Hi, Fred,
On Wed, Oct 16, 2013 at 12:45 PM, Templin, Fred L <Fred.L.Templin@xxxxxxxxxx> wrote:
They do what a middle-box would do in this case: whatever part of the header chain is there, it's up to you to inspect it. As noted by Ole, when you tunnel over IP, IP becomes a link layer... so whatever "defense in depth" techniques you're using, they should be prepared to deal with scenarios in which a tunneled packet does not contain the entire IPv6 header chain, since not all "link layers" (as IPv4 when IPv6 is tuneled over IPv4) guarantee a minimum MTU of 1280 bytes.
I disagree with the header chain terminating as soon as another IPv6
header is encounter. That defeats defense-in-depth, since outer
perimeter middleboxes would be forced to admit packets with unexamined
header chains inward to inner perimeter middleboxes. And, if the
unexamined header chains contain bad stuff inserted by an attacker,
the attack is successful.
The same is true if the underlying encapsulation layer is IPv4. Once you're encapsulating, all bets are off in that respect. You should do defense-in-depth at the tunnel ingress or egress points. Or, if you wish, a middle-box could reassemble-inspect/filter-re-fragment
Note that no matter what hardcoded limit you enforce, an attacker willing to evade you're security controls can always add an extra-layer of encapsulation (assuming he controls that). Also, common security policy is "default deny" -- so if the attacker plays dirty, his packets will be dropped.
That requirement is also not observed by common middlebox systems
such as Wireshark and tcpdump. Both will blast past encapsulating
IPv6 headers through to the header chain inserted by the original
host without stopping at the outermost IPv6 header.
Thanks,
Fernando