Experimental Physics and
Industrial Control System

Sean Leavey via Tech-talk <tech-talk at aps.anl.gov> · Thu, 8 Sep 2022 09:20:17 +0100

Hi tech talkers,

I don't seem to be able to change the behaviour of caget using

EPICS_CA_ADDR_LIST. I'm probably misunderstanding something.

I've got EPICS Base installed on my machine. In one terminal I've got

tcpdump monitoring port 5064. In another terminal I run caget with a

made-up channel. Here's their output after I run caget:

$ caget FAKE:CHANNEL
Channel connect timed out: 'FAKE:FAKE' not found.

$ tcpdump port 5064
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode

listening on wlan0, link-type EN10MB (Ethernet), snapshot length 262144

bytes

09:11:07.345310 IP totoro.41026 > 172.16.15.255.ca-1: UDP, length 48
09:11:07.377158 IP totoro.41026 > 172.16.15.255.ca-1: UDP, length 48
09:11:07.441438 IP totoro.41026 > 172.16.15.255.ca-1: UDP, length 48
09:11:07.569269 IP totoro.41026 > 172.16.15.255.ca-1: UDP, length 48
09:11:07.825253 IP totoro.41026 > 172.16.15.255.ca-1: UDP, length 48

So far, so good? Looks like caget is sending 5 broadcast packets then

giving up. 172.16.15.255 is my link's broadcast address, verified with

`ip addr`.

But what if I know the IP of the IOC that hosts a channel, and want to

specify it directly? I read in [1]:

> To reach IOCs on one or more additional subnets, the environment

variable EPICS_CA_ADDR_LIST needs to be configured. It can list either

the specific IP addresses of each IOC, or the broadcast address of their

subnet. Note, however, that routers will often not forward broadcast

requests, which suggests using specific IP addresses.

This sounds useful to me because my IOC is in a different subnet. From

the quoted text above, I understand that, by setting EPICS_CA_ADDR_LIST

to the IP of my IOC, caget should talk directly to it. But apparently

this is not the case. Keeping tcpdump open, and running the caget

command again but this time with the address of my IOC set in

EPICS_CA_ADDR_LIST, I get the same output:

$ EPICS_CA_ADDR_LIST=195.194.120.115 caget FAKE:FAKE
Channel connect timed out: 'FAKE:FAKE' not found.

$ tcpdump port 5064
[...]
09:14:18.995465 IP totoro.35942 > 172.16.15.255.ca-1: UDP, length 48
09:14:19.027341 IP totoro.35942 > 172.16.15.255.ca-1: UDP, length 48
09:14:19.091486 IP totoro.35942 > 172.16.15.255.ca-1: UDP, length 48
09:14:19.219462 IP totoro.35942 > 172.16.15.255.ca-1: UDP, length 48
09:14:19.475597 IP totoro.35942 > 172.16.15.255.ca-1: UDP, length 48

Clearly I'm misunderstanding how all of this works. Can someone shed

some light on what EPICS_CA_ADDR_LIST does in the context of caget, if

anything?

Cheers,
Sean

[1]

https://epics-controls.org/resources-and-support/documents/howto-documents/configure-channel-access/#IOCs_on_different_subnets

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System