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Subject: RE: CA question with two identical servers
From: Emmanuel Mayssat <emayssat@outlook.com>
To: "Goff, Steven J" <steven.j.goff@boeing.com>, "tech-talk@aps.anl.gov" <tech-talk@aps.anl.gov>
Date: Fri, 26 Jul 2013 14:17:31 -0700
Interesting ... EPICS is used at boeing... Just curious, what are you working on?

Although I am not an expert on the CA protocol, I believe this is not possible because client-server are connected using the reliable TCP protocol. (UDP broadcast are first used by the client to find the PV on a server, but thereafter it is a TCP one-to-one connection)

That being said, I can see 2 ways to do this which consists mostly in 'hacking' your network.
Those 2 methods are fundamentally the same though: have exactly the same machine (same IP) and duplicate the TCP packets.

1/ You need a computer. Use it as a gateway, i.e. like in a 'man in the middle attack'. That's relatively easy with a dual-interface linux box.
2/ If you have access to a > $1K network switch, log into one of them, use port mirroring. That's a common feature. 

--
Emmanuel







From: steven.j.goff@boeing.com
To: tech-talk@aps.anl.gov
Subject: CA question with two identical servers
Date: Fri, 26 Jul 2013 14:32:53 +0000

I would like to use one CA application or MEDM display that will connect to two identical  servers running the same PVs. I would like both servers to receive the Channel Access ca_put at the same time. This might mean I have to modify EPICS code for each server to receive the data simultaneously.


Any ideas on how I would achieve this?


References:
CA question with two identical servers Goff, Steven J

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