We currently have a backup site at a different location to our main site. This backup site mirrors (as closely as possible) our main services, particularly web serving. Is there a way to have the backup site act as a failover for the main site using something like Linux-HA? They are on seperate internet connections with different IP ranges. Thanks -- Tim Edwards
Tim Edwards <tim at registriesltd.com.au> wrote:> We currently have a backup site at a different location to > our main site. This backup site mirrors (as closely as > possible) our main services, particularly web serving. > Is there a way to have the backup site act as a failover > for the main site using something like Linux-HA? > They are on seperate internet connections with different IP > ranges.Yes and no. Yes in that you have a couple of options -- one common, one pretty much a hack. The common one is to have your own autonomous system number and run BGP. That way you control your IP assignments, failover, etc... in ways that are efficient and quickly propogated. The hack is to put routers and/or 1-to-1 NAT devices at each site, which can redirect traffic to the other site. That is less efficient and can cause some headaches. No in the fact that there's really no "software" or "service" facility to deal with this. Round robin DNS does nothing to solve this. Name propogation is always an issue. So it's something you can only address at the IP-level -- either by having your own, Internet-recognized autonomous system number, or redirecting IPs from each site to the other when servers/sites go down. -- Bryan J. Smith Professional, Technical Annoyance b.j.smith at ieee.org http://thebs413.blogspot.com ---------------------------------------------------- *** Speed doesn't kill, difference in speed does ***
I agree, BGP is important to route the IP's, but I've been exploring this same option with a different thought. I'd like to hear what others say about this! Here is my plan (although not implemented or tested) for Web Services. At our main data center we have the primary DNS server and our primary web server. The remote location houses the secondary DNS server and our secondary web server. Also at that second location is "hidden" master DNS server. Your registrar name records only point to the primary and secondary, therefore, the internet knows nothing of the hidden master. Then, the hidden master contains a similar set of DNS records that point to the secondary site. Here's the trick! The secondary DNS server syncs with the primary DNS server every x minutes. If the secondary DNS server cannot communicate with the primary DNS server, it then looks at the "hidden" master DNS server and syncs the records (which is pointing at your secondary site). Then, when your data center site comes back up, the secondary tries to communicate with the true master DNS server...it can...therefore it updates its records. That is the theory in a nutshell. I've read that this is possible, but I haven't had a chance to test it. What do others think about this? This is no substitution for BGP, but for those that don't run BGP or need to re-route the IP networks, this may work. --Todd -----Original Message----- From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On Behalf Of Bryan J. Smith Sent: Thursday, January 05, 2006 12:22 AM To: CentOS mailing list Subject: Re: [CentOS] High Availability using 2 sites Tim Edwards <tim at registriesltd.com.au> wrote:> We currently have a backup site at a different location to > our main site. This backup site mirrors (as closely as > possible) our main services, particularly web serving. > Is there a way to have the backup site act as a failover > for the main site using something like Linux-HA? > They are on seperate internet connections with different IP > ranges.Yes and no. Yes in that you have a couple of options -- one common, one pretty much a hack. The common one is to have your own autonomous system number and run BGP. That way you control your IP assignments, failover, etc... in ways that are efficient and quickly propogated. The hack is to put routers and/or 1-to-1 NAT devices at each site, which can redirect traffic to the other site. That is less efficient and can cause some headaches. No in the fact that there's really no "software" or "service" facility to deal with this. Round robin DNS does nothing to solve this. Name propogation is always an issue. So it's something you can only address at the IP-level -- either by having your own, Internet-recognized autonomous system number, or redirecting IPs from each site to the other when servers/sites go down. -- Bryan J. Smith Professional, Technical Annoyance b.j.smith at ieee.org http://thebs413.blogspot.com ---------------------------------------------------- *** Speed doesn't kill, difference in speed does *** _______________________________________________ CentOS mailing list CentOS at centos.org http://lists.centos.org/mailman/listinfo/centos -------------- next part -------------- A non-text attachment was scrubbed... Name: smime.p7s Type: application/x-pkcs7-signature Size: 3022 bytes Desc: not available URL: <http://lists.centos.org/pipermail/centos/attachments/20060105/7e3c93bb/attachment.bin>
> I agree, BGP is important to route the IP's, but I've been exploring this > same option with a different thought. I'd like to hear what others say > about this! > > Here is my plan (although not implemented or tested) for Web Services. > > At our main data center we have the primary DNS server and our primary web > server. The remote location houses the secondary DNS server and our > secondary web server. Also at that second location is "hidden" master DNS > server. Your registrar name records only point to the primary and > secondary, therefore, the internet knows nothing of the hidden master. > Then, the hidden master contains a similar set of DNS records that point to > the secondary site. Here's the trick! The secondary DNS server syncs with > the primary DNS server every x minutes. If the secondary DNS server cannot > communicate with the primary DNS server, it then looks at the "hidden" > master DNS server and syncs the records (which is pointing at your secondary > site). Then, when your data center site comes back up, the secondary tries > to communicate with the true master DNS server...it can...therefore it > updates its records. > > That is the theory in a nutshell. I've read that this is possible, but I > haven't had a chance to test it. > > What do others think about this? This is no substitution for BGP, but for > those that don't run BGP or need to re-route the IP networks, this may work.propagation
On Thu, 2006-01-05 at 00:10, Tim Edwards wrote:> We currently have a backup site at a different location to our main > site. This backup site mirrors (as closely as possible) our main > services, particularly web serving. Is there a way to have the backup > site act as a failover for the main site using something like Linux-HA? > They are on seperate internet connections with different IP ranges.Web browsers (IE at least) tend to be very good about handling failures if you give out multiple IP addresses for a name and one or more locations does not respond. When both work the load will balance across them. If you provide the client software for other services you can build in similar robustness by getting the list from DNS and trying each until you get a connection (don't retry too fast if you expect to have a lot of clients...). There are expensive commercial DNS servers like F5's 3dns that will test for service availability and modify the response if a location is down. Some free variations may also be available. For a few services you could probably write your own fairly easily - you just have to use a short TTL on the DNS records. However, most applications cache the DNS response internally regardless of the TTL and won't automatically pick up a change unless you exit the app and restart it. IE does this too, but if you have given out 2 addresses and one subsequently stops working it seems to do the right thing where if you give out one address first then change it you have to exit and restart to pick up the new one. -- Les Mikesell lesmikesell at gmail.com
On Jan 5, 2006, at 1:10 AM, Tim Edwards wrote:> We currently have a backup site at a different location to our main > site. This backup site mirrors (as closely as possible) our main > services, particularly web serving. Is there a way to have the > backup site act as a failover for the main site using something > like Linux-HA? They are on seperate internet connections with > different IP ranges.take a look at Super Sparrow, which i believe does what you're trying to do: http://www.supersparrow.org/ i haven't seen it widely deployed, but i have seen it occasionally in production. here's a paper on how it works: http://www.supersparrow.org/ss_paper/index.html -steve --- If this were played upon a stage now, I could condemn it as an improbable fiction. - Fabian, Twelfth Night, III,v
Todd Reed <treed at astate.edu> wrote:> I agree, BGP is important to route the IP's, but I've been > exploring this same option with a different thought.I guess you missed my point. It's _not_ just a matter of using BGP for your dynamic routing. It's a matter of getting an assigned, autonomous system number so the Internet addressing your multiple networks as the same network. [ There's a lot more to the Internet than just IPs ;-] That's the proper way to do it.> I'd like to hear what others say about this!I also made the suggestion to enable 1-to-1 NAT at each facility. Should the servers on one site go down, your 1-to-1 NAT devices would redirect requests to servers at the other site. That doesn't require an additional, "external" registration/administration. Of course it means packets are now routed to your first site first, then your second site, so if the first site is wiped out (with no equipment), that doesn't help you.> Here is my plan (although not implemented or tested) for > Web Services. > At our main data center we have the primary DNS server and > our primary web server. The remote location houses the > secondary DNS server and our secondary web server. Also at > that second location is "hidden" master DNS server. > .. cut ... > That is the theory in a nutshell. I've read that this is > possible, but I haven't had a chance to test it.The problem with the theory is that names are cached all over the Internet. That's why DNS server/name changes don't do squat when it comes to failover. Now you could _consider_ setting a very low time-to-live (TTL) on your servers -- like 5 minutes. But that doesn't always work either.> What do others think about this? This is no substitution > for BGP, but for those that don't run BGP or need tore-route> the IP networks, this may work.Again, it's _more_ than just BGP. ;-> You have to modify how the Internet sees you. Not just what you provide to the Internet. ;-> That's a key distinction that most people don't consider. -- Bryan J. Smith Professional, Technical Annoyance b.j.smith at ieee.org http://thebs413.blogspot.com ---------------------------------------------------- *** Speed doesn't kill, difference in speed does ***
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