On 07/29/2014 09:42 PM, Andrew Hanushevsky wrote:
> On 7/21/14, 3:05 AM, Tommaso Boccali wrote:
>> Ciao,
>> we are in the situation where we would like to add another redirector to
>> the DNS-RR setup (so a third one under the same DNS entry). If I
>> understand correctly, everyone (servers or local cmsd) using the DNS-RR
>> would need to be restarted, since they resolve hostname -> IPs at the
>> start. This is quite painful, clearly.
[..]
> DNS caching does not exist in xrootd until 4.0.0 and later. We added
> that because people were running into DNS issues when a lot of worker
> machines would all of a sudden connect at the same time. While using the
> nodnr option would completely avoid the DNS; some sites were annoyed
> that it leaves the log with nothing but IP addresses. Anyway, the
> default is 3 hours but you can set it to whatever you want (see
> xrd.network cache)....
My impression is that the question was specifically on multihomed DNS
aliases, in particular those used with the '+'-syntax ("all.manager meta
any DNSALIAS.DOMAIN+").
It looks like xrootd/cmsd does resolve the alias at start, then keeps
track of all machines behind such an alias via their IP
address/individual hostname. I think this is independent of the generic
Hostname->IP caching in Xrootd4.
Is there a way to get the daemons to re-resolve the DNS alias at some
stage? Ideally this would happen automatically e.g. on connection
failures, or once/day etc.
Use case is of course new machine being brought into the service,
usually without taking over the IP address of a previous machine -
simple DNS-aliasing is much easier than fullblown IP-level HA.
For the specific case of federations, the clients (aka local
redirectors) are in different administrative domains, so scheduling a
full restart requires talking to lots of people - not just some SSHing.
Regards,
jan
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