Re: [LARTC] Simplest method for 2 external lines?

[Whit Blauvelt <whit@transpect.com>]

On Tue, Oct 30, 2001 at 01:05:56PM -0500, Adrian Chung wrote:

> > network. Since the DNS round robin should take care of bringing, for
> > instance, http requests in on whichever line is up - or both if both lines
> 
> Will it?  My understanding is that it will (fairly) equally distribute
> the load between both incoming lines, regardless of whether or not
> they're up.  DNS doesn't check to see if the line is up before sending
> a response back...

The trick in Leghart's article is using dynamic DNS zones (which was a new
feature in Bind 8) along with a script he provides that adds and removes IPs
from the round robin according to whether the lines are up, using a short
TTL setting and frequent checks so that adjustment, while not instant,
should be within a few minutes, at most, and not run into problems with
downstream caching servers. I don't mind a Website not working half the time
for a minute or two - people are used to that much fussiness from the Web.

> > are - what I want ip route to do is send the response back out on the same
> > interface the request came in on. What's the easiest way to do that?
> 
> I've been thinking about this for a while too, and haven't been able
> to come up with any bulletproof.
> 
> If the requests are for the box that both external lines run into,
> then policy routing should be able to do what you want.
> 
> Take a look at:
> 
> http://ds9a.nl/2.4Routing/HOWTO/cvs/2.4routing/output/2.4routing-4.html

Thanks. I've been looking at that. I wish it were more fleshed out with
examples - but am certainly glad to have that much.

> Things get hairy when you try to use policy routing, and the packets
> don't terminate at the router, but are portforwarded further inbound,
> like this case:
> 
> > Also, port forwarding is being used to an internal NT mail server. I'd like
> > to handle redundancy to that by having a secondary MX on the second public
> > interface, and also forwarding that to the server. Again, when the server
> > comes back through the masq in negotiations I'd like it to connect out
> > through whichever interface/IP it was connected to from.
> 
> It's easy enough to forward both incoming SMTP MX addresses on both
> external lines to the same internal host, but I don't know how to get
> the responses from the internal host to go back out whichever
> interface they originally came in from.  If anyone's got any ideas,
> I'm all ears. :)

Hmm. Considering that the secondary MX won't be used when the first is
working, guess it would mostly work even if the remote mail daemon is fussy
about having the response come back the same way - have to confess I don't
know the degree of fussiness on this - sure would like the avoid the issue
entirely, but may well be you're right and the hooks just aren't there to
control this yet. Thanks for helping me think at it!

Whit
@transpect.com

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