I had to check my packet generator source code, because I thought the TCP header was calculated the same way you do. But when I checked the source you have to feed both the TCP and IP headers into the calculation to get a correct checksum. It uses some information from the IP header in the calculation (source and destination IP for example). So you don't actually use the entire IP header, just a part of it. -----Original Message----- From: Antony Stone [mailto:Antony@xxxxxxxxxxxxxxxxxxxx] Sent: Friday, November 21, 2003 14:27 To: netfilter@xxxxxxxxxxxxxxxxxxx Subject: Re: Checksumming Issues On Friday 21 November 2003 8:15 pm, Leonid Veytser wrote: > A general question about checksumming. If I change the destination address > in the IP header, then of course I need to recalculate the IP header > checksum. "You" the user (or system administrator) do not need to recalculate any checksums - the SNAT or DNAT code does this for you. > But do I have to recalculate the checksum of the next protocol > header (ie TCP, UDP, etc)? No. Everything is handled for you in the background. Anyway, if you only change the IP address (source and/or destination), the TCP or UDP checksum remains valid. Antony. -- Feeling bad at breakfast because you don't have a hangover is evidence of a complex emotional life it can take many years to perfect. - Pete McCarthy, The Road to McCarthy Please reply to the list; please don't CC me. ---------------------------------------- The information transmitted in this message is intended only for the person or entity to whom it is addressed and may contain confidential and/or privileged material. Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon this information by persons or entities other than the intended recipient is prohibited. If you received this in error, please contact the sender and destroy any copies of this document.
