Re: [libvirt] [PATCH[ nwfilter: Discard class D and E IP addresses when sniffing
On 08/13/2010 01:45 PM, Stefan Berger wrote:
>> - // skip eth. bcast and mcast addresses,
>> + // skip eth. bcast and mcast addresses (224.0.0.0
-
>> + // 239.255.255.255), class E (255.*)
>> // and zero address in DHCP Requests
>> - if ((ntohl(vmaddr) & 0xc0000000) || vmaddr == 0)
http://en.wikipedia.org/wiki/Classful_network
Class D addresses have highest bits with pattern 1110 0000 -> 0xe0
Class E addresses have highest bits with pattern 1111 0000 -> 0xf0
I think my masks are fine and the masking with 0xf0 00 00 00 should also
include 254.* = 0xfe.* .
In that case, the comments are wrong. Class E is more than 255.*, it is
240.0.0.0-255.255.255.255. And in that case, the bit operations can be
simplified:
if ((ntohl(vmaddr) & 0xc0000000) == 0xc0000000) || vmaddr == 0)
In other words, the logic bug is that we were rejecting IP addresses
that had 1 or 2, but not all three, of the top three bits set. The
desired action is to reject IP packets if all three of the top bits are
simultaneously set.
Let's see a v2 that gets the comments right, and uses the simpler logic.
--
Eric Blake eblake(a)redhat.com +1-801-349-2682
Libvirt virtualization library http://libvirt.org
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