Re: Fw: [NTBUGTRAQ] Win 2003 DNS requests makes replies over 512 byte PIX limit

[Derek Atkins <derek () ihtfp com>]

Mathias Gerber <mathias () intergga ch> writes:

> Hello hggdh,
> On Thu, 8 May 2003 12:09:22 -0500 you wrote:
> > FYI. Any ideas?
> > > We are running the latest version (6.3.1) on our Cisco PIX and it
> > > appears that there is hard limit of 512 bytes on ANY UDP packets
> > > arriving on port 53.  Everything exceeding that is dropped.
>
> AFAIK the DNS uses TCP for larger replys.

Yea, but resolvers normally use a response with the TC-bit set in
order to signal the fact that the response was truncated and TCP
should be used!  If the UDP response is dropped, then a resolver will
never see the response and never fall back to TCP.  It will timeout
and fail instead.

Also, it's possible to negotiate larger-than-512-byte UDP packets.
For example with EDNS(0) you can use larger UDP packets.  Just
dropping larger packets is a PIX bug and can cause a DNS black-hole.

> mathias

-derek

-- 
       Derek Atkins
       Computer and Internet Security Consultant
       derek () ihtfp com             www.ihtfp.com
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