Re: [LARTC] Help: howto config mail server with 2 internet provider?

[Darryl Miles <lartc-list@the-morg.org>]

masroor wrote:

>  I want to send/receive email thru second internet line when the first line 
>is disconnected and back to first line when it get on again automattically.
>
>my mail server is  postfix on redhat 9. and it have 2 ethernet cards which 
>everyone connected to it's internet line.
>
>i tried this:
>http://linux-ip.net/html/adv-multi-internet.html
>but it didn't work. 
>
>please someone give me more advice about doing it step by step.
>  
>

I've glanced over your quoted document, it looks like a policy routing 
example for a multihomed NATed internet site.  Your description of need 
doesn't indicate you want agregate bandwidth just failover.


Some food for thought:

* Is the mail server and LANs default router the same box ?
* Are the interfaces for both primary and backup directly connected to 
the same box ?  If so, How ?
* Are both internet connections NATed ? or routed ?
* You need to confirm the reliability of the mechanism the router uses 
to know when the primary Internet link has failed.
* If your requirement is as simplictic as wanting a backup to be used 
only when the primary link fails, maybe setup two default routes in your 
routing table with a lower metric on the primary route.  In the event 
the link goes down I presume the interface will also go into down state, 
and therefore the routing entry will be removed implicitly by the 
kernel.  The backup route with the higher metric will then be selected 
for default routing.
* Actually test the situation where your primary link fails and confirm 
the host it's directly connected to can detect this and does bring that 
interface down.


In the situation where the mail host and LANs default router is the same 
box and the two Internet links are suppled on dedicated ethernet interfaces:

# Primary link via gateway 10.0.0.1 on eth0
route add -net default gw 10.0.0.1 dev eth0

# Backup link via gateway 10.0.1.1 on eth1
route add -net default gw 10.0.1.1 metric 1 dev eth1


One area to look at is how the default route is added when the backup 
link comes up, this would usualy mean altering the configuration to how 
the backup link add's its default route, you simply add the "metric 1" 
into the route command at this point.


Darryl


_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/