为什么这个 send-IPv6-local-node-multicast 程序在 MacOS/X 下可以工作，而在 Linux 下不行？

【问题标题】：Why does this send-IPv6-local-node-multicast program work under MacOS/X but not under Linux?为什么这个 send-IPv6-local-node-multicast 程序在 MacOS/X 下可以工作，而在 Linux 下不行？
【发布时间】：2021-03-31 18:12:33
【问题描述】：

以下是在 MacOS/X 和 Linux 下编译的最小示例程序。其目的是通过机器的环回设备发送单个节点范围的 UDP/IPv6 多播数据包。我相信它是正确的，它在 MacOS/X 下确实可以正常工作，但在 Linux 下它会失败并显示以下输出：

Attempting to send an IPv6/UDP packet to multicast address [ff11:0:1:0:94a4:2318:6300:4d51] on interface at index 1 (aka lo)
FAILURE!  sendto() returned -1, errno=101 aka [Network is unreachable]

...但是，如果我修改第 26 行以设置常量 interfaceIdx = 0 而不是 interfaceIdx = if_nametoindex(ifaceName)，它在 Linux 下成功发送数据包：

Attempting to send an IPv6/UDP packet to multicast address [ff11:0:1:0:94a4:2318:6300:4d51] on interface at index 0 (aka lo)
SUCCESS!  sendto() returned 18

...所以我可以输入一个#ifdef __linux__ 来强制interfaceIdx 变量为0，但我觉得这不是正确的解决方法，因为AFAIK 正确设置接口索引是IPv6 范围的多播。有谁知道为什么将interfaceIdx 设置为正确的值会破坏 Linux 下的程序？

这是程序：

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <net/if.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>

// This function is here solely to make sure I pass in the right pointer to inet_pton's third argument
static int Inet_PtoN(int af, const char * src, struct in6_addr * dst)
{
   return inet_pton(af, src, dst);
}

int main(void)
{
   const char * dest = "ff11:0:1:0:94a4:2318:6300:4d51";  // my local-node-scoped IPv6 multicast address
#ifdef __linux__
   const char * ifaceName = "lo";   // name of loopback device under Linux
#else
   const char * ifaceName = "lo0";  // name of loopback device under MacOS/X
#endif
   const int interfaceIdx = if_nametoindex(ifaceName);

   printf("Attempting to send an IPv6/UDP packet to multicast address [%s] on interface at index %i (aka %s)\n", dest, interfaceIdx, ifaceName);

   int s = socket(AF_INET6, SOCK_DGRAM, 0);
   if (s<0) {perror("socket()"); return 10;}

   if (setsockopt(s, IPPROTO_IPV6, IPV6_MULTICAST_IF, &interfaceIdx, sizeof(interfaceIdx)) != 0) {perror("setsockopt(IPV6_MULTICAST_IF)"); return -1;}

   struct sockaddr_in6 toAddr; memset(&toAddr, 0, sizeof(toAddr));
   toAddr.sin6_family = AF_INET6;
   toAddr.sin6_port   = htons(12345);
   if (Inet_PtoN(AF_INET6, dest, &toAddr.sin6_addr) <= 0) {perror("inet_pton()"); return -1;}
   toAddr.sin6_scope_id = interfaceIdx;

   char buf[] = "dummy payload text";
   const int r = sendto(s, buf, strlen(buf), 0, (struct sockaddr *)&toAddr, sizeof(toAddr));
   if (r >= 0) printf("SUCCESS!  sendto() returned %i\n", r);
          else printf("FAILURE!  sendto() returned %i, errno=%i aka [%s]\n", r, errno, strerror(errno));

   close(s);
   return 0;
}

...这里仅供参考，是我的 Linux VM（Ubuntu 18.0.5）中ifconfig -a 的输出：

ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 172.16.158.154  netmask 255.255.255.0  broadcast 172.16.158.255
        inet6 fe80::bcf8:ae62:d420:b850  prefixlen 64  scopeid 0x20<link>
        ether 00:0c:29:d5:92:67  txqueuelen 1000  (Ethernet)
        RX packets 41456  bytes 60446720 (60.4 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 15899  bytes 1058502 (1.0 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 5442  bytes 983303 (983.3 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 5442  bytes 983303 (983.3 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

....这是ifconfig -a 在我的 Mac（通过 VMWare Fusion 托管 Linux Ubuntu VM）上的输出：

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
            options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP>
    inet 127.0.0.1 netmask 0xff000000
    inet6 ::1 prefixlen 128
    inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
    inet 127.94.0.2 netmask 0xff000000
    inet 127.94.0.1 netmask 0xff000000
    nd6 options=201<PERFORMNUD,DAD>
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
XHC0: flags=0<> mtu 0
XHC1: flags=0<> mtu 0
XHC20: flags=0<> mtu 0
VHC128: flags=0<> mtu 0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=50b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV,CHANNEL_IO>
    ether f0:18:98:e8:e4:81
    inet6 fe80::4ef:a7f5:734c:5a82%en0 prefixlen 64 secured scopeid 0x8
    inet 10.0.1.26 netmask 0xffffff00 broadcast 10.0.1.255
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (<unknown type>)
    status: active
en6: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    ether ac:de:48:00:11:22
    inet6 fe80::aede:48ff:fe00:1122%en6 prefixlen 64 scopeid 0x9
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (100baseTX <full-duplex>)
    status: active
en8: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=400<CHANNEL_IO>
    ether 52:de:06:3a:29:9f
    inet6 fe80::1438:7fb7:7145:e06c%en8 prefixlen 64 secured scopeid 0xa
    inet 169.254.73.131 netmask 0xffff0000 broadcast 169.254.255.255
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (100baseTX <full-duplex>)
    status: active
ap1: flags=8802<BROADCAST,SIMPLEX,MULTICAST> mtu 1500
    options=400<CHANNEL_IO>
    ether f2:18:98:a8:a3:b4
    media: autoselect
    status: inactive
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=400<CHANNEL_IO>
    ether f0:18:98:a8:a3:b4
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (<unknown type>)
    status: inactive
p2p0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> mtu 2304
    options=400<CHANNEL_IO>
    ether 02:18:98:a8:a3:b4
    media: autoselect
    status: inactive
awdl0: flags=8902<BROADCAST,PROMISC,SIMPLEX,MULTICAST> mtu 1484
    options=400<CHANNEL_IO>
    ether 2a:72:ad:a2:71:2b
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect
    status: inactive
llw0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=400<CHANNEL_IO>
    ether 2a:72:ad:a2:71:2b
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect
    status: inactive
en5: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
    options=460<TSO4,TSO6,CHANNEL_IO>
    ether 82:32:b3:81:34:04
    media: autoselect <full-duplex>
    status: inactive
en2: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
    options=460<TSO4,TSO6,CHANNEL_IO>
    ether 82:32:b3:81:34:01
    media: autoselect <full-duplex>
    status: inactive
en3: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
    options=460<TSO4,TSO6,CHANNEL_IO>
    ether 82:32:b3:81:34:00
    media: autoselect <full-duplex>
    status: inactive
en4: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
    options=460<TSO4,TSO6,CHANNEL_IO>
    ether 82:32:b3:81:34:05
    media: autoselect <full-duplex>
    status: inactive
bridge0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=63<RXCSUM,TXCSUM,TSO4,TSO6>
    ether 82:32:b3:81:34:01
    Configuration:
            id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
            maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
            root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
            ipfilter disabled flags 0x0
    member: en2 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 17 priority 0 path cost 0
    member: en3 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 18 priority 0 path cost 0
    member: en4 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 19 priority 0 path cost 0
    member: en5 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 16 priority 0 path cost 0
    nd6 options=201<PERFORMNUD,DAD>
    media: <unknown type>
    status: inactive
utun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1380
    inet6 fe80::d1a3:48e0:11d1:fc27%utun0 prefixlen 64 scopeid 0x15
    nd6 options=201<PERFORMNUD,DAD>
utun1: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 2000
    inet6 fe80::ee71:43c3:4648:31b0%utun1 prefixlen 64 scopeid 0x16
    nd6 options=201<PERFORMNUD,DAD>
utun2: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1500
    inet 172.27.224.165 --> 172.27.224.165 netmask 0xffffffc0
vmnet1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    ether 00:50:56:c0:00:08
    inet 172.16.158.1 netmask 0xffffff00 broadcast 172.16.158.255

【问题讨论】：

  • 也许与您在 Linux 上的 ifconfg 似乎在告诉您 lo 的 IPv6 地址的范围 ID 是 0x10 有关，而您程序的输出表明该接口是 1。我不清楚这些属性之间是否应该有任何特定的关联。
  • @JohnBollinger 我也这么认为，但是 AFAICT 0x10 和 0x20 不是有效的区域 ID（即，如果我打印出 if_nameindex() 返回的结构，它只显示 1/lo 和 2/ens33，如果我尝试使用interfaceIdx=16 发送多播数据包，我会收到“无效设备索引”类型的错误）。所以我不清楚后面的 0 应该表示什么，但 AFAICT 它不是实际区域 ID 的一部分。
  • 我承认有点超出了我通常的区域，但据我所知或可以确定，区域 ID 仅适用于链接本地地址。您没有发送到链接本地地址。当您省略设置区域 id 时，您的程序是否在 Mac 上按预期工作（最初清除了整个结构，就像您所做的那样）？
  • 不，如果我在 Mac 上设置interfaceIdx=0，我会得到setsockopt(IPV6_MULTICAST_IF): Can't assign requested address，如果我也注释掉setsockopt(IPV6_MULTICAST_IF) 调用并重试，我会得到FAILURE! sendto() returned -1, errno=65 aka [No route to host]。因此，Mac 上似乎需要适当的区域 ID。
  • 那么，我希望在这方面比我有更多专业知识的人感兴趣。同时，检查每台机器上getaddrinfo() 返回的一些环回地址可能会很有趣。可能是一个或另一个网络堆栈有问题，但也许它们只是不同。

标签： c linux macos ipv6 multicast


【解决方案1】：

由于ffx1: 不应该离开系统，通常应该可以在系统默认界面上会合（除非在客户端和服务器设置之间界面可能会上下波动。）如果可以添加防火墙规则操作系统可能会意外地通过网络发送或接受这些地址。

如果您想让实际的 lo 接口充当有效的多播以在专用链路上显式地集合，它需要与普通接口相同的配置。如何在启动时保持这种状态会有所不同，但我认为大多数现代 Linux 都会理解这些命令：

ifconfig lo multicast
ip -6 route show table local
# find an interface route and copy it for lo and create it with a lower metric:
ip -6 route add table local ff00::/8 dev lo metric 255 pref medium

【讨论】：

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