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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 redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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