snmp

SNMP is good for collecting router information, not good for making configuration changes.

oid = object identifier
mib = management information base (provides definition of snmp objects (oids, human readable names, interpretation of the data sent back)

There are two main snmp library choices:
	pysnmp (entirely python)
	netsnmp (c library with python hooks). Good to walk and find oids in the snmp tree

You can use a MiB browser to find this info:
	.iso(1).org(3).dod(6).internet(1).mgmt(2).mib2(1).system(1).sysDescr(1)
	
	Scalar data needs .0 append on the end
	Tabular data does not need this

From linux cli to test snmp:
	snmpwalk -v 2c -c <commstring> 10.0.0.15 1.3.6.1  # this returns everything starting at that oid position.
	snmpget -v 2c -c <commstring> 10.0.0.15 1.3.6.1.2.1.1.1.0  # to return a specific value
	snmpget -v 2c -c <commstring> 10.0.0.15 <oid_description_name_returned_by_walk>
	
You install the MIB definition on the server doing the SNMPWalk to get a human readable description back
On Centos is /usr/share/snmp/mibs



Python Example:

# need to have this helper script available for the import
# https://github.com/ktbyers/pynet/blob/master/snmp/snmp_helper.py
from snmp_helper import snmp_get_oid,snmp_extract

COMMUNITY_STRING = 'mycommstr'
SNMP_PORT = 161
IP = '10.0.0.15'

# this is a tuple (unchangable list)
a_device = (IP, COMMUNITY_STRING, SNMP_PORT)

# system description oid
OID = '1.3.6.1.2.1.1.1.0'

snmp_data = snmp_get_oid(a_device, oid=OID)

# Converts from Hex to Human Readable Output
output = snmp_extract(snmp_data)

print output


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SNMPv3 - adds authentication and encryption

IOS Router Config Example:

# Create a view of what can be seen in the SNMP tree (this view actually
# specifies the entire tree is visible).
snmp-server view VIEWSTD iso included

# Create an SNMP Group using SNMPv3 with authentication and encryption, 
# read-only, using VIEWSTD, filtered by ACL98.  Group name is 'READONLY'.  
# 'priv' indicates authentication and encryption is used.
snmp-server group READONLY v3 priv read VIEWSTD access 98

# Bind a user to the SNMP group, specify SHA1 for auth and AES128 for encryption.
# Specify the authentication and encryption keys
snmp-server user <user> READONLY v3 auth sha <password> priv aes 128 <key>

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Using kbyers snmp_helper.py program for SNMPv3 queries

import snmp_helper
# this assumes the defaults for encyption AES and authentication of SHA

ip = '10.11.12.55'
a_user = 'mysnmpuser'
auth_key = 'myauthkey'
encrypt_key = 'myencryptkey'
snmp_user = (a_user, auth_key, encrypt_key)
router_1 = (ip, <port>)
router_2 = (ip, <port>)

snmp_data = snmp_helper.snmp_get_oid_v3(router_1, snmp_user, oid='1.3.6.1.2.1.1.5.0')

output = snmp_helper.snmp_extract(snmp_data)
print output