Subject: Re: [RFC][PATCH] IP: Send a fragment reassembly time exceeded packet when enabling connection track

From: Shan Wei <[email-masked]>
Date: Wed, 13 Jan 2010 11:15:31 +0800

Patrick McHardy wrote, at 01/05/2010 01:44 PM:
> Shan Wei wrote:
>> Default, a host may send a fragment reassembly time exceeded packet
>> (ICMP Time Exceeded Message with code value of 1) when defraging fragments timeout.
>> But, when enabling connection track, a host can't send the packet.
>>
>> Because, the module of nf_defrag_ipv4 selected by connection track is registered 
>> in PRE_ROUTING HOOK and assembles all accepted fragments(here, not begin to routing).
>> After defrag timeout, the host can't send fragment reassembly time exceeded packet, 
>> because of lack of router information.
>>
>> RFC 792 says:
>>>>>>   If a host reassembling a fragmented datagram cannot complete the
>>>>>>   reassembly due to missing fragments within its time limit it
>>>>>>   discards the datagram, and it may send a time exceeded message.
>>>>>>
>>>>>>   If fragment zero is not available then no time exceeded need be
>>>>>>   sent at all.
>>>>>>
>>>>>>
>>>>>> Read more: http://www.faqs.org/rfcs/rfc792.html#ixzz0aOXRD7Wp
>> So, the patch try to fix it with filling router information before sending fragment reassembly
>> time exceeded packet when defrag timeout.
> 
> I guess the question is whether we really want to send an ICMP
> message in this case. The above quote applies to end hosts,

Yes, what you guess is what i want to ask. :-)
Should end hosts which are using conntrack send a fragment reassembly time exceeded message?

> while conntrack is also (probably more commonly) used on routers,
> which normally shouldn't attempt reassembly.  

There are two point:
1.Take security into account, end hosts also used conntrack. 

  For example: When a host is attacked by denial of service TCP flaws, RedHat used the conntrack&recent
  match to limit the TCP connections.
  
  About details, see the phenomenon description:
    http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-4609
    http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2008-4609

  See RedHat's solution:
    http://kbase.redhat.com/faq/docs/DOC-18730

2.On the latest kernel, a router on which the conntrack is used, reassemble fragments and 
  forward reassembled intact packet. This implementation is not coincide with what you said.

  nf_defrag_ipv4 module is registered on PRE_ROUTING hook with the highest priority. So, search router table 
  after completing the reassembly and forward it to destination host.

If I miss something, please tell me.

Thanks.
-----
Shan Wei