Shan Wei wrote: > 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? Yes, they should. >> 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 I'm not sure I get the connection to this patch. > 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. Yes, thats a necessity for conntrack to work, but its not what a router usually does. But it actually does refragment the packet if it exceeds the MTU of the outgoing interface. > 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. -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
