[j-nsp] Odd issue with ARP in different subnet

[Gordon Smith]

 Put the /32 on a loopback instead of a secondary address.

 See 
 http://www.netlinxinc.com/netlinx-blog/45-dns/119-anycast-dns-part-2-using-static-routes-.html

 You could always fire up WireShark and watch exactly what's going on on 
 the wire



 On Wed, 9 Mar 2011 18:56:44 -0600, Chris Adams wrote:
> Once upon a time, Gordon Smith <gordon at gswsystems.com> said:
>> Check the default router config.
>>
>> When the server sends the arp request, the router should reply with
>> it's own MAC address
>> Does it not have a route back to the switch?
>
> No, the router isn't proxy ARPing.  Let me put some IPs to the 
> problem:
>
> EX switch: 10.1.1.5/27
> Linux server eth0: 10.1.1.10/27
> router (M10i): 10.1.1.30/27
> DNS IP: 10.2.2.2/32 (secondary IP on Linux server eth0)
>
> EX wants to reach 10.2.2.2, so it sends the packet to the M10i at
> 10.1.1.30.  Router has route for 10.2.2.2/32 pointing to 10.1.1.10, 
> so
> it sends the packet to the Linux server.  Linux server realizes it
> doesn't need to route back to EX in the same subnet and is going to 
> send
> a packet directly from 10.2.2.2 to 10.1.1.5.  Linux server doesn't 
> have
> an ARP entry for 10.1.1.5, so it sends an ARP request, using a source 
> IP
> of 10.2.2.2 (since that's the source of the desired packet).
>
> At this point the EX sees the ARP request for its IP, but doesn't
> respond to it.  I'm guessing it is ignoring the ARP request because 
> the
> source IP is in a different subnet (but that's just a guess).
>
> There's also an old Cisco switch on the same segment, and it replies 
> to
> out-of-subnet ARP requests just fine.  I also tried a FreeBSD host in 
> a
> similar setup with a different Linux server, and it also works okay.  
> I
> don't have any other OSes handy to try.
>
> Per another email, I tried setting the Linux server to put the DNS IP 
> on
> a loopback interface instead of the ethernet, but it still sent the 
> ARP
> request with the DNS IP as the source.