Installing and using Net-SNMP On Linux

tooj

On Redhat 7.1 or above, NetSnmp has become the default snmp...
But on other linux version. It is still a good guide.
http://www.rpmfind.net
is a good site for finding rpm package for linux.
In this tutorial we will

• download and install net-snmp,
• write and install a simple MIB,
• write a subagent to handle to mib.

Download and install net-snmp package
This package was previously known as ucd-snmp

Download the source from
here
, or if this link is broken try the
net-snmp homepage
, to your local directory which we will now refer to as ${HOME}. For this tutorial we will use net-snmp-5.0-pre2.

change directory to ${HOME}, untar and unzip the package using: $gunzip net-snmp-5.0.pre2.tar.gz
$tar xvf net-snmp-5.0.pre2.tarThis will dump all the souce into ${HOME}/net-snmp-5.0.pre2

To compile to package: $./configure --with-mib-modules="agentx"
$make
$make install
$cd local; make install; cd ..
$cd mibs; make install; cd ..The "configure" command configure the agent to use the AgentX protocol. This is a IETF defined protocol that allows a master/client relationship between agents and subagents. The last two command should not theoretically have to be to used ... but without them .. things do not seem to work. Now, we have to setup the snmpd configuration file, before "snmpd" can work properly.

Copy the example configuration file: $ cp ${HOME}/EXAMPLE.conf /usr/local/share/snmp/snmpd.conf

Now we need to modify /usr/local/share/snmp/snmpd.conf as follows:

Replace COMMUNITY with "democommunity". This is your community string.

Comment out 2nd "com2sec" line. We do not allow network access for now.

On a new line at the end of the file add "master agentx". This tells the agents to behave as the master in the master/client AgentX protocol.

We now need to fix some library links (this is truely awful ... is this a Redhat or a net-snmp "problem"/"feature" ?)(Note: On some machines this is not required e.g RedHat 7.1 ... use your judgement :-)) $ln -s /usr/local/lib/libnetsnmp-0.5.0.0.2.so /lib/libnetsnmp-0.5.0.0.2.so
$ln -s /usr/local/lib/libnetsnmpagent-0.5.0.0.2.so /lib/libnetsnmpagent-0.5.0.0.2.so
$ln -s /usr/local/lib/libnetsnmphelpers-0.5.0.0.2.so /lib/libnetsnmphelpers-0.5.0.0.2.so
$ln -s /usr/local/lib/libnetsnmpmibs-0.5.0.0.2.so /lib/libnetsnmpmibs-0.5.0.0.2.so

To check "snmpd" do:
become root $ ps awwux | grep snmpif you see an earlier snmpd deamon ... kill it $ cd ${HOME}/net-snmp-5.0.pre2/agent
$ ./snmpd -f -LThis should start the "snmpd" agent but keep it attached to the current terminal (which is useful since we want to kill it very soon).

On another window: $snmpget -v 1 -c democommunity localhost system.sysUpTime.0If snmpd was installed correctly, this gives up the timeticks the snmpd agent has been up (NOT how long your system was up !!). If you get an error .. retrace your steps from the beginning.
You can now use "^C" to kill the snmpd deamon in the first window.
Writing and installing a MIBIn this part of the the tutorial we will write and install a simple MIB.

First write the mib that you want to implement. For our tutorial we write a simple MIB called
JM-TEST-1-MIB.txt
: JM-TEST-1-MIB DEFINITIONS ::= BEGIN
IMPORTS
        MODULE-IDENTITY,
        OBJECT-TYPE,
        INTEGER
                FROM SNMPv2-SMI;
jmtest MODULE-IDENTITY
    LAST-UPDATED "200203210000Z"
    ORGANIZATION "Temple U"
    CONTACT-INFO
        "None yet."
    DESCRIPTION
        "AgentX testing MIB"
    REVISION     "200203210000Z"
    DESCRIPTION
        "None yet."
    ::= { experimental 72}
firstKey OBJECT-TYPE
    SYNTAX      INTEGER (0..100)
    MAX-ACCESS  read-write
    STATUS      current
    DESCRIPTION
        "Value initialized to 0 and on each
         access:
         - Return current val.
         - increment"
    ::= { jmtest 1 }
END
This mib represents a resource "firstKey" whose initial value is 0 and whose value gets incremented every time it is queried.
Note that this MIB will be registered under the 1.3.6.1.3.72 heirarchy. This choice is arbitrary and should only be used for experiments. For real world implementation you should get a "real" OID. Existing OIDs and guidelines on getting new ones can be fo und
here
. This page is maintained by the current (March 2002) IETF/IESG chair, so he probably knows what he is talking about.
Copy this mib into the mibs directory
$cp JM-TEST-1-MIB.txt /usr/local/share/snmp/mibs

Now we need to get the SNMP tools to recognise this MIB. So: $echo "mibs +JM-TEST-1-MIB" >> /usr/local/share/snmp/snmp.confThis adds a directive to the snmp tools configuration asking them load our mib.

To verify if our MIB is loaded use the verstile snmptranslate command: $snmptranslate -IR -Tp experimentalThis command will draw the present tree under the experimental branch. Omit the last parameter and you will get the whole tree (as currenly understood by the snmp tools). The output should look like: Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
+--experimental(3)
   |
   +--jmtest(72)
      +-- -RW- INTEGER   firstKey(1)
               Range: 0..100If you get the above output, then we are in good shape so far. The real work, howeve still remains. We must now write the subagent that will handle queries on under this OID branch. Writing and installing a subagentIn this section we will write and install a subagent that serves the mib we installed in the previous section.
The subagent is an independent program that communicates with the master agent (snmpd in our case) using the AgentX protocol.
The basic steps of writing the code is as follows:

Write the agent code in a C file, say example.c. This can be done using the mib2c tool.

Create the subagent executable using the net-snmp-config tool. mib2c is (supposed to) take in the MIB definition as spit out the subagent code. However (as far as I could figure out) the mib2c program distributed with this version of net-snmp does not generate code for simple scalar mibs, instead dealing with mibs th at have tables.
So for an example of subagent code for simple scalar objects look at ${HOME}/agent/mibgroups/example/example.c.
We have adapted example.c for our mib (JM-TEST-1-MIB.txt) as
example2.c
.
Download
example2.c
and
example1.h
$mkdir ${HOME}/agent/mibgroup/examples/subagent
$cp example2.c ${HOME}/agent/mibgroup/examples/subagent
$cp example.h ${HOME}/agent/mibgroup/examples/subagentNow we create the executable using the net-snmp-config tools as: $net-snmp-config --compile-subagent example2 example2.c -I../../../mibgroupThis produces a executable called example2. example2 is our subagent. Voila !
To test whether things are still working.
In first window:
$cd ${HOME}/agent
$./snmpd -f -L -DThis will start the snmpd deamon in the debugging mode (you will see LOTS of messages).
In the second window:
$cd agent/mibgroup/examples/subagent
$./example2Finally in the third window, the command and output should look like (output is shown in bold): [root@x mibs]# snmpget -v 1 -c democommunity localhost firstKey.0
Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
JM-TEST-1-MIB::firstKey.0 = 1
[root@x mibs]# snmpget -v 1 -c democommunity localhost 1.3.6.1.3.72.1.0
Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
JM-TEST-1-MIB::firstKey.0 = 2
[root@x mibs]# snmpset -v 1 -c democommunity localhost 1.3.6.1.3.72.1.0 i 10
Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
JM-TEST-1-MIB::firstKey.0 = 10
[root@x mibs]# snmpget -v 1 -c democommunity localhost firstKey.0
Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
JM-TEST-1-MIB::firstKey.0 = 10
[root@x mibs]# snmpget -v 1 -c democommunity localhost 1.3.6.1.3.72.1.0
Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
JM-TEST-1-MIB::firstKey.0 = 11
[root@x mibs]# snmpget -v 1 -c democommunity localhost 1.3.6.1.3.72.1.0
Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
JM-TEST-1-MIB::firstKey.0 = 12
[root@x mibs]# snmpwalk -v 1 -c democommunity localhost firstKey        
Unlinked OID in JM-TEST-1-MIB: jmtest ::= { experimental 72 }
JM-TEST-1-MIB::firstKey.0 = 13In the output above notice that every snmpget query returns an increasing value of "firstKey" and snmpset lets us set "firstKey" to an arbitrary value (within a defined range).
We now have a working subagent. In real applications the subagent could be embedded in the application to be managed.
--by
Jaiwant Mulik
, March 2002.
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