IPv6组播接口选择

用于为具有IPv4的传出流量选择接口的setsockopt方法是IP_MULTICAST_IF,它接受两个参数.在ip(4)手册页中：

Set the local device for a multicast socket. The argument for
setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq
structure similar to IP_ADD_MEMBERSHIP, or an in_addr structure.

尝试对IPv6流量执行类似操作时,该选项将更改为接口索引.从ipv6(4)手册页：

Set the device for outgoing multicast packets on the socket.
This is allowed only for SOCK_DGRAM and SOCK_RAW socket. The
argument is a pointer to an interface index (see netdevice(7))
in an integer.

在网络接口(例如eth0)分配了多个地址的情况下会发生什么情况？ IPv6套接字接口是否消除了单独使用每个地址的可能性？

解决方法:

如果您的接口只有一个本地链接地址(fe80 :: / 10)和一个公共可路由地址,则传出数据包的源地址取决于您要发送到的多播地址的范围.

IPv6多播地址的格式为ffxy :: / 16,其中x是标志字段,y是作用域.如果范围是1(本地接口)或2(本地链接),则源地址将是本地链接地址.如果范围是3或更大,则源地址将是可公开路由的地址.

另一方面,如果接口具有多个可公共路由的地址,则在发送数据报时需要使用sendmsg,以便可以使用IPV6_PKTINFO控制标头设置源地址.

以下是如何执行此操作的完整示例,假设您将2001 :: 1：2：3和2002 :: 1：2：3作为一个接口上的IPv6地址,而ff03 :: 1：2：3是多播您发送到的地址.

#define _GNU_SOURCE   // needed for some IPv6 datatypes to be visible

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

// multicast address to send to
const char *maddr = "ff03::1:2:3";

// uncomment the line for the source address you want to use
const char *srcaddr = "2001::1:2:3";
//const char *srcaddr = "2002::1:2:3";

int main()
{
    int sock;
    struct sockaddr_in6 dstaddr;

    struct iovec iovec[1];
    struct msghdr msg;
    struct cmsghdr* cmsg;
    char msg_control[1024];
    char udp_packet[] = "this is a test";
    int cmsg_space;
    struct in6_pktinfo *pktinfo;

    dstaddr.sin6_family = AF_INET6;
    inet_pton(AF_INET6, maddr, &dstaddr.sin6_addr);
    dstaddr.sin6_port = htons(5555);
    dstaddr.sin6_flowinfo = 0;
    dstaddr.sin6_scope_id = 0;

    if ((sock=socket(AF_INET6, SOCK_DGRAM, 0)) == -1) {
        perror("socket failed");
        exit(1);
    }

    // set up the msghdr structure with the destination address, 
    // buffer to send, and control info buffer
    iovec[0].iov_base = udp_packet;
    iovec[0].iov_len = strlen(udp_packet);
    msg.msg_name = &dstaddr;
    msg.msg_namelen = sizeof(dstaddr);
    msg.msg_iov = iovec;
    msg.msg_iovlen = sizeof(iovec) / sizeof(*iovec);
    msg.msg_control = msg_control;
    msg.msg_controllen = sizeof(msg_control);
    msg.msg_flags = 0;

    // add IPV6_PKTINFO control message to specify source address
    cmsg_space = 0;
    cmsg = CMSG_FIRSTHDR(&msg);
    cmsg->cmsg_level = IPPROTO_IPV6;
    cmsg->cmsg_type = IPV6_PKTINFO;
    cmsg->cmsg_len = CMSG_LEN(sizeof(*pktinfo));
    pktinfo = (struct in6_pktinfo*) CMSG_DATA(cmsg);
    pktinfo->ipi6_ifindex = 0;
    inet_pton(AF_INET6, srcaddr, &pktinfo->ipi6_addr);
    cmsg_space += CMSG_SPACE(sizeof(*pktinfo));
    msg.msg_controllen = cmsg_space;

    // send packet
    if (sendmsg(sock, &msg, 0) == -1) {
        perror("send failed");
    }

    return 0;
}

标签：sockets,ipv6,c-3,linux,multicast
来源： https://codeday.me/bug/20191109/2012075.html