snmp proxy in python

Question

I need kind of snmp v2c proxy (in python) which:

• react on snmpset command
• read value from command and write it to yaml file
• run custom action (prefer in different thread and somehow reply success to snmpset command):
  • run another snmpset to different machine, or
  • ssh to user@host and run some command, or
  • run some local tool

and:

• react on snmpget command
• check value for requested oid in yaml file
• return this value

I'm aware of pysnmp but documentation just confuse me. I can image I need some kind of command responder (I need snmp v2c) and some object to store configuration/values form yaml file. But I'm completely lost.

Answer 1

I think you should be able to implement all that with pysnmp. Technically, that would not be SNMP proxy but SNMP command responder.

You can probably take this script as a prototype and implement your (single) cbFun which (like the one in the prototype) receives the PDU and branches on its type (GET or SET SNMP command in your case). Then you can implement value read from the .yaml file in the GetRequestPDU branch and .yaml file write, along with sending SNMP SET command elsewhere, in the SetRequestPDU branch.

The pysnmp API we are talking here is the low-level one. With it you can't ask pysnmp to route messages between callback functions -- it always calls the same callback for all message types.

However, you can also base your tool on the higher-level SNMP API which was introduces along with the SNMPv3 model. With it you can register your own SNMP applications (effectively, callbacks) based on PDU type they support. But given you only need SNMPv2c support I am not sure the higher-level API would pay off in the end.

Keep in mind that SNMP is generally time sensitive. If running local command or SSH-ing elsewhere is going to take more than a couple of seconds, standard SNMP manager might start retrying and may eventually time out. If you look at how Net-SNMP's snmpd works - it runs external commands and caches the result for tends of seconds. That lets the otherwise timing out SNMP manager eventually get a slightly outdated response.

Alternatively, you may consider writing a custom variation plugin for SNMP simulator which largely can do what you have described.