Slashdot: Providers Ignoring DNS TTL?

Wed Apr 20 20:04:45 UTC 2005

> While that setup may have worked well, it's not an anycast implementation 
> I would suggest that others follow.  Having the same set of servers 
> announcing multiple IP addresses (assuming those addresses are both in the 
> same set of addresses given out to those doing dns lookups) leaves you 
> open to a failure mode where a single server stops responding to queries 
> but doesn't withdraw its routing announcement.  If a user sees that server 
> as the closest instance of both DNS server IP addresses, they will see 
> that as a failure of "both" of their DNS servers.

Agreed, that's a possibility. In practice it hasn't really turned out to
be a problem for us.

> A more reliable way of doing this is to have multiple anycast clouds with 
> their own servers, each serving a single service address.  That way, an 
> incomplete failure (on query response but no route withdrawl) of a local 
> server for one service address won't have any effect on access to the 
> second service address.

Yes, we have been doing some of this also. Part of the problem is the
desire to spread the load among the servers: Some of this comes naturally
from the different server locations in our network - but with 2 addresses
per server we can also balance the load somewhat by announcing either one
or two addresses per server.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no