[Linux-ha-dev] Failure of a network connection.

[Alan Robertson]

Harold Naparst wrote:
> I would like to ensure that visitors to my website always receive a
> response.
> The problem is that the ISP that the server is connected to is very
> unreliable.
> So I purchased another IP address from another ISP.  Now I have two
> different
> connections to the internet, on totally separate networks.  I also
> happen to have
> another server.  I connected the second network connection to my spare
> server.
> 
> I understand that I can use DRBD, heartbeat, ipfail, and MySQL
> replication to make sure that both computers are able to serve requests.
> This would mean, for instance, that if the first server were dead, and a
> client happened
> to make a request on the second IP (through the luck of a round-robin A
> record draw),
> then he would be able to write records to the database, view the web
> site/forums, etc.
> 
> But still, the basic problem is that user's DNS servers all over the
> world are caching
> both public IP addresses as A records.
> 
> Half the requests that clients make will go to a dead server, if a node
> has died.
> Sure I can use heartbeat to take over services, but still, what is done
> to avoid having
> client requests (from outside my network) go into a black hole?
> 
> Sorry for such a simple question.  The only thing I can see is to put a
> 60 second TTL on
> the DNS record.  Then, if a node is down, I can republish the DNS tables
> to not include the dead node.
> But even so, that means that for up to 60 seconds, some requests will go
> to a black hole.

DNS is not an HA mechanism.

You really need to buy your own netblock (256 addresses) and get
together with both of your ISPs and have them allow you to supply them
with routing information on which connection you currently have serving.

This is the only real way to fix it in anything like a timely fashion.

PS:  Some DNS clients ignore short TTL requests, so it's probably worse
than you think.

-- 
    Alan Robertson <alanr at unix.sh>

"Openness is the foundation and preservative of friendship...  Let me
claim from you at all times your undisguised opinions." - William
Wilberforce