apcupsd pcnet and different subnets

Scenario:

  • Subnet A: 192.168.0.0/24
  • Subnet B: 10.10.10.0/24
  • No firewalls blocking communication between them

Problem: An apcupsd instance running on a machine residing on subnet “B” cannot communicate with the APC NMC that resides on subnet “A”

Solution (probably): Make sure to specify the port in your DEVICE declaration in /etc/apcupsd/apcupsd.conf:

DEVICE 192.168.0.5:username:password:3052

Snippet from the default apcupsd.conf:

# pcnet ipaddr:username:passphrase:port
# PowerChute Network Shutdown protocol which can be
# used as an alternative to SNMP with the AP9617
# family of smart slot cards. ipaddr is the IP
# address of the UPS management card. username and
# passphrase are the credentials for which the card
# has been configured. port is the port number on
# which to listen for messages from the UPS, normally
# 3052. If this parameter is empty or missing, the
# default of 3052 will be used.

The question is: Why does explicitly defining something that is supposed to be the default (even when left undefined), change the behavior of the program? I do not have an answer at present. Good luck though!