Re: Understanding EPICS_CA_ADDR_LIST and caget

[Jure Varlec via Tech-talk <tech-talk at aps.anl.gov>]

Hello Sean,

As long as you have EPICS_CA_AUTO_ADDR_LIST set to YES, broadcast searches will be done in addition to what you have specified in EPICS_CA_ADDR_LIST. In other words, both broadcast searches to broadcast-capable interfaces and unicast searches to explicitly listed addresses will happen. More information on how this works is available in the CA Reference Manual, which is linked at the bottom of the page that you referenced in your message. And also below 🙂

https://epics.anl.gov/base/R3-14/12-docs/CAref.html#EPICS

This explains why you still see broadcast searches done on your first interface. You should also see broadcasts on the other interfaces, and unicast searches as well. But you do not, because tcpdump will only listen on the first interface by default. You should tell it to listen on any interface, it will give you a wider picture of what's going on.

Best,

Jure

---

From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of Sean Leavey via Tech-talk <tech-talk at aps.anl.gov>
Sent: Thursday, September 8, 2022 10:20
To: tech-talk at aps.anl.gov <tech-talk at aps.anl.gov>
Subject: Understanding EPICS_CA_ADDR_LIST and caget

 

Caution: This email originated from outside of Cosylab.


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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