Chris's Wiki :: blog/sysadmin/PrometheusSnmpExporterConfigNotes
A variety of devices (including DSL modems) expose interesting information via SNMP (which is not simple, despite its name). If you have a Prometheus environment, it would be nice to get (some of) this information from your SNMP capable devices into Prometheus. You could do this by hand with scripts and commands like 'snmpget', but there is also the officially supported SNMP exporter. Unfortunately, in practice the Prometheus SNMP exporter pretty much has a sign on the front door that says "no user serviceable parts, developer access only". Understanding how to do things even a bit out of standard with it is, well, a bit tricky. So here are some notes.
The SNMP exporter ships with a 'snmp.yml' configuration file that's what the actual 'snmp_exporter' program uses at runtime (possibly augmented by additional files you provide). As you'll read when you look at the file, this file is machine generated. As far as I can tell, the primary purpose of this file is to tell the exporter what SNMP OIDs it could try to read from devices, what metrics generated from them should be called, and how to interpret the various sorts of values it gets back over SNMP (for instance, network interfaces have a 'ifType' that in raw format is a number, but where the various values correspond to different types of physical network types). These SNMP OIDs are grouped into 'modules', with each module roughly corresponding to a SNMP MIB (the correspondence isn't necessarily exact). When you ask the SNMP exporter to query a SNMP device, you normally tell the exporter what modules to use, which determines what OIDs will be retrieved and what metrics you'll get back.
The generated file is very verbose, which is why it's generated, and its format is pretty underdocumented, which certainly does help contribute to the "no user serviceable parts" feeling. There is very little support for directly writing a new snmp.yml module (which you can at least put in a separate 'snmp-me.yml' file) if you happen to have a few SNMP OIDs that you know directly, don't have a MIB for, and want to scrape and format specifically. Possibly the answer is to try to write a MIB yourself and generate a snmp-me.yml from it, but I haven't had to do this so I have no opinions on which way is better.
