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<== Date ==> <== Thread ==>
Subject: Re: Using CP in PV links vs. routing
From: Ralph Lange via Tech-talk <tech-talk at aps.anl.gov>
To: EPICS Tech Talk <tech-talk at aps.anl.gov>
Date: Wed, 15 Apr 2020 15:18:53 +0200
The localhost network (usually 127.0.0.0/8) is by default disabled for CA name resolution, otherwise any CA client asking for a locally hosted PV would get two replies: one through the network IP, one through localhost.
Any CA name resolution request will by default be broadcast on the network as a UDP broadcast. (Unless EPICS_CA_AUTO_ADDR_LIST=NO, which switches off automatic name resolution broadcasting on all interfaces.) Broadcasts to a network will obviously always be sent to the physical network interface.

When the client IOC connects to the server, it will use the IP that it reads from the CA name resolution reply to open the TCP connection. Kernel routing will decide how this connection is being handled. Normally, the kernel's routing table will mark this target IP address as being local and feed outgoing packets back into the TCP/IP stack without sending them to the network interface (which often is a physical device).

So, in my understanding, the traffic will leave the machine logically as far as TCP/IP is concerned, but never be sent to the network interface.

Cheers,
~Ralph

References:
Using CP in PV links vs. routing Hinko Kocevar via Tech-talk
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