Re: maximum data over link

[Bowie Owens <bowie owens csiro au>]

Bart Vanherck wrote:

> On Mon, 2004-09-27 at 03:31, Bowie Owens wrote:
>
>  
>
> > As I understand it, the purpose of GIOP_INITIAL_MSG_SIZE_LIMIT is to 
> > prevent a DOS attack from consuming memory on a server via large invalid 
> > messages. The first time a server receives a valid message it is 
> > supposed to record that the client is legit and ignore the size of the 
> > messages from then on. So you can send a short "hello" message to the 
> > server and then after that send messages of any size. A while ago ORBit 
> > was always applying the size limit. I don't know if it still does this 
> > as I ended up working around the problem using the command line argument:
> >
> > --ORBInitialMsgLimit=4000000000
> >    
> I did some tests with this and I saw that after first sending an hello 
> message, and afterwards retrieving a huge buffer does not work. The 
> command line argument does work. 
> However I have a little question. This GIOP_INITIAL_MSG_SIZE_LIMIT is 
> for me at the "client" side that is preventing to receive a large
> buffer. And so I did set the argument for the client only. How does this
> prevent a possible DOS ?
The client still has to listen for messages coming in on its 
connections. The point I didn't mention is that the size check is done 
only on the header. The message body is not stored in memory if the size 
check fails. If ORBit were to read in the entire message before 
establishing its validity, a rogue program could send your client a very 
large message to consume resources on your machine. By sending lots of 
large messages the rogue program should be able to force the machine to 
thrash uselessly. By rejecting large messages before reading them this 
kind of attack is prevented.

> I did not have to set this argument at the
> server side.
>
>  
Sending is a bit different from receiving a message. The sending program 
can trust itself. However, the receiving program has to be doubtful 
about the nature of anything it receives.


--
Bowie Owens

CSIRO Mathematical & Information Sciences
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