在實際工作中碰到一個問題：設備有兩個網口，當把這兩個網口橋接到br0上時，之后如果設置了mac地址，就會發現數據包不能正常收發。
如下是相關解釋
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轉自（http://blog.csdn.net/fanwenbo/article/details/2131193）
先說現象
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
結論：
br0如果沒有指定hw MAC, br0的MAC地址會根據bridge中port的變化，自動選擇port中最小的一個MAC地址作為br0的MAC地址。
br0只能指定port中有的interface的MAC作為br0的MAC地址。
源代碼分析：
source code dir is: linux-2.4.x/net/bridge
- br_device.c
br_dev_setup() 注冊了一些函數，其中 dev->set_mac_address = br_set_mac_address; //這個就是ifconfig br0 hw ether調用的函數了
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為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地址與首選MAC是否相符
break;
}
if (addr == br_mac_zero ||
memcmp(p->dev->dev_addr, addr, ETH_ALEN) < 0)
//尋找MAC最小的那一個
addr = p->dev->dev_addr;
p = p->next;
}
//如果沒有Port的話，就為0；
//如果沒有設置首選MAC，就等於最小的那一個。
//如果設置了首選MAC，則首選MAC必須同其中一個port的MAC匹配，才等於首選MAC
if (memcmp(br->bridge_id.addr, addr, ETH_ALEN))
br_stp_change_bridge_id(br, addr);
}
- 什么時候會執行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，該事件是任意port有修改addr就會觸發的
看過代碼就很容易理解bridge在處理自身MAC地址時的行為了
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針對這種情況，目前采用的方法就是先去設置網口的mac地址，然后再橋接成br0,這樣mac地址就不需要操心了，網橋端口的mac地址設置時需要留意一下。