TCP checksum includes the psuedo-header (some fields of the IP header - see Richard stevens or the RFC for details). If the psuedo-header changes, TCP checksum needs to be re-calculated or "adjusted". ----- Original Message ----- From: Abhijit Gadgil <gabhijit@ee.iitb.ac.in> To: Amit Shah <shahamit@gmx.net> Cc: <linux-india-programmers@lists.sourceforge.net>; <kernelnewbies@nl.linux.org> Sent: Monday, February 18, 2002 1:43 PM Subject: Re: tcp checksum calculation > > Read rfc793 page 15, carefully. I think you will have to recalculate the > checksum. Because you are changing the destination address field. Perhaps > the nat code pointed to by someone may help you find out how exactly to do > it. > > -abhijit > > > Amit Shah wrote : > > >Hi, > > I have a function(in the kernel, ip_rcv_finish... before the ip > >forwarding code.) that takes in packets, replaces the dest. ip:port, and > >sends to that machine. (ip forwarding is enabled.) I need to recalculate > >the checksum..... ip checksumming works fine(ip_fast_csum)... but, the tcp > >checksum is shown as incorrect on ethereal. The packets are not delivered > >to the new dest.... > > > >Also, what is the skb->csum, in case there is no data (e.g. for ACK or > >FIN -only packets)? > > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > IRC Channel: irc.openprojects.net / #kernelnewbies > Web Page: http://www.kernelnewbies.org/ > > -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ IRC Channel: irc.openprojects.net / #kernelnewbies Web Page: http://www.kernelnewbies.org/
