Comments on: How to Code Raw Sockets in C on Linux

By: Vladyslav

Vladyslav — Tue, 28 Feb 2023 10:38:22 +0000

Hi, thank you for the great example!
One question. You use direct mapping of the structures into memory. How do you know that there will be no problems with alignment? Did you just check it with “sizeof” beforehand?

By: Silver Moon

Silver Moon — Sun, 26 Jul 2020 14:36:33 +0000

In reply to Dev. if you need to receive the data then it is not recommended to use raw sockets. Just use normal tcp or udp sockets to send and receive data.

By: Majid

Majid — Sat, 05 May 2018 10:24:41 +0000

also you can using below checksum function in your codes instead of this checksum function because this function may not work correctly on all processor architecture.
https://locklessinc.com/articles/tcp_checksum/

proposed checksum is second function in this link. you can read descriptions about different functions in above link.

Thanks.

By: Majid

Majid — Sat, 05 May 2018 10:21:41 +0000

hi dears,
this code has one problem:
1. you must send size of IP header for checksum function instead of iph->tot_len because this field is sum of IP and TCP with data part. (IP header size is 20 byte)

Thanks.

By: Mike

Mike — Tue, 17 Apr 2018 15:57:50 +0000

Why is the payload in hexadecimal form?

By: Dev

Dev — Sun, 18 Feb 2018 18:37:02 +0000

If i want receive the data then what should i have to do ?

By: SAURAV SEN

SAURAV SEN — Sat, 21 Oct 2017 11:26:52 +0000

sir why datagram is of size 4096…have u taken it randomly or thorugh some calculation??

struct iphdr *iph = (struct iphdr *) datagram;

//TCP header
struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ip));

please explain sir this it will be helpful if u mail me… also

By: Joseph

Joseph — Wed, 22 Jun 2016 14:08:21 +0000

Nice example: I am a little confused about the IPH checksum. you are passing iph->tot_len to csum(), which looks to include the data. I thought the IPH checksum was confined to only the header. Tx.

By: Sruthi

Sruthi — Tue, 10 May 2016 10:25:26 +0000

Can I get the code for SMTP raw socketing?

By: Abhishek Sagar

Abhishek Sagar — Tue, 03 May 2016 11:31:03 +0000

In reply to Nicolai Pascal Großer.

I began reading this article, because i wanted to learn how user can define his own L4 layer protocol, but you ended up preparing standard TCP packet. Anyways….
Suppose if i define some XYZ layer 4 protocol, how the recieving machine would going to unwrap the layer 4 header as it may not aware of this non standard L4 header ??

By: Imed Ontario

Imed Ontario — Tue, 29 Dec 2015 04:09:00 +0000

thannnnks, can you give us the modified raw tcp code for the ipv6 please ?

By: Alex

Alex — Sat, 11 Jul 2015 15:32:00 +0000

Hello,

Very good example and explanations. Since I make use of raw sockets to send crafted TCP packets in my program, I was looking for an example on the web for exact variables, structures etc. and after reading many of those, I can tell you sir, Your’s explanation is excellent. Not only you’re mentioning all the correct fields. you’re also demonstrating using it with some excellent example.

However, since you’re using “malloc” in your example, shouldn’t you “free” it before the program ends?

By: Nicolai Pascal Großer

Nicolai Pascal Großer — Mon, 02 Feb 2015 15:53:00 +0000

In reply to Nicolai Pascal Großer. Okay. I just checked out the header files. It is actually the same. Probably some Linux/BSD stuff.

By: Nicolai Pascal Großer

Nicolai Pascal Großer — Mon, 02 Feb 2015 01:48:00 +0000

where is the difference between “struct iphdr” and “struct ip”
Why do you use “struct ip” in line 74 ?

By: Ivan

Ivan — Wed, 19 Feb 2014 11:59:00 +0000

Thanks for the example. How do I send a complete http get request to a particular uri?

By: Vijay Kanta

Vijay Kanta — Fri, 28 Jun 2013 13:17:00 +0000

Wow, I knew a bit of socket programming with pure C, but this is taking it too deeper. Thanks for letting me know it even existed. :-)

By: Silver Moon

Silver Moon — Thu, 27 Jun 2013 12:37:00 +0000

In reply to John Lauro. yeah right, thanks for pointing it out. shall fix the code to be more portable and post soon.

By: John Lauro

John Lauro — Thu, 27 Jun 2013 12:33:00 +0000

The code makes some architecture assumptions that will probably work 99% of the time, but isn’t completely portable. For example, the checksum calculation assumes short is 2 bytes, and will likely segfault on anything where it is larger.

By: Silver Moon

Silver Moon — Thu, 27 Jun 2013 03:30:00 +0000

In reply to patrick h. thanks for pointing it out. the total_length value should be passed through htons. the code works well because the kernel fills in the correct total length in the ip header.

By: patrick h

patrick h — Thu, 27 Jun 2013 03:08:00 +0000

Shouldn’t iph->tot_len be hton’d?

Seems like there are several fields in the headers that need to be hton’d and they dont seem to be present in this example…

does this example work? or am i missing something?