Subject: Re: rpc xid randomness
To: <>
From: David Laight <>
List: tech-userlevel
Date: 09/06/2003 13:56:12
On Sat, Sep 06, 2003 at 09:23:29PM +0900, wrote:
> >	does it have any negative sideeffects?  i mean, is there any code that
> >	relies on xid being generated from timestamp?
> clnt_vc.c:      call_msg.rm_xid = ((u_int32_t)++disrupt) ^ __RPC_GETXID(&now);
> 	how important "disrupt" is?

Does it even matter if RPC xids are predictable?
IIRC They are only used to tie responses to commands in the client.
So what is really, really important is that there aren't two outstanding
requests with the same xid - and I'm not even sure the code above does that!

The only possible use of a predecicted xid is to send an early response
from an alien system.  Now the xid isn't encrypted, so if you can see the
outbound packet then you can generate a correct response anyway.
If you can't see the outbound packet then you will need a lot of luck on
getting the timing of the response right!


David Laight: