Is there any particular reason why reassembled packets were not
checked? If the answer is no, I'll send in a PR.
I think it was just a random decision -- either pass packets to
the firewall before reassembly as we do, or after reassembly, as
linux does. Both have pros and cons.
Going through the firewall twice, however, is problematic because
far too many things (counters, dummynet, etc.) expect to see each
packet only once.
ok, thanks for letting me know.
I think that a patch like the one you propose is very useful (for
ipv4 as well) but it requires a sysctl or other mechanism to make
sure that when it is enabled we don't pass fragments through the
firewall.
i've looked further into this and I now wonder if is a byproduct of my
use of ipfw. the problem seems to be that offset will always be
non-zero with any packet with a v6 fragment header, so a rule requiring
offset to be zero is never run. i'll spend a bit more time on this
tomorrow, and come back with a patch for ipfw.
Note this code:
offset = ((struct ip6_frag *)ulp)->ip6f_offlg & IP6F_OFF_MASK;
/* Add IP6F_MORE_FRAG for offset of first
* fragment to be != 0. */
offset |= ((struct ip6_frag *)ulp)->ip6f_offlg & IP6F_MORE_FRAG;
if (offset == 0) {
printf("IPFW2: IPV6 - Invalid Fragment Header\n");
if (fw_deny_unknown_exthdrs)
return (IP_FW_DENY);
break;
}
This code seems to be incorrect, per rfc 2460:
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.
A stack can send an IPv6 packet with a fragment header attached that
does not have the MF bit set. I'm 90% sure that FreeBSD itself will do
this when it receives a PTB with an MTU value of (say) 1000.
Matthew
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