Keep sending malformed TCP header using bsd/socket api in C

Question

long story short I'm currently experimenting with BSD/Socket C API .

I want to try to emulate a full TCP Handshake. The first thing that I have to do is send the first SYN packet to the host, in my case the router (192.168.1.1) .

Here is the snippet of the code that generates the SYN packet .

https://paste.ubuntu.com/23758682/

Here is what I'm getting from wireshark when I'm running this program as

sudo ./sendsyn -h 192.168.1.1 -p 80 -i enp5s0

http://prnt.sc/dsnmgq

The code is compiled on linux x86, 64 bit with GCC

$ gcc --version
gcc (GCC) 6.2.1 20160830

I want the code to be compliant just for the x86 platform and run only on Linux. I don't care about portability that much.

Also, I'm using in gcc this flags.

gcc  -DDEBUG_MODE  -std=gnu11 -fno-strict-aliasing 
-Wall -Wextra -pedantic -Wshadow -Wpointer-arith 
-Wcast-align -Wmissing-prototypes -Wmissing-declarations 
-Winline -Wuninitialized -Wstrict-prototypes 
-Werror -Wno-variadic-macros

So be aware that when compiling I'm turning off the -fno-strict-aliasing rules.

The problems

Why the tcp header dosen't show as tcp header in wireshark?

Previously I tried sending the packet as a struct like

typedef struct packet_t {
  struct iphdr iph;
  struct tcphdr tcph;
}__attribute__((packed)) packet_t;

And now in the code I first create a uint8_t datagram and after I've filled all the information in my IP Header and TCP Header I memcpy into the datagram and then after send them over the wire.

Why the ip header flags are not set to 0x4000(don't fragment) ?

Can anyone provide a solution to my problem in a snippet along with some good explanations?

Answer 1

I found a solution to my problem.

iph.frag_off = 0x4000;

this line should be

iph.frag_off = htons(0x4000);

Because 0x4000 is 16 bit wide and all values that needs more than 8 bits to be stoder should always be transformed into network byte order(big endian).

Now this is working, see wireshark tcp syn