CODEWITHSUNDEEP
csocketsrecv
user1016711
user1016711

Reputation: 139

recv with raw socket

I am trying to recv raw packets from socket and failed. Message is printing only while sending packets on server site. When no packets are transferred - program hungs in recv (socket in synchronous mode).

The problem is that printing message is "buffer" but without received data.

#include <sys/socket.h>
#include <linux/if_packet.h>
#include <linux/if_ether.h>
#include <linux/if_arp.h>

#define ETH_FRAME_LEN 1400

int main(){

    int s; /*socketdescriptor*/

    s = socket(PF_PACKET, SOCK_RAW, htons(0x88b5));
    if (s == -1) { perror("socket"); }

        struct sockaddr_ll socket_address;
    int r;
    char ifName[IFNAMSIZ] = "eth0";

    struct ifreq ifr;  
        strncpy((char *)ifr.ifr_name ,device , IFNAMSIZ); 

    /* Get the index of the interface to send on */
    memset(&if_idx, 0, sizeof(struct ifreq));
    strncpy(if_idx.ifr_name, ifName, IFNAMSIZ-1);
    if (ioctl(sockfd, SIOCGIFINDEX, &if_idx) < 0)
        perror("SIOCGIFINDEX");

    /* Get the MAC address of the interface to send on */   
    memset(&if_mac, 0, sizeof(struct ifreq));
    strncpy(if_mac.ifr_name, ifName, IFNAMSIZ-1);
    if (ioctl(sockfd, SIOCGIFHWADDR, &if_mac) < 0)
        perror("SIOCGIFHWADDR");

        memset(&socket_address, 0, sizeof(socket_address));
        socket_address.sll_ifindex = ifr.if_idx;
        socket_address.sll_protocol = htons(0x88b5);
        socket_address.sll_family = PF_PACKET;
        socket_address.sll_pkttype = PACKET_OUTGOING;

        r = bind(s, (struct sockaddr*)&socket_address,
                    sizeof(socket_address));
    if ( r < 0) { perror("bind")};

    void* buffer = (void*)malloc(ETH_FRAME_LEN); /*Buffer for ethernet frame*/
    int length = 0; /*length of the received frame*/ 

    length = recv(s, buffer, ETH_FRAME_LEN, 0,);
    if (length == -1) { perror("recvfrom"); }
    printf ("buffer %s\n", buffer);

}

Upvotes: 1

Views: 5661

Answers (1)

David Schwartz
David Schwartz

Reputation: 182855

You can only use the %s format specifier for C-style strings. You can't use it for arbitrary binary data. How would it know how many characters to print? You have the length in a variable called length. You need to print that many characters. For example:

for (int i = 0; i < length; ++i)
     putchar(((char *)buffer)[i]);

This will probably look like garbage because you're outputting a bunch of non-printable characters. Maybe you want something like:

void print(void *buf, int length)
{
      char *bp = (char *) buf;
      for (int i = 0; i < length; ++i)
          putchar( isprintf(bp[i]) ? bp[i] : '.' );
      putchar('\n');
}

This will replace non-printable characters with dots.

Upvotes: 3

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