IDS: RE: IDS bypassing

["charles lindsay" <frostbackeng () lycos com>]

Could you be more explicit as to which NAT devices support this evasion technique?

All NAT/PAT devices I am familiar with are either complete TCP proxies, in which case they verify the checksum coming 
in, and then re-calculate it as it goes out, or they only implement the "quick-update" algorithm (RFC 1624 et alia).  
In the first case, your evil packets get dropped at the first NAT, in the second case, they always have an incorrect 
checksum.


================ On Sun 12/29/02 at 6:44 PM ========================
============== Ed3f [ed3f () overminder com] spake: =====================

> > Systems Affected
> >
> >      NAT/PAT/load_balancing/packet_manipulation implementations
> >
> > Overview
> >
> >      Multiple vendors' implementations of
> > NAT/PAT/load_balancing/packet_manipulation
> >      calculate level 4 checksum from scratch.
<< snip>>



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