如何用Python获取SNMP数据？

Question

如何用Python从fdb表中获取MAC地址和VLAN信息？
在bash中，使用snmpwalk可以正常工作：

snmpwalk -v2c -c pub 192.168.0.100 1.3.6.1.2.1.17.7.1.2.2.1.2

使用pysnmp：

import os, sys
import socket
import random
from struct import pack, unpack
from datetime import datetime as dt

from pysnmp.entity.rfc3413.oneliner import cmdgen
from pysnmp.proto.rfc1902 import Integer, IpAddress, OctetString

ip='192.168.0.100'
community='pub'
value=(1,3,6,1,2,1,17,7,1,2,2,1,2)

generator = cmdgen.CommandGenerator()
comm_data = cmdgen.CommunityData('server', community, 1) # 1 means version SNMP v2c
transport = cmdgen.UdpTransportTarget((ip, 161))

real_fun = getattr(generator, 'getCmd')
res = (errorIndication, errorStatus, errorIndex, varBinds)\
    = real_fun(comm_data, transport, value)

if not errorIndication is None  or errorStatus is True:
       print "Error: %s %s %s %s" % res
else:
       print "%s" % varBinds

输出结果：[(ObjectName(1.3.6.1.2.1.17.7.1.2.2.1.2), NoSuchInstance(''))]

import netsnmp

def getmac():
    oid = netsnmp.VarList(netsnmp.Varbind('.1.3.6.1.2.1.17.7.1.2.2.1.2'))
    res = netsnmp.snmpgetbulk(oid, Version = 2, DestHost='192.168.0.100',
                           Community='pub')
    return res

print getmac()

输出结果：('27', '27', '25', '27', '27', '27', '24', '27', '25', '18', '4', '27', '25', '27', '27', '25', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '23', '25', '27', '27', '27', '25', '27', '25', '27', '27', '25', '27', '27', '27', '27', '27', '27', '27', '27', '27', '25', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '25', '25', '25', '7', '27', '27', '9', '25', '27', '20', '19', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '11', '25', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '27', '25', '27', '27', '27', '27', '27', '27', '27', '27', '27', '2', '27', '5', '27', '0', '27', '27', '27', '27', '27')

第一个脚本（pysnmp）返回了NoSuchInstance。第二个脚本（netsnmp）返回了一系列端口，但没有MAC和VLAN信息。到底出了什么问题？

Answer 1

在这个pysnmp的例子中，你正在进行的是SNMPGET（snmpget），而不是GETNEXT（snmpwalk）。如果你把

real_fun = getattr(generator, 'getCmd')

改成

real_fun = getattr(generator, 'nextCmd')

你就会开始看到有用的结果。

至于你在结果中看到的snmpwalk和python的net-snmp绑定之间的差异：snmpwalk和snmpbulkget的表现是不同的。如果你在命令行中用和snmpwalk相同的选项执行snmpbulkget，你会得到和你的python net-snmp例子一样的结果。

如果你在你的python net-snmp例子中更新以下这一行，

res = netsnmp.snmpgetbulk(oid, Version=2, DestHost='192.168.0.100', 
                          Community='pub')

改成

res = netsnmp.snmpwalk(oid, Version=2, DestHost='192.168.0.100', 
                       Community='pub')

那么你现在应该能从python net-snmp例子中得到和在命令行上执行snmpwalk时看到的相同结果列表。