Pass persist

The pass_persist option is very useful if you need to implement some specific OID subtree, but it has a few traps and features which are poorly communicated, or not even mentioned in the snmpd.conf man page.

1. the shutdown protocol: when the snmpd server wants to shutdown, it tells your script by sending a blank line on stdin. (http://dreness.com/wikimedia/index.php?title=Net_SNMP)
2. snmpd sends a PING at the start of every request (I read the docs to mean that it is sent once, on script startup).
3. the list of types you can return is quite limited. It is actually described in the docs for pass, which says that the type must be "one of the text strings: integer, gauge, counter, timeticks, ipaddress, objectid, or string"
4. because you cannot return DateAndTime (RFC 2579), for datetime values I've resorted to returning two values (two separate OIDs):
   1. seconds-since-epoch (type counter)
   2. datetime formatted as ISO-8601 string (type string)
5. there doesn't seem to be a way for getnext to distinguish between an invalid input OID, and running past the end of the supported tree.

Other things I've learned:

1. your MIB must be valid, otherwise snmpget won't even query the daemon.
2. you will probably need to add a line to the security section of snmpd.conf, to allow access to the OID subtree e.g. something like (where nnnnn is your Private Enterprise Number):

   view all    included    .1.3.6.1.4.1.nnnnn

3. I used python to implement my pass_persist script. You must flush stdout after sending each line, otherwise snmpd will timeout after waiting for your response. Or invoke python with -u option (unbuffered stdin/stdout).
4. python stdin.readline() preserves the newline, so you need to strip it.

My pass_persist daemon main loop looks like this:

    def getline():
        return sys.stdin.readline().strip()


    def output(line):
        sys.stdout.write(line + "\n")
        sys.stdout.flush()


    def main():
        logger.info("starting pass_persist daemon")

        try:
            while True:
                command = getline()

                if command == "":
                    logger.info("stopping pass_persist daemon")
                    sys.exit(0)

                # snmpd 5.4.2.1 sends a PING before every snmp command
                elif command == "PING":
                    output("PONG")

                elif command == "set":
                    oid = getline()
                    type_and_value = getline()
                    logger.info("%s %s %s" % (command, oid, type_and_value))
                    #snmp_set(oid, type_and_value)
                    # set not supported yet...
                    output("not-writable")

                elif command == "get":
                    oid = getline()
                    logger.info("%s %s" % (command, oid))
                    snmp_get(oid)

                elif command == "getnext":
                    oid = getline()
                    logger.info("%s %s" % (command, oid))
                    snmp_getnext(oid)

                else:
                    logger.error("Unknown command: %s" % command)

        # If we get an exception, spit it out to the log then quit
        # (by propagating exception).
        # snmpd will restart the script on the next request.
        except Exception, e:
            logger.exception("")
            raise