We have a nagios server and I would add snmp requests for our Linksys/Cisco RVS4000s. They are connected via ipsec tunnels. Problem is that the RVS4000's do not seem to support SNMP over the WAN interface, only the LAN. Even going through a vpn tunnel.
Is there any can of small agent I can install on a server on the remote LAN to proxy requests from the nagios server to the router? That way the requests come from the LAN
I think you might need to install another Nagios server in that particular LAN and set up a distributed server.
The function of a distributed server is to actively perform checks all the services you define for a "cluster" of hosts. I use the term "cluster" loosely - it basically just mean an arbitrary group of hosts on your network. Depending on your network layout, you may have several cluters at one physical location, or each cluster may be separated by a WAN, its own firewall, etc. The important thing to remember to that for each cluster of hosts (however you define that), there is one distributed server that runs Nagios and monitors the services on the hosts in the cluster. A distributed server is usually a bare-bones installation of Nagios. It doesn't have to have the web interface installed, send out notifications, run event handler scripts, or do anything other than execute service checks if you don't want it to.
Not sure what your reason is for requiring the SNMP request to come from the inside, however I have RV042's, and created a firewall rule to allow incoming SNMP requests from my IP over the WAN to be passed to ANY as destination, and SNMP works.