[Nut-upsuser] Master Works, Slave Does Not

Charles Lepple clepple at gmail.com
Sun Nov 9 21:31:22 UTC 2014

On Nov 9, 2014, at 3:48 PM, Steve Read <sd_read at hotmail.com> wrote:

> I would like to verify my understanding keeping in mind I have a one master and one slave computer.
> When the master gets the low batt/noAC signal it initiates a broadcast packet(s) on port 3493.
> 1) Is this correct?

No, the slave computer's upsmon process connects to the master (via TCP, so point-to-point, not broadcast), and listens for the OB (on battery) and then "OB LB" (on battery; low battery) status.

(Not to say that some other UPS shutdown software couldn't broadcast, but port 3493 is reserved for NUT.)

> Then the slave computer must have this port open and it listens on this port for the shut-down command.
> 2) Is this correct?

It's the other way around: the master (via upsd) listens on port 3493, and the slave connects to that port.

> On the slave if I run the following:
> steve at MyDesktop:~$ nmap -p 3493
> Starting Nmap 6.40 ( http://nmap.org ) at 2014-11-09 15:42 EST
> Nmap scan report for
> Host is up (0.00013s latency).
> 3493/tcp closed nut
> Nmap done: 1 IP address (1 host up) scanned in 11.06 seconds
> Which tells me this port is configured for nut but it is closed?

As mentioned above, that is expected. But if you run it against the master (, if I understand correctly) from the slave, you should see that it is open.

> I have rebooting and starting and stopping the service using:
> sudo service ups-monitor stop 
> sudo service ups-monitor start
> But it remains closed.
> 3) Assuming this should be open how do I fix this?

You mentioned that you have a "LISTEN" line in upsd.conf on the master - has upsd been restarted since that change was made? Other than that, you should be set. Do things match the "netstat" output I mentioned in a previous email?

Charles Lepple
clepple at gmail

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