Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Thu, 27 Jun 2013 11:49:36 -0500

Hi Brian,

On 2013-06-27 Brian McAllister wrote:
> I still don't understand why it didn't work from the local host.  iptables
> accepts all traffic from the loopback.  Why doesn't that work for CA ?

It would have if you had added the loop-back address to EPICS_CA_ADDR_LIST; 
by default CA deliberately excludes that from the list of interfaces it sends 
searches to.  That exclusion makes perfect sense for VxWorks IOCs since there 
is an internal short-cut between the client library and the RSRV for local 
channels so there is no point sending search requests to yourself, that would 
just be wasting CPU time.

For workstations where the CA server is in a different process sending 
searches to the sub-net's broadcast address will still result in local 
servers seeing the searches, so loop-back is not needed there either.  Maybe 
CA should include the loop-back address if there are no other interfaces 
available, but that wouldn't have helped your case anyway because there was 
one, it was just fire-walled off.

You can find out more about the CA protocol in these slides:
    http://www.aps.anl.gov/epics/docs/AES2013/11-Protocol.pdf

- Andrew
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System