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Im trying to create an SNMP setup to monitor 500-1K machines. Each will be joined to the SNMPTRAPD server via a VPN.

When the client system sends its messages, it's sending its internal address instead of the VPN address even though the 'trapsync' command is using the VPN. This will be a problem as many of these system will have DHCP addresses based on location so there will be overlap.

What I'd like to figure out is how to pass extra values with the trap so I can identify which system a given trap comes from.

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up vote 0 down vote accepted

Found it: in the 'monitor' line you can specify values from other MIBs. If you preface them with '-o' it will apply any applicable wildcards. Use '-i' to pass them as-is.

monitor -r 10 -i SNMPv2-MIB::sysName.0 -o prErrMessage "process table" prErrorFlag != 0   

This will check the process table MIB for any values (among those specified in your snmpd.conf with 'proc' directives) for which the corresponding 'prErrorFlag' is non-zero. At the snmptrapd system it will pass the corresponding 'prErrorMessage' value for this entry. It will also send the 'sysName.0' entry AS-IS. (If you put a -o instead of -i you'll see an error as it will append the proc list value to the end and it won't be defined.)

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