Failover for internet line through DNS

Posted on 2004-10-18
Medium Priority
Last Modified: 2013-12-14
Hi all,

let me first explain what we are trying to do:

We have one apache server and two netgear routers. Each router has it's own static external IP adress. Each router uses a different internet line (aka ISP) so if one ISP goes down, the other one should still operate (of course this depends on which backbones the ISPs are actually using, but let's assume that one line can be up while the other one being down).

The server should be available under the DNS entry server.mycompany.com. It is typically reached through router 1 / internet line 1. But when line 1 goes down, server.mycompany.com should be reached via router 2 / internet line 2.

- router 1 is using a fast line and has IP address
- router 2 is using a slow (backup) line and has IP address
- DNS entry server.mycompany.com is pointing to

now, when the fast line is down, I want server.mycompany.com to be quickly updated to

when talking to my ISP I was told that DNS updates from their side can take some time to propagate.

My questions are as follows
- how comes that services like dyndns.org see to propagate the updates within seconds ?
- can anybody think of another alternative to setup failover for internet lines ?


PS: just for the records - yes we have a backup server as cold stand-by :-)
Question by:Holsche
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Expert Comment

ID: 12344322
Have both of your ISPs propigate the routes to the other ISP's network space that you lease.

ISP 1 - you lease and (your server)
ISP 2 - you lease 200..2.2.2 and (your server second NIC)

Turn on bridging so the Apache will forward packets coming in on ISP 2 destined for to the other interface instead of dropping them and visa versa.

Have each ISP add the following information to their routers.
ISP 1 - Static route to next hop
ISP 2 - Static route to next hop


Author Comment

ID: 12357323
AutoSponge - thanks for your suggestion, but - honestly, I don't understand how this suggestion works.

Accepted Solution

AutoSponge earned 800 total points
ID: 12357761
Well, each ISP's router needs to know about the other network address your server has.  This should then be re-distributed in BGP by those routers into the ISP's AS.  Now each ISP has a route to each NIC on your server.  Because of the way BGP works, people will be directed to the shortest path for them to your server (depending on which ISP's AS is closer).  Then, if one AS is unreachable or your router's connection to the network drops, the other link will continue to route to both addresses beacuse, while it may be a longer path, at least it will complete.

Just make sure that both ISPs are advertising both of your IP addresses and that your router or server will forward the traffic accordingly.  If you don't have a router of your own, you're going to have to test the bridging capabilities to make sure this works.  Best bet is to have a router in front of the server that is connected to both ISP routers and has two Eth ports that plug into the two NICs on the Apache.

Author Comment

ID: 12358001
thanks !!! now it's clear :-)

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