IP 组播

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OSFP 协议会利用组播地址来发送hello 和 keep live 消息

224.0.0.5 all ospf router

224.0.0.6 designated router(DR)

224.0.0.1特指所有主机

224.0.0.2特指所有路由器

IP 组播需要层二数据链路层的支持

默认情况下ethernet 网卡能够接受单播(目标MAC地址为当前网卡的MAC地址)和广播MAC地址(FF:FF:FF:FF:FF:FF)全一。

如果希望当前网卡能够接收来自某个组的包，则需要把当前网卡加入某个组这样就能接收来自某个组的包。

如下系统调用可以支持把当前网卡加入某个组：

/* 设置要加入组播的地址 */
28         bzero(&mreq, sizeof (struct ip_mreq));
29
30         inet_pton(AF_INET,"224.0.1.2",&ia.sin_addr);
31         /* 设置组地址 */
32         bcopy (&ia.sin_addr.s_addr, &mreq.imr_multiaddr.s_addr, sizeof (struct in_addr));
33         /* 设置发送组播消息的源主机的地址信息 */
34         mreq.imr_interface.s_addr = htonl (INADDR_ANY);
35
36         /* 把本机加入组播地址，即本机网卡作为组播成员，只有加入组才能收到组播消息 */
37         if (setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq,sizeof (struct ip_mreq)) == -1)
38         {
39                 perror ("setsockopt");
40                 exit (-1);
41         }

现在网卡能够接受的MAC地址包括

1，当前网卡的MAC地址

2，广播MAC地址

3，设置的组播MAC地址 01：00：5e：00：00：01 由 224.0.0.1转换而来。

下面是wireshark抓取的广播包

root@baohua-VirtualBox:~# cat packet
No. Time Source Destination Protocol Length Info
1 0.000000000 10.0.2.15 224.0.0.1 UDP 49 Source port: 23456 Destination port: 7838

Frame 1: 49 bytes on wire (392 bits), 49 bytes captured (392 bits) on interface 0
Ethernet II, Src: CadmusCo_eb:0a:68 (08:00:27:eb:0a:68), Dst: IPv4mcast_00:00:01 (01:00:5e:00:00:01)
    Destination: IPv4mcast_00:00:01 (01:00:5e:00:00:01)
        Address: IPv4mcast_00:00:01 (01:00:5e:00:00:01)
        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
        .... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
    Source: CadmusCo_eb:0a:68 (08:00:27:eb:0a:68)
        Address: CadmusCo_eb:0a:68 (08:00:27:eb:0a:68)
        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
    Type: IP (0x0800)
Internet Protocol Version 4, Src: 10.0.2.15 (10.0.2.15), Dst: 224.0.0.1 (224.0.0.1)
User Datagram Protocol, Src Port: 23456 (23456), Dst Port: 7838 (7838)
Data (7 bytes)

0000 66 64 61 73 66 64 0a fdasfd.
root@baohua-VirtualBox:~#

ifconfig

eth0      Link encap:Ethernet HWaddr 08:00:27:eb:0a:68
          inet addr:10.0.2.15 Bcast:10.0.2.255 Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:feeb:a68/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
          RX packets:56 errors:0 dropped:0 overruns:0 frame:0
          TX packets:124 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:10202 (10.2 KB) TX bytes:15806 (15.8 KB)

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