[PLUG] ARP black magic question

[Eric Harrison]

On Tue, 26 Aug 2003, Zot O'Connor wrote:

>On Tue, 2003-08-26 at 12:45, Eric Harrison wrote:

>> One particularly complex load-balancing setup we have is in need of
>> a little arp black magic: I need server A to answer arp requests on
>> behalf of server B.
>> 
>> That is, when some random host sends out an arp query "who has serverb?",
>> Server A needs to respond "serverb is at 01:02:03:04:05:06", where 
>> "01:02:03:04:05:06" is the real MAC address for server B. 

>The question is, do you want to proxy an IP or a mac address?

I'm not trying to _proxy_ arp. The kernel doc's say that proxy_arp
forwards arp packets between two devices attached to different media.

I'm trying to get one server to respond to arp requests for another
server on the same physical network. Proxy arp doesn't work in this case,
or at least I have not figured out how to bend proxy arp in that fashion...

-Eric

>Lets assume outside and inside of a gateway.
>
>Address 192.168.90.45 is inside
>
>192.168.90.46 is the gateway's nic on the inside (eth1)
>192.168.90.47 is the gateway's nic on the outside. (eth0)
>192.168.90.1 is the real default gateway which is on the outside.
>
>So we want to make 192.168.90.45 look like it available to the outside
>
>we have eth1 for the inside, and eth0 for the outside
>
>To publish to outside that .45 is to be "answered" by eth0
>        arp -Ds 192.168.90.45  eth1 pub -i eth0
>
>To publish to the inside that .1 is to answered by eth1
>        arp -Ds 192.168.90.1  eth0 pub -i eth1
>
>You should be able to do this with MAC Address as well.
>
>Word of warning, this works real well under Linux, but other Unix-like
>OSes including routers do not always support pub.
>
>