网络 – 设置组播mac地址

2.组播mac地址映射？
组播的mac地址： IANA规定，IPv4组播MAC地址的高24位为0x01005E，第25位为0，低23位为IPv4组播地址的低23位。

3.如何判断该组播包是自己的？
   发送端发送组播包时，目的mac地址是根据组播ip和固定格式进行映射，并没有指向固定目标主机，向网络上进行发送。
   接收端接收组播包时，同样需要对组播ip和固定格式进行映射，生成的组播mac地址与接收到的数据包mac比对，若相通往上传。
   接收端同样可以设置混杂模式：接收所有经过它的数据流，这样不论其目的地址是否是他，都会往上传，由网络层进行判断，确定是递交上层（传输层），还是丢弃。

4.网卡驱动中如何设置组播mac地址？

以下是linux中9x25设置组播mac地址的部分代码

static int inline hash_bit_value(int bitnr, __u8 *addr)
{
        if (addr[bitnr / 8] & (1 << (bitnr % 8)))
                return 1;
        return 0;
}

/*
 * The hash address register is 64 bits long and takes up two locations in the memory map.
 * The least significant bits are stored in EMAC_HSL and the most significant
 * bits in EMAC_HSH.
 *
 * The unicast hash enable and the multicast hash enable bits in the network configuration
 *  register enable the reception of hash matched frames. The destination address is
 *  reduced to a 6 bit index into the 64 bit hash register using the following hash function.
 * The hash function is an exclusive or of every sixth bit of the destination address.
 *   hash_index[5] = da[5] ^ da[11] ^ da[17] ^ da[23] ^ da[29] ^ da[35] ^ da[41] ^ da[47]
 *   hash_index[4] = da[4] ^ da[10] ^ da[16] ^ da[22] ^ da[28] ^ da[34] ^ da[40] ^ da[46]
 *   hash_index[3] = da[3] ^ da[09] ^ da[15] ^ da[21] ^ da[27] ^ da[33] ^ da[39] ^ da[45]
 *   hash_index[2] = da[2] ^ da[08] ^ da[14] ^ da[20] ^ da[26] ^ da[32] ^ da[38] ^ da[44]
 *   hash_index[1] = da[1] ^ da[07] ^ da[13] ^ da[19] ^ da[25] ^ da[31] ^ da[37] ^ da[43]
 *   hash_index[0] = da[0] ^ da[06] ^ da[12] ^ da[18] ^ da[24] ^ da[30] ^ da[36] ^ da[42]
 * da[0] represents the least significant bit of the first byte received, that is, the multicast/
 *  unicast indicator, and da[47] represents the most significant bit of the last byte
 *  received.
 * If the hash index points to a bit that is set in the hash register then the frame will be
 *  matched according to whether the frame is multicast or unicast.
 * A multicast match will be signalled if the multicast hash enable bit is set, da[0] is 1 and
 *  the hash index points to a bit set in the hash register.
 * A unicast match will be signalled if the unicast hash enable bit is set, da[0] is 0 and the
 *  hash index points to a bit set in the hash register.
 * To receive all multicast frames, the hash register should be set with all ones and the
 *  multicast hash enable bit should be set in the network configuration register.
 */

/*
 * Return the hash index value for the specified address.
 */
static int hash_get_index(__u8 *addr)
{
        int i, j, bitval;
        int hash_index = 0;

        for (j = 0; j < 6; j++) {
                for (i = 0, bitval = 0; i < 8; i++)
                        bitval ^= hash_bit_value(i*6 + j, addr);

                hash_index |= (bitval << j);
        }

        return hash_index;
}

/*
 * Add multicast addresses to the internal multicast-hash table.
 */
static void at91ether_sethashtable(struct net_device *dev)
{
        struct netdev_hw_addr *ha;
        unsigned long mc_filter[2];
        unsigned int bitnr;

        mc_filter[0] = mc_filter[1] = 0;

        netdev_for_each_mc_addr(ha, dev) {
                bitnr = hash_get_index(ha->addr);
                mc_filter[bitnr >> 5] |= 1 << (bitnr & 31);
        }

        at91_emac_write(AT91_EMAC_HSL, mc_filter[0]);
        at91_emac_write(AT91_EMAC_HSH, mc_filter[1]);
}

/*
 * Enable/Disable promiscuous and multicast modes.
 */
static void at91ether_set_multicast_list(struct net_device *dev)
{
        unsigned long cfg;

        cfg = at91_emac_read(AT91_EMAC_CFG);

        if (dev->flags & IFF_PROMISC)                        /* Enable promiscuous mode */
                cfg |= AT91_EMAC_CAF;
        else if (dev->flags & (~IFF_PROMISC))                /* Disable promiscuous mode */
                cfg &= ~AT91_EMAC_CAF;

        if (dev->flags & IFF_ALLMULTI) {                /* Enable all multicast mode */
                at91_emac_write(AT91_EMAC_HSH, -1);
                at91_emac_write(AT91_EMAC_HSL, -1);
                cfg |= AT91_EMAC_MTI;
        } else if (!netdev_mc_empty(dev)) { /* Enable specific multicasts */
                at91ether_sethashtable(dev);
                cfg |= AT91_EMAC_MTI;
        } else if (dev->flags & (~IFF_ALLMULTI)) {        /* Disable all multicast mode */
                at91_emac_write(AT91_EMAC_HSH, 0);
                at91_emac_write(AT91_EMAC_HSL, 0);
                cfg &= ~AT91_EMAC_MTI;
        }

        at91_emac_write(AT91_EMAC_CFG, cfg);
}

更多精彩内容