - in br_notify.c 中 br_devic_event()的 NETDEV_CHANGEADDR 事件，该事件是任意port有修改addr就会触发的
     
     
看过代码就很容易理解bridge在处理自身MAC地址时的行为了

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针对这种情况，目前采用的方法就是先去设置网口的mac地址，然后再桥接成br0,这样mac地址就不需要操心了，网桥端口的mac地址设置时需要留意一下。