[ipv6hackers] funny FreeBSD bug

Simon Perreault simon.perreault at viagenie.ca
Thu Jul 26 19:36:49 CEST 2012

Le 2012-07-26 10:26, Marc Heuse a écrit :
>> Why do you think it's a bug?
> first it servs no use to add the fragmentation header if the packet is
> not fragmented. second I have not seen this behaviour in other OS,
> however I havent looked for it though.
>> Seems like normal IPv6 behaviour to me. It's in the RFC...
> I cant remember having seen this in any rfc - do you have a pointer?

RFC 2460 section 5:

    In response to an IPv6 packet that is sent to an IPv4 destination
    (i.e., a packet that undergoes translation from IPv6 to IPv4), the
    originating IPv6 node may receive an ICMP Packet Too Big message
    reporting a Next-Hop MTU less than 1280.  In that case, the IPv6 node
    is not required to reduce the size of subsequent packets to less than
    1280, but must include a Fragment header in those packets so that the
    IPv6-to-IPv4 translating router can obtain a suitable Identification
    value to use in resulting IPv4 fragments.  Note that this means the
    payload may have to be reduced to 1232 octets (1280 minus 40 for the
    IPv6 header and 8 for the Fragment header), and smaller still if
    additional extension headers are used.

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