ISP failover and DNS 1

[rpast]

Hello,

I have a basic question regarding Internet link failover and DNS. Assuming I can successfully reroute traffic to an alternate ISP following a service outage with my primary ISP, how do external DNS databases get updated following the change?

I have internal web servers that are mapped on a firewall to ISP-A’s IP addresses. When a failover occurs, I can map those same servers to ISP-B’s IPs, but this covers only outbound traffic. How do external DNS servers become aware of those changes so that remote users know where to go?

As basic as this question seems to be, I cannot find the answer anywhere. I don’t necessarily need to know the details of how to do this, only the general idea. The only solution I can come up with is that I’d need to have the cooperation of the two ISPs, who would update their databases in response to (if nothing else) a phone call from me, following an outage.

But can DNS list alternate host records that can somehow automatically become primary records when the primary address cannot be reached? This question is probably a big ‘duh’ to a lot of people. Any DNS experts out there?

Thank you.

 

[ChrisAC]

You would need to manually change the DNS settings so that hostnames map to address space on your other ISP.

Regards,

Chris.



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Chris A.C, CCNA, CCSA
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[unclerico]

I have a basic question regarding Internet link failover and DNS. Assuming I can successfully reroute traffic to an alternate ISP following a service outage with my primary ISP, how do external DNS databases get updated following the change?

If you are able to multihome with BGP to both of your providers you will be able to route around a link failure. This would really only work if the link fails, not the CE router itself.

But can DNS list alternate host records that can somehow automatically become primary records when the primary address cannot be reached? This question is probably a big 'duh' to a lot of people. Any DNS experts out there?

I know there are offerings out there for dns failover that will automatically update the A records, but your ISP/registrar would need to employ this or else you would need to host your own DNS and use this solution. One such solution is dnsmadeeasy.com. If you use multiple A records with different addresses then you'll have DNS round-robin which, if you're looking for failover/HA is not a good solution.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[rpast]

Since we our ISP (only one currently) manages our external DNS, that means that I'd have to alert him, have him propagate the new info worldwide - maybe doing the same with the second ISP. There has to be a better way of doing this without going to BGP. I've heard vaguely about scripts that can change DNS entries, rrset statements, and TTL in DNS records, more or less automating the process. Don't know how they work in the real world though.

 

[rpast]

My previous response was to ChrisAC -- thank you, unclerico. This is more of what I was thinking of.

Speaking of BGP, I do have a follow-up question. If I register for an AS with ARIN, must I also register for my own public IP subnet? I would think so, but also wonder whether an ISP can issue me an address space, and have it effectively owned by me. This would save $$.

 

[unclerico]

Speaking of BGP, I do have a follow-up question. If I register for an AS with ARIN, must I also register for my own public IP subnet? I would think so, but also wonder whether an ISP can issue me an address space, and have it effectively owned by me. This would save $$.

It is requirement that you specify the CIDR block(s) you will be using. I'm not sure about your second question, you would need to speak with both of your providers. They need to agree to advertise each others prefixes.

In reference to your comment back to ChrisAC

I've heard vaguely about scripts that can change DNS entries, rrset statements, and TTL in DNS records, more or less automating the process. Don't know how they work in the real world though.

This would work if you were hosting your own DNS as you would have the necessary permissions to do the changes. The dnsmadeeasy.com solution effectively does this.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[rpast]

Thanks again.