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

编程问答 2022-06-02

问题描述

下面是一个可以在 MacOS/X 和 Linux 下编译的最小示例程序。它的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 下确实按预期工作，但在 Linux 下它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)，它会在 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 正确设置了接口索引是 IPv6 范围多播的要求。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,sizeof(toAddr));
   toAddr.sin6_family = AF_INET6;
   toAddr.sin6_port   = htons(12345);
   if (Inet_PtoN(AF_INET6,&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),(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;
}

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

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

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

lo0: flags=8049<UP,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,SMART,SIMPLEX,MULTICAST> mtu 1500
    options=50b<RXCSUM,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,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,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,MULTICAST> mtu 1500
    options=400<CHANNEL_IO>
    ether f2:18:98:a8:a3:b4
    media: autoselect
    status: inactive
en1: flags=8863<UP,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,MULTICAST> mtu 2304
    options=400<CHANNEL_IO>
    ether 02:18:98:a8:a3:b4
    media: autoselect
    status: inactive
awdl0: flags=8902<broADCAST,PROMISC,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,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,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,CHANNEL_IO>
    ether 82:32:b3:81:34:01
    media: autoselect <full-duplex>
    status: inactive
en3: flags=8963<UP,CHANNEL_IO>
    ether 82:32:b3:81:34:00
    media: autoselect <full-duplex>
    status: inactive
en4: flags=8963<UP,CHANNEL_IO>
    ether 82:32:b3:81:34:05
    media: autoselect <full-duplex>
    status: inactive
bridge0: flags=8863<UP,MULTICAST> mtu 1500
    options=63<RXCSUM,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,MULTICAST> mtu 1380
    inet6 fe80::d1a3:48e0:11d1:fc27%utun0 prefixlen 64 scopeid 0x15
    nd6 options=201<PERFORMNUD,DAD>
utun1: flags=8051<UP,MULTICAST> mtu 2000
    inet6 fe80::ee71:43c3:4648:31b0%utun1 prefixlen 64 scopeid 0x16
    nd6 options=201<PERFORMNUD,DAD>
utun2: flags=8051<UP,MULTICAST> mtu 1500
    inet 172.27.224.165 --> 172.27.224.165 netmask 0xffffffc0
vmnet1: flags=8863<UP,MULTICAST> mtu 1500
    ether 00:50:56:c0:00:08
    inet 172.16.158.1 netmask 0xffffff00 broadcast 172.16.158.255

解决方法

由于 ffx1: 不应该离开系统，因此通常可以在系统默认界面上进行 rendevouz（除非在客户端和服务器设置之间界面可能会上下波动。）可以添加防火墙规则，如果操作系统可能会意外地在线发送或接受这些地址。

如果你想让实际的 lo 接口作为一个有效的多播来在私有链路上显式地 rendevouz，它需要与普通接口相同的配置。如何在启动时保持这种持久性会有所不同，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
cipv6linuxlinuxmacosmacosmulticast

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