[ipv6hackers] funny FreeBSD bug
fgont at si6networks.com
Thu Jul 26 20:10:50 CEST 2012
On 07/26/2012 12:35 PM, Marc Heuse wrote:
> I found a funny bug in freebsd (9.0 with all updates):
> if you send an ICMP toobig message to it with a too low MTU size,
> FreeBSD will prepend any packet data with an one-shot fragment (or
> atomic fragment as Fernando calls it).
> Frag Hdr Offset 0, No more Frags Bit set
This is not a bug, but intended behaviour. It is meant to handle the
case of v6/v4 translators. A translator may receive an IPv6 packet that
needs to be fragmented before sending it to the IPv4 world. In that
case, it may send an ICMPv6 PTB with a MTU<1280. This will cause the
IPv6 sending host to include a Fragment Header in each packet, thus
selecting an appropriate Fragment ID. When the translator receives such
fragments, it will use the received Fragment ID for the IPv4 fragments.
The aforementioned behavior is required by stateless translators.
See slide 35 of
for a list of systems that support this behavior.
e-mail: fgont at si6networks.com
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