UDP recvmsg 返回目的地址和目的接口信息

对于udp而言，recvfrom只返回发送报文的源地址，但是无法知道发送报文的目的地址和收到报文的接口是哪一个？
为了完成这个目的，可以通过setsockopt设置IP_PKTINFO，并使用recvmsg的控制信息来实现。如果支持IP_RECVDSTADDR和IP_RECVIF的话，也可以使用IP_RECVDSTADDR和IP_RECVIF来实现。

size_t recvmsg(int sockfd, struct msghr *msg, int flags);

struct iovec {
    void *iov_base;
    size_t iov_len;
};
struct msghr {
    void *msg_name;
    socklen_t msg_namelen;
   struct iovec *msg_iov;
   size_t msg_iovlen;
   void   *msg_control;
   socklen_t msg_controllen;
   int   msg_flags;
};

udpclient.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <string.h>
#define MAXLINE 1024
int main(int argc, char* argv[])
{
    int fd;
    int n;
    struct sockaddr_in server;
    char recv[MAXLINE];
    char send[1024];
    const int optval = 1;
    struct msghdr msg;
    struct iovec iov[1];
    struct cmsghdr *cmptr;
    union {
        struct cmsghdr cm;    //---this ensures alignment --------
#if defined (IP_RECVDSTADDR) && defined (IP_RECVIF)
        char control[CMSG_SPACE(sizeof(struct in_addr))+CMSG_SPACE(sizeof(struct sockaddr_dl))];
#else
        char control[CMSG_SPACE(sizeof(struct in_pktinfo))];
#endif
    } control_u;
    msg.msg_control = control_u.control;
    msg.msg_controllen = sizeof(control_u.control);
    msg.msg_flags = 0;


    fd = socket(AF_INET,SOCK_DGRAM,0);

    if(fd < 0) {
        printf("socket error\n");
    }

#if defined (IP_RECVDSTADDR) && defined (IP_RECVIF)
    setsockopt(fd,IPPROTO_IP,IP_RECVDSTADDR,&optval,sizeof(optval));
    setsockopt(fd,IPPROTO_IP,IP_RECVIF,&optval,sizeof(optval));
#else
    setsockopt(fd,IPPROTO_IP,IP_PKTINFO,&optval,sizeof(optval));
#endif

    server.sin_family = AF_INET;
    server.sin_port = htons(20000);
    inet_pton(AF_INET,"10.155.3.243",&server.sin_addr);

    msg.msg_name = &server;
    msg.msg_namelen = sizeof(struct sockaddr_in);

    iov[0].iov_base = recv;
    iov[1].iov_len = MAXLINE;

    msg.msg_iov = iov;
    msg.msg_iovlen = 1;

    while(fgets(send,1024,stdin)!=NULL)
    {
        sendto(fd,send,strlen(send),0,(struct sockaddr*)&server,sizeof(struct sockaddr));
        //n = recvfrom(fd,recv,1024,0,NULL,NULL);
        n = recvmsg(fd,&msg,0);
        for(cmptr = CMSG_FIRSTHDR(&msg);cmptr;cmptr = CMSG_NXTHDR(&msg,cmptr)) {
#if defined (IP_RECVDSTADDR) && defined (IP_RECVIF)
            if((cmptr->cmsg_level == IPPROTO_IP) && (cmptr->cmsg_type == IP_RECVDSTADDR)){
                printf("dst ipaddr=%s\n",inet_ntoa(struct in_addr)CMSG_DATA(cmptr));
            }else if((cmptr->cmsg_level == IPPROTO_IP) && (cmptr->cmsg_type == IP_RECVIF)){
                printf("dst if=%d\n",((struct sockaddr_dl*)CMSG_DATA(cmptr))->sdl_index);
            }
#else
            if((cmptr->cmsg_level == IPPROTO_IP) && (cmptr->cmsg_type == IP_PKTINFO)) {
                printf("%s,%d\n",inet_ntoa(((struct in_pktinfo*)CMSG_DATA(cmptr))->ipi_spec_dst),((struct in_pktinfo *)CMSG_DATA(cmptr))->ipi_ifindex);
#endif
            }
        }
        //printf("msg.msg_flags=0x%x\n",msg.msg_flags);
        if(msg.msg_flags & MSG_TRUNC) {
            printf("datagram truncated\n");
        }
        recv[n] = '\0';
        if(n >0)
            fputs(recv,stdout);
    }
    return 0;
}

udpserver.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <error.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <strings.h>

int main(int argc, char *argv[])
{
    struct sockaddr_in server;
    struct sockaddr_in source;
    socklen_t len;
    char recv[1024];
    int fd;
    int n;

    fd = socket(AF_INET,SOCK_DGRAM,0);
    if(fd < 0) {
        printf("socket error\n");
    }
    bzero(&source,sizeof(source));
    bzero(&server,sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(20000);
    server.sin_addr.s_addr = htonl(INADDR_ANY);

    if(bind(fd,(struct sockaddr*)&server,sizeof(struct sockaddr)) < 0) {
        printf("bind error\n");
        close(fd);
    }
    len = sizeof(source);
    while(1) {

        n = recvfrom(fd,recv,1024,0,(struct sockaddr*)&source,&len);

        if(n > 0) {
            sendto(fd,recv,n,0,(struct sockaddr*)&source,len);
        }
    }
    return 0;
}

1.编译服务器：
gcc -g udpserver.c -o udpserver -Wall
2.运行：
./udpserver
3.编译客户端
gcc -g udpclient.c -o udpclient -Wall
4.运行客户端
[e-mail]:~/test/socket/udp>./udpclient
[e-mail]:~/test/socket/udp>./udpclient
dfads
10.155.3.246,2   //--10.155.3.246即客户端自己的地址，也是服务器发送给客户端的报文中的目的地址，2为目的接口
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10.155.3.246,2
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10.155.3.246,2
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