Configuring LVS TUN with new arp_tables_jf

Graeme Fowler graeme at
Tue Aug 9 14:36:47 BST 2005

On Tue 09 Aug 2005 13:11:46 BST , Nigel Hamilton <nigel at> wrote:
> Does this look sane?
>> Nope.
> Ouch.

No pain intended :)

>>> MTU=1480
>>> BOOTPROTO=none
>> ^^^^^^^^^^^^^^^^^^^^^^^
> I actually copied this from my old cluster config. What are the 
> ramifications of an incorrect broadcast address?

That depends; in this case probably very little actually. In the wider 
case, incorrect broadcasts cause all sorts of shenanigans - especially 
on Windows AD networks where you want to physically separate AD or WINS 
domains, incorrect broadcast addresses can result in all manner of 
information leakage. But that's not relevant here.

> Graeme so just to confirm - I need to set the BROADCAST address for 
> LVS TUN to point back to the VIP?


Here's the simple explanation: the network and broadcast addresses for 
a given netmask correspond to the first and last addresses (not 
_usable_ addresses) in that range.
For a netmask of - a /24 network with which most people 
are most familiar - the last octets of these addresses are 0 and 255. 
Using as an example: mask : network, broadcast mask : network, broadcast mask : network, broadcast

...and so on down to... mask : network, broadcast mask : network, broadcast [Only useful as a point-point link] mask : network, broadcast

So a /32 netmask, equating to, is a network of one 
address - this sounds counterintuitive but is often the best way to add 
VIPs to local adapters, amongst other things. It's also used as a route 
injection hack in various routing protocols when you want most of a 
block in one building or location, but specific /32 prefixes elsewhere.

So it sounds weird but it makes perfect sense :)

Put "CIDR calculator" into Google and have a play with some of the 
online ones.



> And the NETWORK should also point to the VIP?
> Nige
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