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Re: [Xen-users] Migration of Xen Networking Setup to new ISP

On Tue, 2 Nov 2010 07:53:50 +0100, Simon Hobson
<linux@xxxxxxxxxxxxxxxx> wrote:
> Thomas Jensen wrote:
>>I currently use pci hide to hide a physical NIC from the Dom0.  This >NIC is 
>>passed to a firewall DomU.  Two other NICs are passed to the >firewall DomU 
>>to create a standard three NIC firewall.  The fourth >NIC in the Dom0 is on a 
>>separate subnet (not part of the firewall) >and is used only for managment of 
>>the Dom0.
>>I would like to setup a firewall DomU on the new ISP and then >migrate the 
>>DomUs to the new firewall one at a time.  I am getting >tripped up on the 
>>fact that my server can't have two gateway >addresses active at one time.
> Well a lot depends on how you want to migrate. There are different
> approaches, in part a lot depends on whether you intend to keep the
> old ISP going :

I don't plan to keep the old one going once the conversion is

> The Big Bang
> You switch off the old connection, and turn on the new one. No
> complications here, you simply switch configs and wait for the DNS
> changes to propagate. Nothing special here except to have all your
> configs worked out in advance to minimise downtime.

I am going to move forward with this approach.  It has probably been a
little more work on my end, but I have been ratcheting down the TTL on
all of the DNS records this week.  I went from 86400 to 3600 yesterday. 
This evening, I will ratchet down to 600 and commence the in the middle
of the night.  I think this will be the most straight forward move and
won't require any significant networking changes during the change over.

> Dual running
> A bit more work, but you parallel run for a while while DNS changes
> propagate and eventually turn off the old connection (unless you want
> to keep it running).
> Phased migration
> You move one bit (service or server) at a time, probably combined
> with parallel running.

I had originally thought about implementing a phased approach;
essentially having two paths out to the two ISPs running at the same
time and moving a DomU from the one to the other one at a time.

> You can do policy based routing in your firewall. Setup a new
> internal bridge for the new subnet you get from your ISP - it doesn't
> need to have a NIC assigned to it. Add extra virtual NICs to the
> guests.

I'm not real strong on the whole bridging concept yet and how that
translates into Xen space.  All of the bridges I currently have now are
linked to physical NICs.  In at least one case, the physical NIC isn't
even physically connected to anything.  This NIC would likely be a good
candidate for the process you described above.

> You then need to set up routing policies (can't help there, never
> done it but I know it can be done) so that :
> Traffic TO subnet A is routed to the old internal network
> Traffic FROM subnet A is routed out via ISP A
> Traffic TO subnet B is routed to the new network
> Traffic FROM subnet B is routed out via ISP B
> I think you can probably do this by running a shared network with
> both subnet A and subnet B on it - ie the extra internal bridge
> probably isn't necessary. Something to look into.
> This article explains how it's done with Shorewall (installed by
> default on all my systems)
> http://shorewall.net/MultiISP.html

I use Shorewall as well on my three NIC firewall.  I had done multiple
searches online, but hadn't turned up the link you provided.  Thanks for
the information.

> -- 
> Simon Hobson
> Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
> author Gladys Hobson. Novels - poetry - short stories - ideal as
> Christmas stocking fillers. Some available as e-books.
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