2018-08-23-snmp笔记
Table of Contents
1 snmp中的oid和instance
oid是指对象id，instance是指对象实例。
我们在获取时，都是获取的对象实例。
scalar的实例都是 oid.0 。table的实例都是 oid.index ，如果table有
多个索引，则其实例类似这样 oid.index1.index2 。
2 snmp的scalar和table的区别
参考 snmp overview 。
要从管理者中获得对象的值，必须指定对象中的实例。对象的实例可以通过 在对象的oid后添加实例的索引来指定。索引就比如下面例子中最后的0就是 实例的索引：
.iso.3.dod.1.mgmt.mib.1.sysUpTime.0
第一个实例是从0开始索引的。1代表第二个实例，以此类推。由于sysUpTime 是一个scalar对象，它只有一个实例。因此，如果想要获得一个scalar对象 的值，总需要在最后使用0来作为索引。索引超过0的情况只会发生在 columnar对象（table）中，这类对象中可以有多个实例。
总的来说，只有一个实例的管理对象叫作scalar对象。而tabular对象中可以 有多个实例，就好像表中有多行一样。
3 mib table中的index含义
参考 SNMP Table Basics 。
一个table中必须要有索引。用户使用索引才能找到对应的实例的值。
以vlanTable为例，它是以 vlanId 为index的：
fhosVlanTable OBJECT-TYPE SYNTAX SEQUENCE OF FhosVlanEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A table of vlan names and characteristics" ::= { fhosVlan 1 } fhosVlanEntry OBJECT-TYPE SYNTAX FhosVlanEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "" INDEX { fhosVlanId } ::= { fhosVlanTable 1 } FhosVlanEntry ::= SEQUENCE { fhosVlanId Integer32, fhosVlanName DisplayString } fhosVlanId OBJECT-TYPE SYNTAX Integer32(1..4094) MAX-ACCESS read-only STATUS current DESCRIPTION "This is the internal ID of the device to reference its VLAN." ::= { fhosVlanEntry 1 } fhosVlanName OBJECT-TYPE SYNTAX DisplayString (SIZE (1..255)) MAX-ACCESS read-only STATUS current DESCRIPTION "This is the VLAN name" ::= { fhosVlanEntry 2 } fhosVlanType OBJECT-TYPE SYNTAX INTEGER { static (1), dynamic(2) } MAX-ACCESS read-only STATUS current DESCRIPTION "The VLAN type can be static(1) dynamic(2)" ::= { fhosVlanEntry 3 }
要取到vlan1的vid：
snmpget host vlanTable.vlanEntry.1.0
要取到vlan1的name：
snmpget host vlanTable.vlanEntry.2.0
要取到vlan2的name：
snmpget host vlanTable.vlanEntry.2.1
索引可以有多个，如本节参考文档中的tcpConnTable就有多个index：
tcpConnTable OBJECT-TYPE -- ... ::= { tcp 13 } tcpConnEntry OBJECT-TYPE -- ... INDEX { tcpConnLocalAddress, tcpConnLocalPort, tcpConnRemAddress, tcpConnRemPort } ::= { tcpConnTable 1 } TcpConnEntry ::= SEQUENCE { tcpConnState INTEGER, tcpConnLocalAddress IpAddress, tcpConnLocalPort INTEGER (0..65535), tcpConnRemAddress IpAddress, tcpConnRemPort INTEGER (0..65535) } tcpConnState OBJECT-TYPE -- ... ::= { tcpConnEntry 1 } tcpConnLocalAddress OBJECT-TYPE -- ... ::= { tcpConnEntry 2 } tcpConnLocalPort OBJECT-TYPE -- ... ::= { tcpConnEntry 3 } tcpConnRemAddress OBJECT-TYPE -- ... ::= { tcpConnEntry 4 } tcpConnRemPort OBJECT-TYPE -- ... ::= { tcpConnEntry 5 }
如果有这样一个表：
+----------------------------------------------------------------------------------------------------------------------------+ | tcpConnState | tcpConnLocalAddress | tcpConnLocaPort | tcpConnRemAddress | tcpConnRemPort | |------------------------+------------------------+------------------------+------------------------+------------------------| |listen(2) |0.0.0.0 |21 |0.0.0.0 |0 | |------------------------+------------------------+------------------------+------------------------+------------------------| |listen(2) |0.0.0.0 |23 |0.0.0.0 |0 | |------------------------+------------------------+------------------------+------------------------+------------------------| |listen(2) |0.0.0.0 |3306 |0.0.0.0 |0 | |------------------------+------------------------+------------------------+------------------------+------------------------| |listen(2) |0.0.0.0 |6000 |0.0.0.0 |0 | |------------------------+------------------------+------------------------+------------------------+------------------------| |established(5) |127.0.0.1 |1042 |127.0.0.1 |6000 | |------------------------+------------------------+------------------------+------------------------+------------------------| |established(5) |127.0.0.1 |6000 |127.0.0.1 |1042 | |------------------------+------------------------+------------------------+------------------------+------------------------| |closeWait(8) |192.168.1.78 |1156 |192.168.4.144 |80 | +----------------------------------------------------------------------------------------------------------------------------+
要取到最后一行的tcpConnState：
snmpget host tcpConnTable.tcpConnEntry.tcpConnState.192.168.1.78.1156.192.168.4.144.80 等价于： snmpget host tcpConnTable.tcpConnEntry.1.192.168.1.78.1156.192.168.4.144.80
要取到最后一行的tcpConnLocalAddress：
snmpget host tcpConnTable.tcpConnEntry.tcpConnLocalAddress.192.168.1.78.1156.192.168.4.144.80 等价于： snmpget host tcpConnTable.tcpConnEntry.2.192.168.1.78.1156.192.168.4.144.80
注意看两者的区别。
4 如何根据mib文件生成对应的C代码
以命令为例：
env MIBS="+/path/to/your/mib/fhos-vlan.mib" mib2c fhosVlanTable
修改为对应的路径和table的名字就可以了。
使用mib2c时会要求选择code style：
1) ucd-snmp style code 2) Net-SNMP style code
由于当前frr中的snmp实现都是使用原来的ucd-snmp，所以这里我们也都选择1。
这两种格式的区别可以在 Mail List 里面找到。
> Dear:
> I am developting the snmp agent using ..... net-snmp 5.2.3.
> When using the tool of mib2c generate the .c file,Can I
> choose the UCD-SNMP style Code choice?
Yes.
> That is can I developt the agent in UCD-SNMP style with
> the pakage of NET-SNMP5.2.3?
Yes.
Many of the current MIB implementations still use the older
UCD style.
> Are there some faults?
The main disadvantages of the UCD style over the newer
handler-based APIs are:
a) It's slightly less efficient
(though probably not enough to worry about)
b) It doesn't handle as much automatically.
With the newer v5 API, particularly when implementing MIB
tables, the helpers will take care of much of the donkey work
(interpreting indexes, etc). With the UCD style of MIB, this
was all left to the MIB programmer.
But if you're happier working with the UCD approach, that's
fine - it should work OK.
One word of warning about the mib2c interface though.
The code generated is not always as accurate as we might
like - particularly when run on anything more than just a
single table. So do check the OIDs that it's trying to register
carefully.
Dave
5 如何添加一个临时的mib文件
参考 TUT:Using and loading MIBS 。
linux中如果安装好snmp后默认会安装一些标准的mib。snmp相关的程序都是 在指定的路径中搜索mib文件。默认在这两个目录：
1. $HOME/.snmp/mibs 2. /usr/local/share/snmp/mibs
也可以使用下面的命令来查一下：
net-snmp-config --default-mibdirs
当然，可以直接在命令中指定对应的mib文件，比如我把我们cisco交换机对应
的mib文件都下载下来，放到 ~/.snmp/mibs/ 目录下。可以这样来玩：
snmptranslate -m +/home/pengpengxp/.snmp/mibs/CISCOSBvlan.mib -IR -On vlanNameTable snmpwalk -m +/home/pengpengxp/.snmp/mibs/CISCOSBvlan.mib -u xiepeng -l NoauthNoPriv 10.0.0.2 vlan snmpwalk -m +/home/pengpengxp/.snmp/mibs/CISCOSBvlan.mib -u xiepeng -l NoauthNoPriv 10.0.0.2 vlanNameTable
其中 -m 选项就是指定对应的mib文件。就可以直接使用名字，而不是oid来
使用命令了。 xiepeng 这个用户是我新建的测试用户。
6 如何把mib文件添加到系统
参考 TUT:Using and loading MIBS 。
我把自定义的一些mib文件都放到 ~/.snmp/mibs 目录。原则上放在在
net-snmp-config --default-mibdirs 中都可以。
需要注意的是，文件名和文件内的第一行“模块名”应该是相同的。比如
FHOS-VLAN 文件中的第一行如下：
FHOS-VLAN DEFINITIONS ::= BEGIN
mib文件放入目录后，对应的 snmp* 的命令还是找不到的。需要在全局的配
置文件 /etc/snmp/snmp.conf 中加入对应的文件才行，比如我把当前fhos
的mib文件都放入目录后，加入下面几行就可以了：
mibs +FHOS-VLAN mibs +FHOS-PORT mibs +FHOS-TRUNK mibs +FHOS-STATS
这样 snmptranslate,mib2c 这些命令都可以直接使用这些mib了。
7 ubuntu中配置snmpd
8 mib中admin string, display string和octet sring的区别
参考 这里 。
Re: Difference between admin string, display string and octet sring.
From: Dave Shield <D.T.Shield@li...> - 2012-07-18 13:05:22
On 18 July 2012 13:35, Benix Vincent <benixvincent@...> wrote:
> When and how Display string, octet string and snmp admin sting should be
> used in data type?
Octet String is the underlying base type.
Defining a MIB object using this type doesn't say anything about what
sort of value the object should contain (other than that it will be a
sequence of octet values).
In particular, there is no implication that the value will necessarily
be a printable string.
Both DisplayString and SnmpAdminString *are* intended for printable
string values. The difference between them is whether the string is
purely ASCII text (DisplayString), or can potentially contain other
characters (such as accented or non-roman characters).
> For a product model name, what is the correct object type to be used?
If you are working from a pre-existing MIB file, then you should follow
what is defined there.
If you are defining a new MIB file, then it's probably sensible to use
SnmpAdminString. That leaves the way open to support international
values in the future - even if you don't expect to need this at the moment.
Dave
Octet String是基本类型，使用这个类型的mib对象不会指定它将包含的类型。 DisplayingString和SnmpAdminString才是真正指定 可打印 字符的类型。区 别在于DisplayingString只是指ascii码的文本。
9 integer和integer32的区别
参考 这里 。
7.1.1. Integer32 and INTEGER The Integer32 type represents integer-valued information between -2^31 and 2^31-1 inclusive (-2147483648 to 2147483647 decimal). This type is indistinguishable from the INTEGER type. Both the INTEGER and Integer32 types may be sub-typed to be more constrained than the Integer32 type. The INTEGER type (but not the Integer32 type) may also be used to represent integer-valued information as named-number enumerations. In this case, only those named-numbers so enumerated may be present as a value. Note that although it is recommended that enumerated values start at 1 and be numbered contiguously, any valid value for Integer32 is allowed for an enumerated value and, further, enumerated values needn't be contiguously assigned. Finally, a label for a named-number enumeration must consist of one or more letters or digits, up to a maximum of 64 characters, and the initial character must be a lower-case letter. (However, labels longer than 32 characters are not recommended.) Note that hyphens are not allowed by this specification (except for use by information modules converted from SMIv1 which did allow hyphens).
总的来说没有太大区别，INTEGER可能被用于表示枚举类型。所以我觉得如果 是用来做数字使用integer32,如果需要用枚举的就使用INTEGER。