Geek Answers Handbook

Raw Socket Help: Why are UDP packets created using raw sockets not accepted by the UDP kernel?

I study raw sockets. I used the IP_HDRINCL parameter to create my own IP headers. After the IP header, I create a UDP header. Then I send the packet to my loopback address. I have another program that will capture UDP packets as they arrive. To check if packets are correctly formed and received, I have another process that reads the source IP datagrams. My problem is that although the second process (reading the source datagrams) works well (all the IP and UDP fields seem to be in order), the first process (receiving UDP) does not receive any of the packets I created. The protocol field in the IP header is fine, and the port also matches ... I am using Linux 2.6.35-22. I want to know if this is normal in new kernels? Please check the code below for any errors. UDP processwhich should receive packets, listens on a socket connected to port 50,000 on the same machine ...

unsigned short in_cksum(unsigned short *addr, int len)
{
    int nleft = len;
    int sum = 0;
    unsigned short *w = addr;
    unsigned short answer = 0;

    while (nleft > 1) {
        sum += *w++;
        nleft -= 2;
    }

    if (nleft == 1) {
        *(unsigned char *) (&answer) = *(unsigned char *) w;
        sum += answer;
    }

    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    answer = ~sum;
    return (answer);
}


main()
{
    int fd=socket(AF_INET,SOCK_RAW,IPPROTO_UDP);

    int val=1;

    int ret=setsockopt(fd,IPPROTO_IP,IP_HDRINCL,&val,sizeof(val));

    char buf[8192];

    /* create a IP header */

    struct iphdr* ip=(struct iphdr*)buf;//(struct iphdr*) malloc(sizeof(struct iphdr));

    ip->version=4;
    ip->ihl=5;
    ip->tos=0;
    ip->id=0;
    ip->frag_off=0;
    ip->ttl=255;
    ip->protocol=IPPROTO_UDP;
    ip->check=0;
    ip->saddr=inet_addr("1.2.3.4");
    ip->daddr=inet_addr("127.0.0.1");


    struct udphdr* udp=(struct udphdr*)(buf+sizeof(struct iphdr));//(struct udphdr*) malloc(sizeof(struct udphdr));
    udp->source=htons(40000);   
    udp->dest=htons(50000);
    udp->check=0;
    char* data=(char*)buf+sizeof(struct iphdr)+sizeof(struct udphdr);strcpy(data,"Harry Potter and the Philosopher Stone");
    udp->len=htons(sizeof(struct udphdr)+strlen(data));
    udp->check=in_cksum((unsigned short*) udp,8+strlen(data));

    ip->tot_len=htons(sizeof(struct iphdr)+sizeof(struct udphdr)+strlen(data));

    struct sockaddr_in d;
    bzero(&d,sizeof(d));
    d.sin_family=AF_INET;
    d.sin_port=htons(50000);
    inet_pton(AF_INET,"localhost",&d.sin_addr.s_addr);
    while(1)
     sendto(fd,buf,sizeof(struct iphdr)+sizeof(struct udphdr)+strlen(data),0,(struct sockaddr*) &d,sizeof(d));
}
+5
2

, UDP.

udp->check=in_cksum((unsigned short*) udp,8+strlen(data));

UDP - "-" UDP. UDP . UDP - .

Wireshark UDP-.

. :

+3

- . , API , , , WRITTEN , , , wiresharks/tcpdump. , , UDP. , , - . , , ... , .

0

More articles:


All Articles