Re: TCP checksum and device driver eri0

On Fri, Jun 27, 2003 at 04:30:41PM +0200, Giovanni Porcelli wrote:
> During a snoop session on a 280R with solaris 9 and network interface 
> eri0, I noticed that the TCP checksum field of every outgoing packet was 
> incorrect.

The ERI interface calculates the checksum in hardware, so at the point in
the stack where snoop is reading the data the checksum is indeed incorrect,
but it will be calculated and updated before it leaves the physical
interface.  This is done for the fairly obvious performance improvement it
provides.

> If I want to snoop locally *but* with the correct checksum value, how 

The only way to see the correct checksum on the outgoing packets is to
disable hardware checksum calculations by adding the following to
/etc/system and rebooting :
set ip:dohwcksum=0

Obviously doing this will have a performance hit on the system.

  Scott