ARP is sourced from loopback address

Ray Soucy rps at maine.edu
Tue Jan 31 13:09:00 UTC 2012


We ran into a lot of quirkiness with Linux when we started rolling out
Linux-based CPE with XORP as a routing engine.

I've thrown some sane defaults you might want to consider into a text file at:

http://soucy.org/xorp/xorp-1.7-pre/TUNING

Specifically, you prob. want option 2 instead of 1 for arp_ignore,
otherwise you'll see funkiness with ARPs coming from the wrong IP in a
multi-interface configuration.

----8<----
ARP_IGNORE values:

0    - Reply for any local address.
1    - Reply only if the target IP is configured on the receiving interface.
2    - Like 1, but the source IP (sender's address) must belong to the
same subnet as the target IP.
3    - Reply only if the scope of the target IP is not the local host
(e.g., that address is not used to communicate with other hosts).
4-7 - Reserved.
8    - Do not reply.
>8  - Unknown value; accept request.
----8<----

Hope this helps,




On Mon, Jan 30, 2012 at 7:09 PM, William Herrin <bill at herrin.us> wrote:
> On Mon, Jan 30, 2012 at 6:24 PM, Joe Maimon <jmaimon at ttec.com> wrote:
>> Golden.
>> Thank you, William.
>
> Hi Joe,
>
> You're welcome. The flip side of Linux's arp funkiness is that you can
> get it to do some nifty stuff. For example, a /32 ethernet looks more
> or less like this:
>
> ifconfig lo:1 198.51.100.1 netmask 255.255.255.255
> ifconfig eth1 192.168.0.1 netmask 255.255.255.252
> ip route add 198.51.100.44/32 dev eth1 src 198.51.100.1
> arptables --out-interface eth1 -j mangle -s 192.168.0.1 --mangle-ip-s
> 198.51.100.1
>
> The implicit proxy arp takes care of the rest with the machine hanging
> off the interface thinking that it's part of a /24.
>
>
> This sort of thing is how I'm using all 17 of the IP addresses in my
> Cox /28. :-)
>
> Regards,
> Bill Herrin
>
>
>
> --
> William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
> 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
> Falls Church, VA 22042-3004
>



-- 
Ray Soucy

Epic Communications Specialist

Phone: +1 (207) 561-3526

Networkmaine, a Unit of the University of Maine System
http://www.networkmaine.net/




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