Re: Understanding EPICS_CA_ADDR_LIST and caget

[Jörn Dreyer via Tech-talk <tech-talk at aps.anl.gov>]

Hi Sean,

some more suggestions on the network issue. Normally between two subnets there 
is a firewall setup that blocks all traffic besides some ports and all 
broadcasts. To figure out if you can reach the IOC you could use nmap like 
this:

nmap -p 5064  195.194.120.115

If that does not find the port to be in open state, you will either need to ask 
your IT to open that port on the firewall, or set up an EPICS gateway on a PC 
that has two NICs into the two subnets.
Issue the nmap command on both machines. At least on the machine running the 
IOC the port should be reported to be open.
This would be my first tests in such a case.

Regards

Jörn

Am Donnerstag, 8. September 2022, 11:23:37 CEST schrieb Jure Varlec via Tech-
talk:
> 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/c
> onfigure-channel-access/#IOCs_on_different_subnets
 
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