Comments on: How to Program raw UDP sockets in C on Linux

By: Stefan

Stefan — Tue, 14 Sep 2021 14:58:50 +0000

Hi madmat,

Sorry, I think you’re wrong:
You oversee, that a value >0xFFFF can be appear even after the first sum. So the addition of the bits >0xFFFF have to be done twice.
As an example, think about checksum of 0x3FFFF after addition of all bytes to be checksummed.
The first wrap gives you 3 + 0xFFFF = 0x10002, the second wrap gives you 1 + 2 = 0x0003 as final checksum.
This rule is valid for both, IP and UDP.

regards,
Stefan

By: madmat

madmat — Fri, 07 Sep 2018 10:08:10 +0000

in csum() I found a small error here:

sum = (sum>>16)+(sum & 0xffff);
sum = sum + (sum>>16); // this line is useless
I rebuild csum() a little:

uint16_t csum(void *buf, int cb)
{
uint16_t *ptr = (uint16_t *)buf;
//initialize the sum with the last byte in case of odd size, otherwise to zero
int32_t sum = (cb&1)?((uint8_t)buf)[cb-1]:0;
cb/=2;
while(cb–) sum+=*ptr++;
return (uint16_t)~((sum>>16)+(sum & 0xffff));
}
sould work the same way

By: mljli

mljli — Fri, 09 Jun 2017 10:23:18 +0000

For anyone got UDP checksum error, just swap the order of placeholder and protocol in the pseudo_header struct to match the endianness of your OS.

By: Fatemah Alharbi

Fatemah Alharbi — Wed, 27 Apr 2016 01:23:05 +0000

Thanks for the code but I keep getting error on my UDP checksum calculation. How can I fix that? The IP checksum is correct though!

By: Viktor Przebinda

Viktor Przebinda — Wed, 07 Oct 2015 01:05:00 +0000

Also agree with Daniel, tcpdump was giving checksum errors until I changed csum to use sizeof (struct iphdr).

By: James

James — Fri, 27 Jun 2014 23:04:00 +0000

I agree with Daniel, if the data is different as I needed it to be, you must change the code to use this for IP header checksum: iph->check = csum ((unsigned short *) datagram, sizeof (struct iphdr));

By: Franck Chaillat

Franck Chaillat — Thu, 21 Nov 2013 12:40:00 +0000

you realy help me so much dude! thanks!

By: Silver Moon

Silver Moon — Sat, 20 Oct 2012 05:34:35 +0000

In reply to Daniel. the ip checksum is being calculated like this iph->check = csum ((unsigned short *) datagram, iph->tot_len); the second parameter is the ip header length which specifies how much of the whole datagram should be used to calculate the checksum, which is only the ip header in this case.

By: Daniel

Daniel — Fri, 19 Oct 2012 21:10:12 +0000

First of all, you should really change all the comments that say “TCP” to “UDP”. It looks like you’ve just copied some TCP code. Secondly, the IP header checksum is supposed to only sum the IP header itself, whereas in your code it sums the entire datagram. This may still work, but only because the rest of the datagram is set to 0. To do it correctly, you should only sum the length of the IP header.