struct in_pktinfo{unsigned int ipi_ifindex;    /* 接口索引 */struct in_addr ipi_spec_dst; /* 路由目的地址 */struct in_addr ipi_addr;     /* 头标识目的地址 */};

// sock 使用AF_INET协议族,  socket类型SOCK_DGRAM
setsockopt(sock, IPPROTO_IP, IP_PKTINFO, &opt, sizeof(opt));
// 这里，控制数据是脏数据。
char cmbuf[0x100];
// 目标IP地址
struct sockaddr_in peeraddr;//如果你想要获取UDP包中的数据，那么还需要为msg_iovec字段初始化
struct msghdr mh = {.msg_name = &peeraddr,.msg_namelen = sizeof(peeraddr),.msg_control = cmbuf,.msg_controllen = sizeof(cmbuf),
};
recvmsg(sock, &mh, 0);struct cmsghdr *cmsg ;
for ( // 遍历所有的控制头（the control headers）cmsg = CMSG_FIRSTHDR(&mh);cmsg != NULL;cmsg = CMSG_NXTHDR(&mh, cmsg))
{// 忽略我们不需要的控制头（the control headers）if (cmsg->cmsg_level != IPPROTO_IP ||cmsg->cmsg_type != IP_PKTINFO){continue;}struct in_pktinfo *pi = CMSG_DATA(cmsg);// 在这里, peeraddr是本机的地址（the source sockaddr）// pi->ipi_spec_dst 是UDP包中路由目的地址（the destination in_addr）// pi->ipi_addr 是UDP包中的头标识目的地址（the receiving interface in_addr）
}

#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#define BUFLEN 255int main ( int argc, char **argv )
{struct sockaddr_in peeraddr, localaddr;int sockfd;int socklen, n;
//(1)创建UDP数据报socket描述符sockfd = socket ( AF_INET, SOCK_DGRAM, 0 );if ( sockfd<0 ){printf ( "socket creating err in udptalk\n" );exit ( EXIT_FAILURE );}printf ( "IP address Checking!\n" );socklen = sizeof ( struct sockaddr_in );memset ( &peeraddr, 0, socklen );
//(2)设置目标主机IP和端口，这里我们使用广播方式peeraddr.sin_family=AF_INET;peeraddr.sin_port=htons ( atoi ( "8903" ) );peeraddr.sin_addr.s_addr = htonl ( INADDR_BROADCAST );
//(3设置本机IP和端口，这里我们设置可以接收符合端口的所有的包memset ( &localaddr, 0, socklen );localaddr.sin_family=AF_INET;localaddr.sin_addr.s_addr = htonl ( INADDR_ANY );  //设置接收任何主机
printf ( "try to bind local address \n" );localaddr.sin_port=htons ( atoi ( "8904" ) );//(4)设置IPPROTO_IP标志，以便获取UDP包中的信息int opt = 1;setsockopt ( sockfd, IPPROTO_IP, IP_PKTINFO, &opt, sizeof ( opt ) );int nb = 0;
//(5)设置为广播方式nb = setsockopt ( sockfd, SOL_SOCKET, SO_BROADCAST, ( char * ) &opt,sizeof ( opt ) );if ( nb == -1 ){printf ( "set socket error..." );exit ( EXIT_FAILURE );}printf ( "IP address Checking!\n" );char cmbuf[100];// 这里只是为控制数据申请一个空间
//(6)初始化msg_iovec字段,以便获取UDP包数据域char buffer[BUFLEN+1];struct iovec iov[1];iov[0].iov_base=buffer;iov[0].iov_len=sizeof ( buffer );
//(7)初始化struct msghdr,以便获取UDP包中目标IP地址和源地址struct msghdr mh ={.msg_name = &localaddr,.msg_namelen = sizeof ( localaddr ),.msg_control = cmbuf,.msg_controllen = sizeof (cmbuf ),.msg_iov=iov,                                                           .msg_iovlen=1};
//(8)将本机的地址信息与sockfd绑定起来if ( bind ( sockfd, &localaddr, socklen ) <0 ){printf ( "bind local address err in udptalk!\n" );exit ( 2 );}//发一个消息给目标主机if ( sendto ( sockfd, "HELLO", strlen ( "HELLO" ), 0, &peeraddr, socklen )
<0 ){printf ( "sendto err in udptalk!\n" );exit ( 3 );}printf ( "end of sendto \n" );printf ( "start of recv&send message loop!\n" );for ( ;; )//接收消息循环
    {printf ( "Waiting For Message...!\n" );n=recvmsg ( sockfd, &mh, 0 );//判断socket是否有错误发生if ( n<0 ){printf ( "recvfrom err in udptalk!\n" );exit ( 4 );}else{cmbuf[n]=0;printf ( "Receive:%dByte。\tThe Message Is:%s\n", n,buffer );}
//(9)初始化cmsghdr以便处理mh中的附属数据，通过遍历附属数据对象,找出我们感兴趣的信息struct cmsghdr *cmsg ;for ( cmsg = CMSG_FIRSTHDR ( &mh );cmsg != NULL;cmsg = CMSG_NXTHDR ( &mh, cmsg ) ){// 忽略我们不需要的控制头（the control headers）if ( cmsg->cmsg_level != IPPROTO_IP ||cmsg->cmsg_type != IP_PKTINFO ){continue;}struct in_pktinfo *pi = CMSG_DATA ( cmsg );    //(10)将地址信息转换后输出char dst[100],ipi[100];//用来保存转化后的源IP地址，目标主机地址                // pi->ipi_spec_dst 是UDP包中的路由目的IP地址（the destination in_addr）// pi->ipi_addr 是UDP包中的头标识目的地址（the receiving interface in_addr）if ( ( inet_ntop ( AF_INET,& ( pi->ipi_spec_dst ),dst,sizeof ( dst )) ) !=NULL ){printf ( "路由目的IP地址IPdst=%s\n",dst);}if ( ( inet_ntop ( AF_INET,& ( pi->ipi_addr ),ipi,sizeof ( ipi )) ) !=NULL ){printf ("头标识目的地址ipi_addr=%s\n",ipi);}}printf ( "Send Some Message To Server\n" );if ( sendto ( sockfd, "Hello", strlen ( buffer ), 0, &peeraddr, socklen)
<0 ){printf ( "sendto err in udptalk!\n" );exit ( 3 );}}
}

struct in_addr {in_addr_t s_addr;
};

struct iovec {                   /* Scatter/gather array items */void  *iov_base;              /* Starting address */size_t iov_len;               /* Number of bytes to transfer */
};struct msghdr {void         *msg_name;       /* optional address */socklen_t     msg_namelen;    /* size of address */struct iovec *msg_iov;        /* scatter/gather array */size_t        msg_iovlen;     /* # elements in msg_iov */void         *msg_control;    /* ancillary data, see below */size_t        msg_controllen; /* ancillary data buffer len */int           msg_flags;      /* flags on received message */
};

struct cmsghdr {socklen_t cmsg_len;    /* data byte count, including header */int       cmsg_level;  /* originating protocol */int       cmsg_type;   /* protocol-specific type *//* followed by unsigned char cmsg_data[]; */
};

//setsockopt函数原型
#include <sys/types.h>
#include <sys/socket.h>
int setsockopt(int s, int level, int optname,const void *optval, socklen_t optlen);

#include <sys/socket.h>
struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *msgh);
struct cmsghdr *CMSG_NXTHDR(struct msghdr *msgh, struct cmsghdr *cmsg);
size_t CMSG_ALIGN(size_t length);
size_t CMSG_SPACE(size_t length);
size_t CMSG_LEN(size_t length);
void *CMSG_DATA(struct cmsghdr *cmsg);

struct cmsgptr *mptr;
int fd; /* File Descriptor */
. . .
fd = *(int *)CMSG_DATA(mptr);