SOCK_RAW數據包不發送某個源地址

Question

我正在使用一個類似netcat的工具（主要用於自我教育）。我想發送IP和UDP頭，我通過SOCK_RAW套接字構造的數據包。我在Debian VM上運行以下代碼以通過套接字發送數據包。

/* header must already have source IP, destination IP, and protocol number filled in */ 
int send_ip_packet(ipheader_t *header, char *buf, int numbytes) 
{ 
    int sizeofpacket = sizeof(ipheader_t) + numbytes; 
    if(sizeofpacket > MAX_PCKT_LEN) 
    { 
     printf("Cannot send ip packet of len %i. Too large. - FAIL\n", sizeofpacket); 
     return -1; 
    } 

    /* open a raw socket */ 
    int sd; 
    sd = socket(PF_INET, SOCK_RAW, header->ip_p); 
    if(sd < 0) 
    { 
     perror("socket()"); 
     printf("socket() call - FAIL\n"); 
     return -1; 
    } 
    else 
    { 
     printf("socket() call - SUCCESS\n"); 
    } 

    char packet[sizeofpacket]; 
    memset(packet, 0, sizeofpacket); 

    /* set remaining ip header */ 
    header->ip_hl = 5; /* header length is 5 32-bit octets */ 
    header->ip_v = 4; /* IPv4 */ 
    header->ip_tos = 16; /* low delay */ 
    header->ip_len = sizeofpacket; 
    header->ip_id = htons(54321); /* identifier used for fragmentation */ 
    header->ip_off = 0; /* fragmentation options */ 
    header->ip_ttl = 64; /* max num hops */ 
    header->ip_sum = csum((unsigned short*)packet, sizeofpacket); 

    /* fill packet */ 
    memcpy(packet, (char*) header, sizeof(ipheader_t)); 
    memcpy(packet + sizeof(ipheader_t), (char*) buf, numbytes); 

    /* setup socket addresses */ 
    struct sockaddr_in sin, din; 
    sin.sin_family = AF_INET; 
    din.sin_family = AF_INET; 
    memcpy(&sin.sin_addr.s_addr, &header->ip_src, sizeof(in_addr_t)); 
    memcpy(&din.sin_addr.s_addr, &header->ip_dst, sizeof(in_addr_t)); 

    /* send out the packet */ 
    int one = 1; 
    int *val = &one; 
    if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one))) 
    { 
     perror("setsockopt()"); 
     printf("setsockopt() call - FAIL\n"); 
     return -1; 
    } 
    else 
    { 
     printf("setsockopt() call - SUCCESS\n"); 
    } 

    if(sendto(sd, packet, header->ip_len, 0, (struct sockaddr *) &sin, sizeof(sin)) < 0) 
    { 
     perror("sendto()"); 
     printf("sendto() call - FAIL\n"); 
     return -1; 
    } 
    else 
    { 
     printf("Message sent! - SUCCESS\n"); 
    } 

    return 0; 
} 

代碼成功，只要發送可見的數據包在Wireshark的，因爲我提供的不是列爲我的「真」 IP，當我運行ifconfig源IP。任何人都可以告訴我爲什麼這可能是或如何修復它（除了不使用SOCK_RAW）？我會假設操作系統專門處理數據包，但爲什麼？

Answer 1

sendto()預計目的地址爲第5個參數。

從OP中給出的變量命名我強烈地假設源地址被傳入，這將很好地解釋所描述的bahaviour。