当设置 IP_PKTINFO 时，recvmsg returns 意外 in_pktinfo

Question

我正在编写一个使用 UDP 多播的程序。我需要过滤掉来自指定接口的消息。我设置IP_PKTINFO为socket：

int enable = 1;
OS_Result rc = setsockopt(readArg->conn->sock, IPPROTO_IP, IP_PKTINFO, &enable, sizeof(enable));

然后，我打印出in_pktinfo的内容：

for (struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msgHdr); cmsg != NULL; cmsg = CMSG_NXTHDR(&msgHdr, cmsg)) {
    // ignore the control headers that don't match what we want
    if (cmsg->cmsg_level != IPPROTO_IP || cmsg->cmsg_type != IP_PKTINFO) {
        continue;
    }
    struct in_pktinfo *pi = (struct in_pktinfo *)CMSG_DATA(cmsg);
    char interName[20];
    memset(interName, 0, 20);
    if_indextoname(pi->ipi_ifindex, interName);
    printf("! ================================================= !\n");
    printf("ipi_ifindex: %d\n", pi->ipi_ifindex);
    printf("interface name: %s\n", interName);
    printf("ipi_addr: %s\n", inet_ntoa(pi->ipi_addr));
    printf("ipi_spec_dst: %s\n", inet_ntoa(pi->ipi_spec_dst));
    printf("! ================================================= !\n");
}

没想到多次执行程序后会出现ipi_ifindex和ipi_spec_dst：

! ================================================= !
ipi_ifindex: 0                                           <------ exception
interface name: 
ipi_addr: 239.255.0.1
ipi_spec_dst: 0.0.0.0
! ================================================= !
! ================================================= !
ipi_ifindex: 2
interface name: ens3
ipi_addr: 239.255.0.1
ipi_spec_dst: 192.168.122.50
! ================================================= !
! ================================================= !
ipi_ifindex: 2
interface name: ens3
ipi_addr: 239.255.0.1
ipi_spec_dst: 192.168.122.50
! ================================================= !

似乎Linux 没有正确解析出UDP 消息是从哪个接口接收的。这种现象出现在socket收到的前几个包中，概率出现

Answer 1

我已经解决了这个问题。一旦套接字绑定到 IP 地址和端口，它就开始接收数据包。所以UDP Socket的属性应该在调用bind()函数之前设置。

当设置 IP_PKTINFO 时，recvmsg returns 意外 in_pktinfo

When IP_PKTINFO is set, recvmsg returns an unexpected in_pktinfo

c

sockets

linux

udp