Awesome Netcat test:  nc -zvu xx.xx.xx.xx 161
  Doesn't work with some devices that are serving SNMP successfully :(.
Similarly with nmap:  nmap -sU -pU:161 xxx.xxx.xxx.xxx
  Requires root to run.

Fri Feb  3 10:00:48 EST 2012
http://www.dpstele.com/layers/l2/snmp_l2_tut_part1.php
http://www.dpstele.com/white-papers/snmp-mib/

Reference:  http://www.oidview.com/mibs/

Transport LAYERS  (shown as   Layer:  Protocol)
    Application:  SNMP
    Transport:  UDP
    Internet:  IP
    Network Interface:  Physical

SNMP Operations/PDUs (Programming Datagram Unit) {
    GET /  GetRequest (from manager)
    GET-NEXT / GetNext (from manager)
    SET / SetRequest
    GET-RESPONSE / GetResponse (from agent in response to either of previous)
        Sends either requested-info/success indication or error indication.
    TRAP / Trap (notify) (from agent)
    INFORM (notification ack)  [not in some lists]
}

Components { Manager, Agent }
Basically, Agents are the servers that reside with managed devices;
Managers are client applications.

MIB = SNMP Management Information Base

OIDs distinguish each variable/object uniquely in the MIB
???? Half the time they begin with dot and half the time not.
    Each OID has a "readable label" + ...

MIB FILE:
    -- a comment
    newObjCatName OBJECT IDENTIFIER ::= {parentObjName 123}  # 123 is new int
    newObjName OBJECT-TYPE
        SYNTAX DisplayString ACCESS read-only STATUS mandatory DESRIPTION "y"
        ::= {parentObjName 123}  # 123 is new int
    trapSet TRAP-TYPE blab blab

Net-SNMP  (for network addr 'colorlj')
    GENERAL EXCEPTIONS (from which you can infer the normal defaults)
        snmptranslate works locally and does not default to -IR
        snmpwalk honors only the first specified OID and ignores the rest.
    Several ways to load additional MIB definition files.
      See http://www.net-snmp.org/wiki/index.php/TUT:Using_and_loading_MIBS
    With v3, often need to add switch:  -l authpriv


    snmptranslate is the standalone program that reads mib files.
        N.b. resource specification is an expression not a literal.
        snmptranslate .1.3.6.1.2.1.1.3.0
        snmptranslate -On SNMPv2-MIB::sysUpTime
        snmptranslate -IR -On sysUpTime  # -IR default for all other programs
        snmptranslate -TB -On sysUpTime  # Lists all matches.  Safer than -IR.
        snmptranslate -Tp -IR  sysUpTime # Display a tree
    snmpwalk -Os -c public -v 2c colorlj
        Default OID root for snmpwalk is SNMPv2-SMI::mib-2 == 1.3.6.1.2.1
        OID root of ".oid" "would typically retrieve *all*.
        For v3, at least, sometimes need the -Cc switch.
    snmpget -c public -v 2c colorlj sysUpTime.0
    snmptable works only on tables like snmpwalk, but displays row-first and
     with headers.  (I can't get this to list any prt* table).
    snmpbulk* are just a more efficient version of snmpget, snmpwalk.
    snmptrap obviously

    To run normal agent, set v. 2c listen addr + comm string in
     /etc/snmp/snmp.conf and start /etc/init.d/snmpd.  This listens on
     161/udp.  The trap agent runs on 162/udp.  See with "netstat -nlu".
     The "rocommunity" args are source restrictions.  "agentaddress" sets the
     listen addrs.
     snmpconf is a wizard for /etc/snmp/snmp.conf (and maybe other files).
    snmptrapd obviously...

Scalars vs. Tables
    GUESS! Distinction between *Table/Column and *Entry non-leaf nodes:
        If a leaf name *Index with OID ending in .1, then .1 has no accessible
         value, but the .2 and other values are columns with values accessible
         like ...2.1.1, ...2.1.2,....
    You always need to work with leafs MIBs and I know of no way to tell if a
     leaf MIB is a Table/Column or a Scalar.
    Scalar MIB values are obtained like:  s.0
    Table MIB values are obtained from column OID like:  c.1.1, c.1.2
    Table example:
        prtInputTable is 1.3.6.2.1.43.8.2
            prtInputTableEntry is 1.3.6.2.1.43.8.2.1
                ATTRIBUTE LEVEL
                prtInputTableIndex is 1.3.6.2.1.43.8.2.1.1
                prtInputTableType is 1.3.6.2.1.43.8.2.1.2
                    Some level below Attr Lvl are tuples for each attr, one
                    for each element.
                    If there are 4 prtInputs, then there will be four
                        1.3.6.2.1.43.8.2.1.2 and four 1.3.6.2.1.43.8.2.1.3
                        often like
                        1.3.6.2.1.43.8.2.1.3.1.1, 1.3.6.2.1.43.8.2.1.3.1.2
                        (often start with .1 like here but also start with
                        0.0.0.0.80.0.0.0.0.0)
                prtInputTableType is 1.3.6.2.1.43.8.2.1.2

EMPIRICALLY
    If the MIB list lists X, then ether
    X.1.1 retrieves the TYPE: VALUE
        Example:  snmpget -c public -v 2c colorlj 1.3.6.1.2.1.43.8.2.1.13.1.1
    OR X.0 retrieves the TYPE:VALUE
        Example:  snmpget -c public -v 2c colorlj 1.3.6.1.2.1.1.3.0
    But for unknown reason I get "No Such Instance" when I attempt to pull
     index objects.

GENERIC Mib Object Identifiers
    sysDescr    1.3.6.1.2.1.1.1    DO an snmpget on 1.3.6.1.2.1.1.1.0
    sysObjectID 1.3.6.1.2.1.1.2    DO an snmpget on 1.3.6.1.2.1.1.2.0
                                   ID should begin with 1.3.6.1.4.1.<VENDOR>.
    sysName     1.3.6.1.2.1.1.5    DO an snmpget on 1.3.6.1.2.1.1.5.0
    Sys stuff (walk it)    1.3.6.1.2.1
    1.3.6.1.2.1.2.2.1.6.2  (2.2.1 is the Entry, .6 is Mac, .1 must be index?, .2 is real???)
    # Working example: snmpWalk 10.64.102.233 1.3.6.1.2.1.2.2.2.1.6 -v 1 -S

Linux
    To open up v2 requests from everywhere, change the "rocommunity public"
    line in /etc/snmp/snmp.conf to:  rocommunity public default
    (I added a "rocommunity ext" line that I thought worked before, but it
    ain't working now).

SNMPv2-SMI::enterprises == 1.3.6.1.4.1  is parent of all vendor branches.  E.g., a Cisco bfanch is 1.3.6.1.4.1.9.9.156.  NO NO NO 'cause contains 1.3.6.1.4.1.14179.2.2.1.1.