Failover static and dynamic lease

[Shawn Routhier]

> On May 26, 2016, at 12:03 AM, Simon Hobson <dhcp1 at thehobsons.co.uk> wrote:
> 
>> 
> 
> A couple of things to bear in mind.
> 
> BIND doesn't have the ability (AFAIK) to run a master/master system with failover. So regardless of what you do with DHCP, DNS will always have a single master - and if that goes down, DDNS updates will fail but the slave will be able to continue serving it's copy of the zone. Ie, with the master down, the DNS zone will still be served, but won't get updates.
> 
> If you use the second option of two independent DHCP masters, then DNS updates may work "oddly". If a client is unable to renew it's lease with one server, it'll get a lease from the other. So say A is down, B will attempt a DDNS update - but this will fail because the DNS entry is "owned"* by server A. Only when A expires the lease and removes it's DNS entries will B be able to replace them next time the client renews.
> 
> So in practice, DDNS won't work well in a failure scenario as clients will change address but their DNS entries won't follow suit.
> 
> * This is what the TXT record is for. It's a hash allowing a server to identify if it was the server that put a DNS record in - and it'll refuse to remove/replace one that doesn't "belong" to it.

The TXT or DHCID records are linked to the client not the server.
If a client goes through two different severs but both are calculating the record
the same way then the second sever should be able to update the forward
record.  It is basically a hash of the duid or the client id or the hardware address.

The TXT record is used for the older “interim” DDNS code while the DHCID
record is used for the newer “standard” DDNS code defined in 4701-4704.

Shawn
> 
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