Sending UDP packets to port 0

["Christian P. Schmidt" <schmidt@digadd.de>]

Hi all,

I know that the kernel forbids sending UDP packets to port 0. Is there a 
specific reason for this?

I'm asking because I regularly need to access a device that asks for 
tftp files using port 0 as source. To use these devices I'm using a 
kernel with removed port 0 checks.

I read RFC 768, and it does not explicitly forbid the use of port 0 as 
far as I can see. It only says that if unused, 0 is to be inserted.

RFC 1350 (TFTP) even states explicitly:
The transfer identifiers (TID's) used by TFTP are passed to the Datagram 
layer to be used as ports; therefore they must be between 0 and 65,535.

In my understanding, and apparently that of the authors of the 
appliance's tftp client implementation, this marks port 0 as valid.

Please CC: me on any replies as I am not subscribed to the list.

Regards,
Chris