C - raw socket - tcp/ip header problem

[ApEkV2]

I finished this code today and I'm having trouble with the tcp/ip headers.
They are all messed up, and wireshark doesn't even pick them up.
The input for the program is "192.168.1.100 80"
I think I might have htons and htonl mixed up a lot.
Any and all help would be awesome! (strace does show that sendto is being called)

#include <arpa/inet.h>
#include <ctype.h>
#include <limits.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <unistd.h>

struct pseudohdr {
unsigned long saddr;
unsigned long daddr;
char useless;
unsigned char protocol;
unsigned short length;
};
#define PSEUDO sizeof(struct pseudohdr)
#define TCPHDR sizeof(struct tcphdr)
#define Z_NL 4294967295
unsigned short c_sum(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); }

int main(int argc, char *argv[])
{

int h = 0, x = 0, c, s_port; char *zvalue = NULL;
extern int optind, opterr, optopt;
while ((c = getopt (argc, argv, "hxz:")) != -1)
switch (c) {
case 'h':
h = 1;
break;
case 'x':
x = 1;
break;
case 'z':
zvalue = optarg;
break;
case '?':
if (optopt == 'z')
fprintf (stderr, "Option -%c requires an argument.\n", optopt);
else if (isprint (optopt))
fprintf (stderr, "Unknown option `-%c'.\n", optopt);
else
fprintf (stderr, "Unknown option character `\\x%x'.\n", optopt);
default:
return 1; }

while (optind < (argc - 2)) {
optind++; }
char *d_addr = argv[optind];
unsigned int d_port = atoi(argv[++optind]);
if ((d_addr == NULL || d_port < 1 || d_port > 64000) && h == 0)
{ printf("Error:\n"); h = 1; }

if (h > 0) {
printf(
"Usage: [command] [host] [port] [options]\n"
" -h = show help "
" -x = Not used "
" -z = Not used\n"
); return 0; }

srand((unsigned)time(NULL));
s_port = (int)(rand()/(((double)RAND_MAX + 1)/64000));
int s = socket(PF_INET, SOCK_RAW, IPPROTO_TCP);
if (s == -1) {
perror("[open_sockraw] socket()");
return 1; }

char p_data[4096];
struct ip *iph = (struct ip *) p_data;
struct tcphdr *tcph = (struct tcphdr *) p_data + sizeof(struct ip);
struct sockaddr_in sin;
struct pseudohdr pseudo;
sin.sin_family = AF_INET;
sin.sin_port = htons(d_port);
sin.sin_addr.s_addr = inet_addr(argv[--optind]);
memset(p_data, 0, 4096);

iph->ip_hl = sizeof(struct ip) >> 2;
iph->ip_v = 4;
iph->ip_tos = 0;
iph->ip_len = sizeof(struct ip) + sizeof(struct tcphdr);
iph->ip_id = htons((int)(rand()/(((double)RAND_MAX + 1)/14095)));
iph->ip_off = 0;
iph->ip_ttl = 64;
iph->ip_p = IPPROTO_TCP;
iph->ip_sum = htons((c_sum((unsigned short *) p_data, iph->ip_len >> 1)));
iph->ip_src.s_addr = inet_addr("192.168.1.105");
iph->ip_dst.s_addr = sin.sin_addr.s_addr;

tcph->source = htons(s_port);
tcph->dest = htons(d_port);
tcph->seq = htonl((int)(rand()/(((double)RAND_MAX + 1)/Z_NL)));
tcph->ack_seq = htonl(0);
tcph->res1 = 0;
tcph->doff = 5;
tcph->fin = 1;
tcph->syn = 1;
tcph->rst = 0;
tcph->psh = 0;
tcph->ack = 0;
tcph->urg = 0;
tcph->res2 = 0;
tcph->window = htons(512);

pseudo.saddr = inet_addr("127.0.0.1");
pseudo.daddr = sin.sin_addr.s_addr;
pseudo.useless = htons(0);
pseudo.protocol = IPPROTO_TCP;
pseudo.length = htons(TCPHDR);

tcph->check = htons((c_sum((unsigned short *) &pseudo, PSEUDO+TCPHDR)));
tcph->urg_ptr = htons(0);

int tmp = 1, z_stop = 0; const int *val = &tmp;
if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, val, sizeof(tmp)) < 0) {
printf("Error: setsockopt() - Cannot set HDRINCL:\n");
return 1; }

while (z_stop < 2) {
if (sendto(s, p_data, iph->ip_len, 0, (struct sockaddr *) &sin, sizeof(sin)) < 0) {
printf("Error: sendto()\n");
break; }
else
++z_stop;
}

return 0;
}
And this is the strace of sendto.

sendto(3, "E\0(\0\302\5\0@\377\6\244\220\177\0\0\1\300\250\1d \0\0\0\0\0\0\0\0\0\0\0\0\0"..., 40, 0, {sa_family=AF_INET, sin_port=htons(80), sin_addr=inet_addr("192.168.1.100")}, 16) = 40

And this is the strace of sendto for Hping3 (with -SF)

sendto(3, "E\0\0(\370F\0\0@\6\0\0\300\250\1i\300\250\1d\0062\ 0P0B\35l8C\243\10P"..., 40, 0, {sa_family=AF_INET, sin_port=htons(11111), sin_addr=inet_addr("192.168.1.100")}, 16) = 40

[dwhitney67]

I created an app on my system that sets up a raw socket that uses TCP, and I was able to successfully send messages... and more importantly, monitor the messages using WireShark.

Here's a snippet of the code I wrote (which is in C++); extract the relevant part for your C program.


RawData BuildTcpDatagram(const std::string& servAddr, unsigned short servPort, const RawData& msg)
{
// Obtain the destination and source addresses to be inserted into the IP header
sockaddr_in dst_sin;
sockaddr_in src_sin;

int dst_rtn = socketpp::fillAddress(PF_INET, servAddr.c_str(), servPort, &dst_sin);
int src_rtn = socketpp::fillAddress(PF_INET, "127.0.0.1", 1234, &src_sin);
assert(dst_rtn == 0 && src_rtn == 0);

// this buffer will contain IP header, TCP header, and payload. we'll point an ip
// header structure at its beginning, and a TCP header structure after that to write
// the header values into it.
unsigned char* datagram = new unsigned char[sizeof(ip) + sizeof(tcphdr) + msg.size()];

// setup offsets to mark our header and data regions
ip* iph = reinterpret_cast<ip*>(datagram);
int ip_len = sizeof(ip) + sizeof(tcphdr) + msg.size();

iph->ip_hl = sizeof(ip) >> 2; // 5 words (20 bytes), but could be more!
iph->ip_v = 4; // IPv4
iph->ip_tos = 0;
iph->ip_len = htons(ip_len); // the total length of the datagram
iph->ip_id = htons(54321); // the value doesn't matter here
iph->ip_off = 0;
iph->ip_ttl = 255;
iph->ip_p = IPPROTO_TCP;
iph->ip_sum = 0; // set to 0; compute checksum later
iph->ip_src.s_addr = src_sin.sin_addr.s_addr;
iph->ip_dst.s_addr = dst_sin.sin_addr.s_addr;
//iph->ip_src.s_addr = inet_addr(servAddr.c_str());
//iph->ip_dst.s_addr = inet_addr("127.0.0.1");

// compute checksum for the IP header (this is optional)
iph->ip_sum = socketpp::headerChecksum(reinterpret_cast<uint16_t*>(iph), iph->ip_hl << 2);

// now obtain our other data pointers
tcphdr* tcph = reinterpret_cast<tcphdr*>(datagram + (iph->ip_hl << 2));
unsigned char* data = datagram + (iph->ip_hl << 2) + sizeof(tcphdr);

// assign the message data
memcpy(data, msg.data(), msg.size());

// we'll now fill in the TCP header values
tcph->source = 0;
tcph->dest = htons(servPort);
tcph->seq = 1;
tcph->ack_seq = 0;
tcph->res1 = 0;
tcph->doff = sizeof(tcphdr) >> 2;
tcph->fin = 0;
tcph->syn = 0;
tcph->rst = 0;
tcph->psh = 0;
tcph->ack = 0;
tcph->urg = 0;
tcph->res2 = 0;
tcph->window = 0;
tcph->check = 0;
tcph->urg_ptr = 0;

RawData rd(datagram, datagram + ip_len);

delete [] datagram;

return rd;
}


Btw, I sent the message to the localhost (127.0.0.1), port 12345.

[ApEkV2]

Thanks for posting an example, unfortunately after a couple of hours trying to "extract", I tried to follow a tut.
http://packetstormsecurity.nl/programming-tutorials/raw_socket.txt
I updated the code above, and now if ip header length ( iph->ip_hl = 5; ) is above 16 bytes, it yields a sendto error.
sendto(blablabla) = -1 EPERM (Operation not permitted)

[dwhitney67]

Thanks for posting an example, unfortunately after a couple of hours trying to "extract", I tried to follow a tut.
http://packetstormsecurity.nl/programming-tutorials/raw_socket.txt
I updated the code above, and now if ip header length ( iph->ip_hl = 5; ) is above 16 bytes, it yields a sendto error.
sendto(blablabla) = -1 EPERM (Operation not permitted)

I don't understand what you are saying. When iph->ip_hl is set to 5, this implies that the header has a size of 20 bytes. It is used to denote how many long-words are in the header.

20 >> 2 = 5

or better yet...

sizeof(struct ip) >> 2 = 5

[ApEkV2]

My bad........(me and words)....if ip header length is GREATER than 16 bytes, it gives me that error

EDIT - so for example if I use iph->ip_hl = 5; (20bytes) it gives an error
, but if I use iph->ip_hl = 4; (16bytes) it works (although the packet is invalid)

Here is the strace

sendto(3, "E\0(\0\3241\0\0\377\6\277\306\177\0\0\1\300\250\1d \0\0\0\0\0\0\0\0\0\0\0\0\0"..., 40, 0, {sa_family=AF_INET, sin_port=htons(80), sin_addr=inet_addr("192.168.1.100")}, 16) = -1 EPERM (Operation not permitted)

[dwhitney67]

Well, unless you have extended the IP header, it should be 20 bytes long.

Therefore, if you use 5 (20 >> 2) as your IP header length, and you get an error performing the sendto(), the issue lies elsewhere.

The code I posted earlier worked for me, although WireShark was annoyed that I posted multiple packets with the same sequence number.

Just moments ago I augmented my code to include the checksum in the TCP header; what a pain that was. The formula is the same as used for the IP header, however the data that is used is "weird" because it involves creating a temporary pseudo header for the packet. This pseudo header includes the source address, destination address, the IP protocol, and the length of the TCP header and the data that follows.

[ApEkV2]

I've updated the code in the first post. Is there a way to use GDB for debugging this? (or anything other than strace)

[dwhitney67]

I've updated the code in the first post. Is there a way to use GDB for debugging this? (or anything other than strace)

Yes, just compile your code with the '-g' option. Then when you debug, use a statement similar to the following:


sudo gdb myapp

After you see the GDB prompt, then you enter your command-line parameters. For example:


gdb> b main
gdb> r 192.168.1.100 80

[ApEkV2]

Well I give up on gdb for now (can't really get anything useful out of it).
I used sudo gdb --args a.out [192.168.1.100 80], but I just don't know what to do with it.
Right now I'm about to pass out from lack of sleep. Any other ideas for debugging?

EDIT - I'm getting closer to Hping's sendto. The differences are in red
Hping3 (with -SF)

sendto(3, "E\0\0(\370F\0\0@\6\0\0\300\250\1i\300\250\1d\0062\ 0P0B\35l8C\243\10P"..., 40, 0,
And my program - the tcp header isn't working

sendto(3, "E\0(\0\0177\0\0@\6\0\0\300\250\1i\300\250\1d\0\0\0 \0\0\0\0\0\0\0\0\0\0"..., 40, 0,

[ApEkV2]

Well it turns out that the problem with the sendto was firestarter, once I turned firestarter off, it worked.
However, Wireshark says that the IP header is fine, but the tcp header doesn't have anything in it. (it's empty)
What the heck? Here is a screen shot of wireshark.

[dwhitney67]

Can you please post your code again? I am not psychic, and there's nothing in the world that will help me analyze your code by merely viewing a WireShark screen shot. :p

_

[ApEkV2]

#include <arpa/inet.h>
#include <ctype.h>
#include <limits.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <unistd.h>

struct pseudohdr {
unsigned long saddr;
unsigned long daddr;
char useless;
unsigned char protocol;
unsigned short length;
};
#define PSEUDO sizeof(struct pseudohdr)
#define TCPHDR sizeof(struct tcphdr)
#define Z_NL 4294967295
unsigned short c_sum(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); }

int main(int argc, char *argv[])
{

int h = 0, x = 0, c, s_port; char *zvalue = NULL;
extern int optind, opterr, optopt;
while ((c = getopt (argc, argv, "hxz:")) != -1)
switch (c) {
case 'h':
h = 1;
break;
case 'x':
x = 1;
break;
case 'z':
zvalue = optarg;
break;
case '?':
if (optopt == 'z')
fprintf (stderr, "Option -%c requires an argument.\n", optopt);
else if (isprint (optopt))
fprintf (stderr, "Unknown option `-%c'.\n", optopt);
else
fprintf (stderr, "Unknown option character `\\x%x'.\n", optopt);
default:
return 1; }

while (optind < (argc - 2)) {
optind++; }
char *d_addr = argv[optind];
unsigned int d_port = atoi(argv[++optind]);
if ((d_addr == NULL || d_port < 1 || d_port > 64000) && h == 0)
{ printf("Error:\n"); h = 1; }

if (h > 0) {
printf(
"Usage: [command] [host] [port] [options]\n"
" -h = show help "
" -x = Not used "
" -z = Not used\n"
); return 0; }

srand((unsigned)time(NULL));
s_port = (int)(rand()/(((double)RAND_MAX + 1)/64000));
int s = socket(PF_INET, SOCK_RAW, IPPROTO_TCP);
if (s == -1) {
perror("[open_sockraw] socket()");
return 1; }

char p_data[4096];
struct ip *iph = (struct ip *) p_data;
struct tcphdr *tcph = (struct tcphdr *) p_data + sizeof(struct ip);
struct sockaddr_in sin;
struct pseudohdr pseudo;
sin.sin_family = AF_INET;
sin.sin_port = htons(d_port);
sin.sin_addr.s_addr = inet_addr(argv[--optind]);
memset(p_data, 0, 4096);

iph->ip_hl = sizeof(struct ip) >> 2;
iph->ip_v = 4;
iph->ip_tos = 0;
iph->ip_len = sizeof(struct ip) + sizeof(struct tcphdr);
iph->ip_id = htons((int)(rand()/(((double)RAND_MAX + 1)/14095)));
iph->ip_off = 0;
iph->ip_ttl = 64;
iph->ip_p = IPPROTO_TCP;
iph->ip_sum = htons((c_sum((unsigned short *) p_data, iph->ip_len >> 1)));
iph->ip_src.s_addr = inet_addr("192.168.1.105");
iph->ip_dst.s_addr = sin.sin_addr.s_addr;

tcph->source = htons(s_port);
tcph->dest = htons(d_port);
tcph->seq = htonl((int)(rand()/(((double)RAND_MAX + 1)/Z_NL)));
tcph->ack_seq = htonl(0);
tcph->res1 = 0;
tcph->doff = sizeof(struct tcphdr) >> 2;
tcph->fin = 1;
tcph->syn = 1;
tcph->rst = 0;
tcph->psh = 0;
tcph->ack = 0;
tcph->urg = 0;
tcph->res2 = 0;
tcph->window = htons(512);

pseudo.saddr = inet_addr("192.168.1.105");
pseudo.daddr = sin.sin_addr.s_addr;
pseudo.useless = htons(0);
pseudo.protocol = IPPROTO_TCP;
pseudo.length = htons(TCPHDR);

tcph->check = htons((c_sum((unsigned short *) &pseudo, PSEUDO+TCPHDR)));
tcph->urg_ptr = htons(0);

int tmp = 1, z_stop = 0; const int *val = &tmp;
if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, val, sizeof(tmp)) < 0) {
printf("Error: setsockopt() - Cannot set HDRINCL:\n");
return 1; }

while (z_stop < 1) {
if (sendto(s, p_data, iph->ip_len, 0, (struct sockaddr *) &sin, sizeof(sin)) < 0) {
printf("Error: sendto()\n");
break; }
else
++z_stop;
}

return 0;
}

[dwhitney67]

I took your code, blended in my code (which I believe works), and came up with the app below.

I removed some of the non-essentials from your code; so feel free to add them in at your leisure.



#include <arpa/inet.h>
#include <ctype.h>
#include <limits.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <unistd.h>
#include <time.h>
#include <errno.h>

struct PseudoHdr
{
unsigned long saddr;
unsigned long daddr;
char reserved;
unsigned char protocol;
unsigned short length;
};

#define PSEUDO sizeof(struct pseudohdr)
#define TCPHDR sizeof(struct tcphdr)
#define Z_NL 4294967295

unsigned short c_sum(unsigned short* data, int nbytes)
{
unsigned long sum = 0;

for (; nbytes > 1; nbytes -= 2)
{
sum += *data++;
}

if (nbytes == 1)
{
sum += *(unsigned char*) data;
}

sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);

return ~sum;
}

int fillAddress(int domain, const char* address, unsigned short port, struct sockaddr_in* sin);

unsigned char* buildTCPDatagram(struct sockaddr_in* src_sin, struct sockaddr_in* dst_sin, const unsigned char* msg, unsigned int msgSize);

unsigned int buildPseudoHdrBuffer(unsigned int src_addr, unsigned int dst_addr, unsigned int protocol,
const unsigned char* hdrData, unsigned int hdrBytes,
const unsigned char* msgData, unsigned int msgBytes,
unsigned short** buffer);

int main(int argc, char** argv)
{
const char* dest_ip = (argc > 1 ? argv[1] : "127.0.0.1"); // destination IP
const unsigned short dest_port = (argc > 2 ? atoi(argv[1]) : 80); // destination port

srand(time(0));

// root-privileges needed for the following operation
int s = socket(PF_INET, SOCK_RAW, IPPROTO_TCP);

if (s <= 0)
{
perror("[open_sockraw] socket()");
return 1;
}

int enable = 1;
if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, &enable, sizeof(enable)) < 0)
{
printf("Error: setsockopt() - Cannot set HDRINCL:\n");
return 1;
}

// We no longer need root privileges
setuid(getuid());

// build up out source and destination sock addresses
struct sockaddr_in src_sin;
struct sockaddr_in dst_sin;

fillAddress(PF_INET, "127.0.0.1", 1234, &src_sin);
fillAddress(PF_INET, dest_ip, dest_port, &dst_sin);


// build our TCP datagram
char* msg = "Hello World";
unsigned char* datagram = buildTCPDatagram(&src_sin, &dst_sin, (unsigned char*) msg, strlen(msg));
unsigned int datagramSize = sizeof(struct ip) + sizeof(struct tcphdr) + strlen(msg);

for (int i = 0; i < 3; ++i)
{
if (sendto(s, datagram, datagramSize, 0, (struct sockaddr*) &dst_sin, sizeof(dst_sin)) < 0)
{
printf("Error with sendto() -- %s (%d)\n", strerror(errno), errno);
break;
}
}
free(datagram);

return 0;
}


int fillAddress(int domain, const char* address, unsigned short port, struct sockaddr_in* sin)
{
if (!address)
{
memset(sin, 0, sizeof(struct sockaddr_in));

sin->sin_family = domain;
sin->sin_addr.s_addr = htonl(INADDR_ANY);
sin->sin_port = htons(port);
}
else
{
struct addrinfo hints;
struct addrinfo* host_info = 0;

memset(&hints, 0, sizeof(hints));

hints.ai_family = domain;

if (getaddrinfo(address, 0, &hints, &host_info) != 0 ||
!host_info || !host_info->ai_addr || host_info->ai_family != domain)
{
if (host_info) freeaddrinfo(host_info);
return -1;
}

memcpy(sin, host_info->ai_addr, sizeof(struct sockaddr_in));
sin->sin_port = htons(port);

freeaddrinfo(host_info);
}

return 0;
}


unsigned char* buildTCPDatagram(struct sockaddr_in* src_sin, struct sockaddr_in* dst_sin, const unsigned char* msg, unsigned int msgSize)
{
const int ip_len = sizeof(struct ip) + sizeof(struct tcphdr) + msgSize;
unsigned char* datagram = calloc(1, ip_len);

if (!datagram) return 0;

// setup useful pointers to locations within the datagram
struct ip* iph = (struct ip*) datagram;
struct tcphdr* tcph = (struct tcphdr*)(datagram + sizeof(struct ip));
unsigned char* data = datagram + sizeof(struct ip) + sizeof(struct tcphdr);

// build IP header
iph->ip_hl = sizeof(struct ip) >> 2;
iph->ip_v = 4;
iph->ip_tos = 0;
iph->ip_len = htons(ip_len);
iph->ip_id = htons((int)(rand()/(((double)RAND_MAX + 1)/14095)));
iph->ip_off = 0;
iph->ip_ttl = 64;
iph->ip_p = IPPROTO_TCP;
iph->ip_sum = 0;
iph->ip_src.s_addr = src_sin->sin_addr.s_addr;
iph->ip_dst.s_addr = dst_sin->sin_addr.s_addr;

// now we compute the checksum for the IP header (albeit this is optional)
iph->ip_sum = c_sum((unsigned short*) iph, sizeof(struct ip));

// build TCP header
tcph->source = htons(src_sin->sin_port);
tcph->dest = htons(dst_sin->sin_port);
tcph->seq = htonl((int)(rand()/(((double)RAND_MAX + 1)/Z_NL)));
tcph->ack_seq = htonl(0);
tcph->res1 = 0;
tcph->doff = sizeof(struct tcphdr) >> 2;
tcph->fin = 1;
tcph->syn = 1;
tcph->rst = 0;
tcph->psh = 0;
tcph->ack = 0;
tcph->urg = 0;
tcph->res2 = 0;
tcph->window = htons(512);
tcph->check = 0;
tcph->urg_ptr = htons(0);

// now we compute the TCP header checksum, across a pseudo message buffer, not the actual TCP header
unsigned short* buffer = 0;
unsigned int bufferSize = buildPseudoHdrBuffer(src_sin->sin_addr.s_addr, dst_sin->sin_addr.s_addr, IPPROTO_TCP,
(const unsigned char*) tcph, sizeof(struct tcphdr),
msg, msgSize, &buffer);

tcph->check = c_sum(buffer, bufferSize);
free(buffer);

// add message data (if any)
if (msgSize > 0)
{
memcpy(data, msg, msgSize);
}

return datagram;
}


unsigned int buildPseudoHdrBuffer(unsigned int src_addr, unsigned int dst_addr, unsigned int protocol,
const unsigned char* hdrData, unsigned int hdrBytes,
const unsigned char* msgData, unsigned int msgBytes,
unsigned short** buffer)
{
struct PseudoHdr pseudoHdr;

pseudoHdr.saddr = src_addr;
pseudoHdr.daddr = dst_addr;
pseudoHdr.reserved = 0;
pseudoHdr.protocol = protocol;
pseudoHdr.length = htons(hdrBytes + msgBytes);

unsigned int bufSize = sizeof(struct PseudoHdr) + hdrBytes + msgBytes;
unsigned char* buf = calloc(1, bufSize);
int offset = 0;

memcpy(buf + offset, &pseudoHdr, sizeof(struct PseudoHdr)); offset += sizeof(struct PseudoHdr);
memcpy(buf + offset, hdrData, hdrBytes); offset += hdrBytes;
memcpy(buf + offset, msgData, msgBytes);

*buffer = (uint16_t*) buf;

return bufSize;
}

[ApEkV2]

Holy thanks for helping me out, however I had to move the declaration of "i" outside of the for loop.
And once I changed the destination address, wireshark sees three successful packets with both tcp/ip headers intact (plus the cool message).
Now I just gotta see what I am doing wrong in my code. Thanks again for the example.

[dwhitney67]

Holy thanks for helping me out, however I had to move the declaration of "i" outside of the for loop.
And once I changed the destination address, wireshark sees three successful packets with both tcp/ip headers intact (plus the cool message).
Now I just gotta see what I am doing wrong in my code. Thanks again for the example.
You're welcome.

I use the GCC option -std=gnu99 when compiling C source code, thus allowing me to declare variables pretty much wherever I please, including within the initialization section of for-loop constructs.

[ApEkV2]

I found the problem in mine......get ready......My Dear Aunt Sally. An order of operations mistake.

struct tcphdr *tcph = (struct tcphdr *) p_data + sizeof(struct ip);
Now corrected, the tcp header is now being filled.

struct tcphdr *tcph = (struct tcphdr *) (p_data + sizeof(struct ip));
Now all I have to fix is the tcp checksum, (cause it's bad)
And for the record, I wouldn't have seen the problem if dwhitney hadn't of posted his code.