Wireshark doesn't detect any packet sent. sendto return 0

0
votes

I have been trying to send packets using raw socket in following code.This code I found somewhere in the internet. I created my own ipheader and udp header. The whole data packet is sent using sendto() function on raw socket. sendto() returns 0. Which means a packet of 0 length is sent out of it and hence even wireshark doesnt detect any packet. Where is my mistake?

// Must be run by root lol! Just datagram, no payload/data
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <cstdlib>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/udp.h>

// The packet length
#define PCKT_LEN 35

// Can create separate header file (.h) for all headers' structure
// The IP header's structure
struct ipheader {
    unsigned char      iph_ihl:4, iph_ver:4;
    unsigned char      iph_tos;
    unsigned short int iph_len;
    unsigned short int iph_ident;
    unsigned char      iph_flag;
    unsigned short int iph_offset;
    unsigned char      iph_ttl;
    unsigned char      iph_protocol;
    unsigned short int iph_chksum;
    unsigned int       iph_sourceip;
    unsigned int       iph_destip;
};

// UDP header's structure
struct udpheader {
    unsigned short int udph_srcport;
    unsigned short int udph_destport;
    unsigned short int udph_len;
    unsigned short int udph_chksum;
};
// total udp header length: 8 bytes (=64 bits)

// Function for checksum calculation. From the RFC,
// the checksum algorithm is:
//  "The checksum field is the 16 bit one's complement of the one's
//  complement sum of all 16 bit words in the header.  For purposes of
//  computing the checksum, the value of the checksum field is zero."
unsigned short csum(unsigned short *buf, int nwords)
{       //
    unsigned long sum;
    for(sum=0; nwords>0; nwords--)
        sum += *buf++;
    sum = (sum >> 16) + (sum &0xffff);
    sum += (sum >> 16);
    return (unsigned short)(~sum);
}

// Source IP, source port, target IP, target port from the command line arguments
int main(int argc, char *argv[])
{
    int sd;
    // No data/payload just datagram
    char buffer[PCKT_LEN];
    // Our own headers' structures
    struct ipheader *ip = (struct ipheader *) buffer;
    struct udpheader *udp = (struct udpheader *) (buffer + sizeof(struct ipheader));
    // Source and destination addresses: IP and port
    struct sockaddr_in sin, din;
    int one = 1;
    const int *val = &one;

    memset(buffer, 0, PCKT_LEN);

    if(argc != 5)
    {
        printf("- Invalid parameters!!!\n");
        printf("- Usage %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]);
        exit(-1);
    }

    // Create a raw socket with UDP protocol
    sd = socket(PF_INET, SOCK_RAW, IPPROTO_UDP);
    if(sd < 0)
    {
        perror("socket() error");
        // If something wrong just exit
        exit(-1);
    }
    else
        printf("socket() - Using SOCK_RAW socket and UDP protocol is OK.\n");

    // The source is redundant, may be used later if needed
    // The address family
    sin.sin_family = AF_INET;
    din.sin_family = AF_INET;
    // Port numbers
    sin.sin_port = htons(atoi(argv[2]));
    din.sin_port = htons(atoi(argv[4]));
    // IP addresses
    sin.sin_addr.s_addr = inet_addr(argv[1]);
    din.sin_addr.s_addr = inet_addr(argv[3]);

    // Fabricate the IP header or we can use the
    // standard header structures but assign our own values.
    ip->iph_ihl = 5;
    ip->iph_ver = 4;
    ip->iph_tos = 16; // Low delay
    ip->iph_len = sizeof(struct ipheader) + sizeof(struct udpheader);
    ip->iph_ident = htons(54321);
    ip->iph_ttl = 64; // hops
    ip->iph_protocol = 17; // UDP
    // Source IP address, can use spoofed address here!!!
    ip->iph_sourceip = inet_addr(argv[1]);
    // The destination IP address
    ip->iph_destip = inet_addr(argv[3]);

    // Fabricate the UDP header. Source port number, redundant
    udp->udph_srcport = htons(atoi(argv[2]));
    // Destination port number
    udp->udph_destport = htons(atoi(argv[4]));
    udp->udph_len = htons(sizeof(struct udpheader));
    // Calculate the checksum for integrity
    ip->iph_chksum = csum((unsigned short *)buffer, sizeof(struct ipheader) + sizeof(struct udpheader));
    // Inform the kernel do not fill up the packet structure. we will build our own...
    if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
    {
        perror("setsockopt() error");
        exit(-1);
    }
    else
        printf("setsockopt() is OK.\n");

    // Send loop, send for every 2 second for 100 count
    printf("Trying...\n");
    printf("Using raw socket and UDP protocol\n");
    printf("Using Source IP: %s port: %u, Target IP: %s port: %u.\n", argv[1], atoi(argv[2]), argv[3], atoi(argv[4]));

    int count;
    int i;
    for(count = 1; count <=20; count++)
    {
        if(i = sendto(sd, buffer, PCKT_LEN, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0)
        // Verify
        {
            perror("sendto() error");
            exit(-1);
        }
        else
        {
            printf("Count #%u - sendto() is OK. Data Length#%d\n", count,i);
            sleep(2);
        }
    }
    close(sd);
    return 0;
}
3
c++csocketswiresharkraw-sockets
I edited it to make it "better", but I think some of the original spaces were lost. Perhaps Shashi could edit and use the {} to mark the code rather than ` `. - Mats Petersson
I think, however, that 0 packet len means that "nothing was sent", so wireshark won't see anything, because you never sent a packet. As to why that is, I'm not sure. - Mats Petersson
Don't reinvent the wheel, use libtins :D - mfontanini

3 Answers

6
votes

Aha! I've got at least part of it.

i = sendto(sd, buffer, PCKT_LEN, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0 is the same as i = (sendto(sd, buffer, PCKT_LEN, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0)

you probably want: (i = sendto(sd, buffer, PCKT_LEN, 0, (struct sockaddr *)&sin, sizeof(sin))) < 0

You may want to:

  1. Turn on warnings in your compiler - at least if you use gcc, that should give you a warning for comparing and assigning in the same if-statement.
  2. Retry with the fixed code.

I'm sure there may be other problems in your code too - I'm no network expert.

3
votes

It is really hard to read this piece of code and to understand what and why you're doing. So I can recoomend you look at my piece of code: dhcp client implementation

Look at function getSock() to see how socket is created, and on function talker() on how to form and send completed packet.

0
votes

Local IP header structure is wrong... my suggestion is to include the IP header provided with your distro (are you using linux? don't you?).

What i did is just include linux/ip.h, rename ipheader structure reference to iphdr, and rename the ip header fields according to the structure described in the latter file.

I tried to sniff packets with tcpdump and it works now (i didn't try with wireshark but it must work too)

Try this fixed code:

// Must be run by root lol! Just datagram, no payload/data
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/udp.h>
#include <linux/ip.h>

// The packet length
#define PCKT_LEN 35

// UDP header's structure
struct udpheader {
    unsigned short int udph_srcport;
    unsigned short int udph_destport;
    unsigned short int udph_len;
    unsigned short int udph_chksum;
};
// total udp header length: 8 bytes (=64 bits)

// Function for checksum calculation. From the RFC,
// the checksum algorithm is:
//  "The checksum field is the 16 bit one's complement of the one's
//  complement sum of all 16 bit words in the header.  For purposes of
//  computing the checksum, the value of the checksum field is zero."
unsigned short csum(unsigned short *buf, int nwords)
{       //
    unsigned long sum;
    for(sum=0; nwords>0; nwords--)
        sum += *buf++;
    sum = (sum >> 16) + (sum &0xffff);
    sum += (sum >> 16);
    return (unsigned short)(~sum);
}

// Source IP, source port, target IP, target port from the command line arguments
int main(int argc, char *argv[])
{
    int sd;
    // No data/payload just datagram
    char buffer[PCKT_LEN];
    // Our own headers' structures
    struct iphdr *ip = (struct iphdr *) buffer;
    struct udpheader *udp = (struct udpheader *) (buffer + sizeof(struct iphdr));
    // Source and destination addresses: IP and port
    struct sockaddr_in sin, din;
    int one = 1;
    const int *val = &one;

    memset(buffer, 0, PCKT_LEN);

    if(argc != 5)
    {
        printf("- Invalid parameters!!!\n");
        printf("- Usage %s <source hostname/IP> <source port> <target hostname/IP> <target port>\n", argv[0]);
        exit(-1);
    }

    // Create a raw socket with UDP protocol
    sd = socket(PF_INET, SOCK_RAW, IPPROTO_UDP);
    if(sd < 0)
    {
        perror("socket() error");
        // If something wrong just exit
        exit(-1);
    }
    else
        printf("socket() - Using SOCK_RAW socket and UDP protocol is OK.\n");

    // The source is redundant, may be used later if needed
    // The address family
    sin.sin_family = AF_INET;
    din.sin_family = AF_INET;
    // Port numbers
    sin.sin_port = htons(atoi(argv[2]));
    din.sin_port = htons(atoi(argv[4]));
    // IP addresses
    sin.sin_addr.s_addr = inet_addr(argv[1]);
    din.sin_addr.s_addr = inet_addr(argv[3]);

    // Fabricate the IP header or we can use the
    // standard header structures but assign our own values.
    ip->ihl = 5;
    ip->version = 4;
    ip->tos = 16; // Low delay
    ip->tot_len = sizeof(struct iphdr) + sizeof(struct udpheader);
    ip->id = htons(54321);
    ip->ttl = 64; // hops
    ip->protocol = 17; // UDP
    // Source IP address, can use spoofed address here!!!
    ip->saddr = inet_addr(argv[1]);
    // The destination IP address
    ip->daddr = inet_addr(argv[3]);

    // Fabricate the UDP header. Source port number, redundant
    udp->udph_srcport = htons(atoi(argv[2]));
    // Destination port number
    udp->udph_destport = htons(atoi(argv[4]));
    udp->udph_len = htons(sizeof(struct udpheader));

    // Calculate the checksum for integrity
    ip->check = csum((unsigned short *)buffer, sizeof(struct iphdr) + sizeof(struct udpheader));

    // Inform the kernel do not fill up the packet structure. we will build our own...
    if(setsockopt(sd, IPPROTO_IP, IP_HDRINCL, val, sizeof(one)) < 0)
    {
        perror("setsockopt() error");
        exit(-1);
    }
    else
        printf("setsockopt() is OK.\n");

    // Send loop, send for every 2 second for 100 count
    printf("Trying...\n");
    printf("Using raw socket and UDP protocol\n");
    printf("Using Source IP: %s port: %u, Target IP: %s port: %u.\n", argv[1], atoi(argv[2]), argv[3], atoi(argv[4]));

    int count;
    int i;
    for(count = 1; count <=20; count++)
    {
        if((i = sendto(sd, buffer, PCKT_LEN, 0, (struct sockaddr *)&sin, sizeof(sin))) < 0)
        // Verify
        {
            perror("sendto() error");
            exit(-1);
        }
        else
        {
            printf("Count #%u - sendto() is OK. Data Length# %d\n", count,i);
            sleep(2);
        }
    }
    close(sd);
    return 0;
}