[plug] IP Alias/Arp dilemma...

[Richard Sharpe]

At 07:38 AM 6/28/00 +0000, you wrote:
>Here's a curly one:
>
>I'm using IP Aliases to shift services from one host to another without 
>needing
>a DNS entry. ie; each Server has its primary unique name/IP, and then another
>ip via an Alias for the service. Moving the service involves downing the
Alias
>interface on one, and upping it on the other.
>
>At least, that's the theory.
>
>The problem is, the router still has the ARP entry for the old alias, not the
>new one, so no traffic from outside that subnet can find the new location.
One
>solution is to flush the arp cache on the router, but it's not in my power to
>do that automatically (or to do it at all other than going through the COMMS
>guys).

Hmmm, you may need to do a gratuitous ARP to the router. If you look in the
Linux-HA stuff, I think you will find that there is a tool to do gratuitous
ARP which should fix the router's ARP cache.

>Any ideas on how to poke the Gateway to update the ARP entry? On Solaris
it was
>easy - outgoing traffic went out on a random/rolling interface, so traffic
>going out on the new Alias would prod the gateway into revising its info.
>
>However, Linux has all traffic going out the primary interface, which for
most
>things is a GOOD thing (eg; only have to add the one IP to your access list),
>but it means it isn't getting that "poke" to get the gateway to update.
>
>So, any ideas? Is there a way to force a Ping or something to go out a
>particular aliased interface?
>
>-- 
>. Trevor Phillips             -           http://jurai.murdoch.edu.au/ . 
>: CWIS Systems Administrator     -           T.Phillips at murdoch.edu.au : 
>| IT Services                       -               Murdoch University | 
> >------------------- Member of the #SAS# & #CFC# --------------------<
>| On nights such as this, evil deeds are done. And good deeds, of     /
>| course. But mostly evil, on the whole.                             /
> \      -- (Terry Pratchett, Wyrd Sisters)                          /
>
>

Regards
-------
Richard Sharpe, sharpe at ns.aus.com
Samba (Team member, www.samba.org), Ethereal (Team member, www.zing.org)
Co-author, SAMS Teach Yourself Samba in 24 Hours
Author: First Australian 5-day, intensive, hands-on Linux SysAdmin course
Author: First Australian 2-day, intensive, hands-on Samba course