This is a small issue that took me weeks to sort out. We run a SonicWall NSA 240, which supports SNMP. The service was very easy to set up, with settings located under the "System" category and the "Administration" sub-category. There are only a few fields to configure, so you can imagine my dismay when it didn't seem to work.
I ran a few
snmpwalk commands to try and get information, using both v1 and v2c of SNMP (both of which are supposedly supported), but had no luck.
Finally, I ran upon a very important piece of information: By default, the SonicWALL appliance responds only to SNMP Get messages received on its LAN interface. I was sending requests to the LAN interface, but it was just the lead I needed to get to the root of the problem.
It turns out, SNMP can also be enabled/disabled at the interface level! If you open the "Network" category, select the "Interfaces" sub-category, and open one of the interfaces, there are checkboxes for several different management protocols. You can enable/disable HTTP, HTTPS, SSH, and SNMP per-interface. Lo and behold, the SNMP checkbox was un-checked on our LAN interface.
I hope this saves someone the several days of trouble I spent!