浅析linux网桥设置MAC地址时的行关
linux 网桥 MAC
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
