print ip address and udp port from c code

Question

I am using a raw udp socket. i have an IP address and a udp port assigned in the following mannar by terminal insertion. Id like to print them in a classic string format (eg. 192.168.16.2)

ip->iph_sourceip = inet_addr(argv[1]);
udp->udph_srcport = htons(atoi(argv[2]));

I tryed printf ("Packet Send. iph_sourceip: %d \n" , ip->iph_sourceip); but I just get random numbers.

I also tried printf ("Packet Send. iph_sourceip: %c \n" , inet_ntoa(ip->iph_sourceip)); and here I get a strange sign. They don't work.

Here is the full code: (im interested in the printf at line 138)

// ----rawudp.c------
// Must be run by root lol! Just datagram, no payload/data
#include <unistd.h>
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/udp.h>

// The packet length
#define PCKT_LEN 8192

// Can create separate header file (.h) for all headers' structure
// The IP header's structure
struct ipheader {
    unsigned char iph_ihl :5, 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;
    for (count = 1; count <= 20; count++) {
        if (sendto(sd, buffer, ip->iph_len, 0, (struct sockaddr *) &sin,
                sizeof(sin)) < 0)
// Verify
                {
            perror("sendto() error");
            exit(-1);
        } else {
            printf("Count #%u - sendto() is OK.\n", count);
            printf ("Packet Send. iph_sourceip: %s \n" , inet_ntoa(ip->iph_sourceip));
            sleep(2);
        }
    }
    close(sd);
    return 0;
}

Answer 1

IP address is a string, you need to print with %s.

Answer 2

You need to use inet_ntoa() function to convert the network-order binary data to a dotted decimal notation and then you've to use %s format specifier to print the result.