[CentOS] Multiple WAN link -- CentOS Suitability

Ross S. W. Walker rwalker at medallion.com
Thu Jul 19 13:40:45 UTC 2007

> -----Original Message-----
> From: centos-bounces at centos.org 
> [mailto:centos-bounces at centos.org] On Behalf Of Feizhou
> Sent: Thursday, July 19, 2007 3:13 AM
> To: CentOS mailing list
> Subject: Re: [CentOS] Multiple WAN link -- CentOS Suitability
> > --  Is it possible/hard/easy/trivial to share the load 
> between the two 
> > connections?  Have either link fail and things still work correctly?
> http://en.wikipedia.org/wiki/Autonomous_system_(Internet)
> Two connections from two different ISPs? You need a ASN. (not 
> for load 
> sharing...this is primarily to handle link failures)

Well you don't really need to go as far as ASNs and BGP routing to
make it work, but it is tricky. ASNs and BGP routing really plays
into incoming connections during a link failure, but there are
ways to work around that via DNS tricks. Think about running 2
instances of bind on the host, one for internal DNS/caching, the
other for external DNS queries to your host.

The tricky part is to make a host entry appear and disappear when
a link goes up/down, which will need to be verified somehow.

> > 
> > --  What are the implications of two pipes for incoming 
> connections such 
> > as DynDNS based  remote desktop or VNC, or web server, FTP, etc
> Incoming connections will hit either IP and use that IP for 
> the duration 
> of the connection provided that you have a DNS entry that 
> round robins...

Yes, here lies the tricks, you will need round-robin DNS for
just about every site you publish via DNS. For records that
take a weight (MX, SRV, etc) publish 2 entries with equal

Like Feizhou said these will be per-connection load-balanced and
not per-packet, which would be impossible in this scenario and
load-balanced will not mean that the load will be evenly
distributed either as DNS lookups are cached everywhere.

> > 
> >  
> > 
> > The basic hardware layout I see is 3 nics, 1 GB RAM, 60 GB 
> disk space.  
> > 1 NIC for each WAN port, 1 NIC for my local net, some recent CPU.
> > 
> >  
> > 
> > I have been browsing through the "Linux Advanced Routing & Traffic 
> > Control HOWTO," but am still not on top of how to get done what I'm 
> > looking for.  I understand that there are probably products 
> that I could 
> > buy to do this, but my preference is to do it myself.
> I do have a box that has two connections from two different ips. I 
> basically forget about load sharing. I setup multiple routing tables, 
> some ip rules and basically assigned one link for vpn and server 
> activity while the other link is used for office Internet 
> connectivity 
> and a few small things are shared like DNS. Nothing fancy...

I believe there may be a way with later kernels to put entries for
2 default routes of equal weight to each interface that will
round-robin, but I haven't tried that, as when I have that kind of
scenario I usually go to Cisco. I don't know what magic would be
required though in ip tables to get this to work...

If not you will have to look into Squid and it's bag of tricks to
help balance outbound web/ftp traffic and pick a primary/backup
route for all non-proxied traffic.


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