C - Function not executing. calling error? - c

Question

I am trying to build a program that can send TCP packets, but when I compiled my code to test the send function it just doesn't run the way it should. When given arguments it skips to the getchar and exits the program as if the function wasn't called at all. Now I am not particularly experienced and this is probably a stupid question, but I haven't been able to find a solution elsewhere. My guess is I'm calling it wrong somehow and that maybe the send_tcp function can't be inside of the forgepacket function.
The following is my code (not finished at all but should be good enough for a little test run) and the function giving me problems is forgepacket.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <getopt.h>
#include <sys/socket.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#define VERSION "1.0"
#define PCKT_LEN 8192
/* Prototypes */
void forgepacket (unsigned int, unsigned int, unsigned short, unsigned short);
void usage();
/* Checksum */
unsigned short checksum (unsigned short *pac, int len)
{
unsigned long sum;
for (sum = 0; len > 0; len--)
sum += *pac++;
sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
return(~sum);
}
in_addr_t sip;
in_addr_t dip;
int sport;
int dport;
int main(int argc,char **argv)
{
int c;
/* Are we in root? */
if(geteuid() !=0)
{
printf("Root access is required to run this program.\n\n");
exit(0);
}
while (1)
{
static struct option long_options[] =
{
/* Options */
{"send", no_argument, 0, 's'}, /* args s, r and f have no function yet */
{"recieve", no_argument, 0, 'r'},
{"file", required_argument, 0, 'f'},
{"destip", required_argument, 0, 'i'},
{"destport", required_argument, 0, 'p'},
{"sourceip", required_argument, 0, 'o'},
{"sourceport", required_argument, 0, 't'},
{0, 0, 0, 0}
};
int option_index = 0;
c = getopt_long (argc, argv, "srf:d:i:p:o:t:",
long_options, &option_index);
/* Detect the end of the options. */
if (c == -1)
break;
switch (c)
{
case 0:
/* If this option set a flag, do nothing else now. */
if (long_options[option_index].flag != 0)
break;
printf ("option %s", long_options[option_index].name);
if (optarg)
printf (" with arg %s", optarg);
printf ("\n");
break;
case 's':
puts ("option -s\n");
break;
case 'r':
puts ("option -r\n");
break;
case 'f':
printf ("option -f with value `%s'\n", optarg);
break;
case 'i':
dip = inet_addr(optarg);
break;
case 'p':
dport = htons(atoi(optarg));
/* Add handling of bad/non number input here */
break;
case 'o':
sip = inet_addr(optarg);
break;
case 't':
sport = htons(atoi(optarg));
break;
case '?':
/* Error message printed */
break;
default:
abort ();
}
}
/* Print any remaining command line arguments (not options). */
if (optind < argc)
{
printf ("non-option ARGV-elements: ");
while (optind < argc)
printf ("%s ", argv[optind++]);
putchar ('\n');
}
/* check if all mandatory options are set and for unknown arguments */
/* This REALLY needs changing... */
if (dip, sip, dport, sport == 0)
{
usage();
return (-1);
}
forgepacket(dip, sip, dport, sport);
getchar ();
exit (0);
}
void forgepacket(unsigned int sourceip, unsigned int destip, unsigned short sourceport,
unsigned short destport)
{
/* IP header structure */
struct ipheader {
unsigned char iph_ihl:5,
iph_ver:4;
unsigned char iph_tos;
unsigned short int iph_len;
unsigned short int iph_id;
unsigned char iph_flags;
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;
};
/* TCP header structure */
struct tcpheader {
unsigned short int tcph_sourceport;
unsigned short int tcph_destport;
unsigned int tcph_seqnum;
unsigned int tcph_acknum;
unsigned char tcph_reserved:4, tcph_offset:4;
unsigned int
tcp_res1:4,
tcph_hlen:4,
tcph_fin:1,
tcph_syn:1,
tcph_rst:1,
tcph_psh:1,
tcph_ack:1,
tcph_urg:1,
tcph_res2:2;
unsigned short int tcph_win;
unsigned short int tcph_chksum;
unsigned short int tcph_urgptr;
};
int send_tcp()
{
int sock, one = 1;
char buffer[PCKT_LEN];
struct sockaddr_in sin, din;
const int *val = &one;
sock = socket(PF_INET, SOCK_RAW, IPPROTO_TCP);
if (sock < 0)
{
printf("\nError: socket()\n\n");
exit (-1);
}
else
printf ("\nsocket() - Using SOCK_RAW and TCP protocol is OK.\n\n");
/* Size of the headers */
struct ipheader *ip = (struct ipheader *) buffer;
struct tcpheader *tcp = (struct tcpheader *) (buffer + sizeof (struct ipheader));
memset (buffer, 0, PCKT_LEN);
/* IP attributes */
ip->iph_ihl = 5;
ip->iph_ver = 4;
ip->iph_tos = 16;
ip->iph_len = sizeof(struct ipheader) + sizeof(struct tcpheader);
ip->iph_id = htons(54321);
ip->iph_offset = 0;
ip->iph_ttl = 64;
ip->iph_protocol = 6;
ip->iph_chksum = 0;
ip->iph_sourceip = sip;
ip->iph_destip = dip;
/* TCP attributes */
tcp->tcph_sourceport = sport;
tcp->tcph_destport = dport;
tcp->tcph_seqnum = htonl(1);
tcp->tcph_acknum = 0;
tcp->tcph_offset = 5;
tcp->tcph_syn = 1;
tcp->tcph_ack = 0;
tcp->tcph_win = htons(32767);
tcp->tcph_chksum = 0;
tcp->tcph_urgptr = 0;
ip->iph_chksum = checksum ((unsigned short *) buffer, (sizeof (struct ipheader )+ sizeof (struct tcpheader)));
/* Address family */
sin.sin_family = AF_INET;
din.sin_family = AF_INET;
/* Source port */
sin.sin_port = sport;
din.sin_port = dport;
/* Source IP */
sin.sin_addr.s_addr = sip;
din.sin_addr.s_addr = dip;
/* Tell the Kernel we're building our own packet */
if ((setsockopt(sock, IPPROTO_IP, IP_HDRINCL, (char *)&one, sizeof (one))) < 0)
{
printf("\nError: Can't set socketoptions\n\n");
return (-1);
}
/* Send */
if (sendto(sock, buffer, ip->iph_len, 0, (struct sockaddr *)&sin, sizeof(sin)) < 0)
{
printf("\nError: Can't send packet\n\n");
return (-1);
}
else
printf("Packet sent to %d", dip);
close(sock);
}
}
void usage()
{
/* This is the user manual */
printf("\nTCP project %s\n\n", VERSION);
printf("Usage: -s -f <file> -i <destip> -p <destport> -o <sourceip> -t <sourceport>\n\n");
printf("-s, --send, Puts program in send mode\n");
printf("-r, --recieve, Puts program in recieve mode\n");
printf("-f, --file, Specify file containing steganographic message\n");
printf("-i, --destip, Destination IP address\n");
printf("-p, --destport, Destination port\n");
printf("-o, --sourceip Source IP address\n");
printf("-t, --sourceport Source port\n");
}

Answer 1

It appears the function definition of send_tcp() is within the function definition of forgepacket() (I didn't know this was even possible??), but there is no call to it. Replicate the arrangement you have for forgepacket() for send_tcp():
/* Prototype. */
int send_tcp(); /* add whatever parameters are required. */
and move definition of send_tcp() outside of forgepacket() and add a call to send_tcp() within forgepacket().

Answer 2

C doesn't allow nested functions by default. Furthermore, even if it did, I am not seeing a call to send_tcp in the code (it may be there, but a quick scan doesn't show it).
What happens if you break the send_tcp function out of forge_packet and make sure to pass the required data in as parameters?