TUT:SNMPv3 Options

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SNMPv3 Options

Introduction

The 3rd version of the SNMP protocol introduced a whole slew of new security related features that have been missing from the previous versions. In SNMPv1 and SNMPv2c, a simple community string was put in clear text into the packet to authenticate the request. This is obviously highly insecure. (If it's not obvious, then just trust me).

SNMPv3 introduces advanced security which splits the authentication and the authorization into two pieces:

The USM is the default Security Module for SNMPv3. The U stands for User-based, as it is contains a list of users and their attributes. The USM is described by RFC 2574.

The VACM is the default Access Control Module and determines which users (and SNMPv1/v2c communities) are allowed to access MIB information. The V stands for View-based, and allows different levels of access for different sections of the MIB tree. The VACM is described by RFC 2575.

This document will discuss how to use the net-snmp tools to get and set data from a remote host.

Users

A user's profile contains the following data: 

 % snmptranslate -Tp -IR usmUserTable
 +--usmUserTable(2)
    |
    +--usmUserEntry(1)
       |
       +-- ---- String    usmUserEngineID(1)
       |        Textual Convention: SnmpEngineID
       |        Size: 5..32
       +-- ---- String    usmUserName(2)
       |        Textual Convention: SnmpAdminString
       |        Size: 1..32
       +-- -R-- String    usmUserSecurityName(3)
       |        Textual Convention: SnmpAdminString
       |        Size: 0..255
       +-- CR-- ObjID     usmUserCloneFrom(4)
       |        Textual Convention: RowPointer
       +-- CR-- ObjID     usmUserAuthProtocol(5)
       |        Textual Convention: AutonomousType
       +-- CR-- String    usmUserAuthKeyChange(6)
       |        Textual Convention: KeyChange
       +-- CR-- String    usmUserOwnAuthKeyChange(7)
       |        Textual Convention: KeyChange
       +-- CR-- ObjID     usmUserPrivProtocol(8)
       |        Textual Convention: AutonomousType
       +-- CR-- String    usmUserPrivKeyChange(9)
       |        Textual Convention: KeyChange
       +-- CR-- String    usmUserOwnPrivKeyChange(10)
       |        Textual Convention: KeyChange
       +-- CR-- String    usmUserPublic(11)
       |        Size: 0..32
       +-- CR-- EnumVal   usmUserStorageType(12)
       |        Textual Convention: StorageType
       |        Values: other(1), volatile(2), nonVolatile(3), permanent(4), readOnly(5)
       +-- CR-- EnumVal   usmUserStatus(13)
                Textual Convention: RowStatus
                Values: active(1), notInService(2), notReady(3), createAndGo(4), createAndWait(5), destroy(6)


Well, thats nice but what does it mean?

To summarize, most importantly each user has a name (called a securityName) an authentication type (authProtocol) and a privacy type (privProtocol) as well as associated keys for each of these (authKey and privKey).

Authentication is performed by using a user's authKey to sign the message being sent. The authProtocol can be either MD5 or SHA at this time. authKeys (and privKeys) are generated from a passphrase that must be at least 8 characters in length.

Encryption is performed by using a user's privKey to encrypt the data portion the message being sent. The privProtocol can be either AES or DES.

Messages can be be sent unauthenticated and unencrypted (noAuthNoPriv), authenticated but unencrypted (authNoPriv), or authenticated and encrypted (authPriv) by setting the securityLevel to use.

All of this information is passed to commands using the command line arguments described in the table below. Additionally, you can put default values in your ~/.snmp/snmp.conf files using the tokens specified in the 3rd column.

Parameter Command Line Flag snmp.conf token
securityName -u NAME defSecurityName NAME
authProtocol -a (MD5|SHA) defAuthType (MD5|SHA)
privProtocol -x (AES|DES) defPrivType DES
authKey -A PASSPHRASE defAuthPassphrase PASSPHRASE
privKey -X PASSPHRASE defPrivPassphrase PASSPHRASE
securityLevel -l (noAuthNoPriv|authNoPriv|authPriv) defSecurityLevel (noAuthNoPriv|authNoPriv|authPriv)
context -n CONTEXTNAME defContext CONTEXTNAME

Examples

Here is a completely unauthenticated request (which still needs a user name, nonetheless): 

 % snmpgetnext -v 3 -n "" -u noAuthUser -l noAuthNoPriv test.net-snmp.org sysUpTime
 system.sysUpTime.0 = Timeticks: (83467131) 9 days, 15:51:11.31


Here is a authenticated request: 

 % snmpgetnext -v 3 -n "" -u MD5User -a MD5 -A "The Net-SNMP Demo Password" -l authNoPriv test.net-snmp.org sysUpTime
 system.sysUpTime.0 = Timeticks: (83491735) 9 days, 15:55:17.35


And finally, here is an authenticated and encrypted request: 

 % snmpgetnext -v 3 -n "" -u MD5DESUser -a MD5 -A "The Net-SNMP Demo Password" -x DES -X "The Net-SNMP Demo Password" -l authPriv test.net-snmp.org system
 system.sysUpTime.0 = Timeticks: (83493111) 9 days, 15:55:31.11

Of course, they don't look much different since they all worked identically. But, the host above allows us to look at it using any level of authentication. Any hosts you set up should be more restricted than that and require at least a level of authNoPriv when you configure the VACM access control.

Finally, consider a snmp.conf file that looks like this: 

 defContext none
 defSecurityName MD5User
 defAuthPassphrase The Net-SNMP Demo Password
 defVersion 3
 defAuthType MD5
 defSecurityLevel authNoPriv

This sets up the defaults for you so that your snmp commands can boil down to something as simple as: 

 % snmpgetnext test.net-snmp.org sysUpTime
 system.sysUpTime.3.0 = Timeticks: (83517052) 9 days, 15:59:30.52

Or: 

 % snmpset test.net-snmp.org ucdDemoPublicString.0 s "I changed  something"
 enterprises.ucdavis.ucdDemoMIB.ucdDemoMIBObjects.ucdDemoPublic.ucdDemoPublicString.2.0 = "I changed something"

Then: 

 % snmpget test.net-snmp.org ucdDemoPublicString.0
 enterprises.ucdavis.ucdDemoMIB.ucdDemoMIBObjects.ucdDemoPublic.ucdDemoPublicString.2.0 = "I changed something"

Tutorial Sections

About the SNMP Protocol

These tutorial links talk about SNMP generically and how the protocol itself works. They are good introductory reading material and the concepts are important to understand before diving into the later tutorials about Net-SNMP itself.

Net-SNMP Command Line Applications

These tutorial pages discuss the command line tools provided in the Net-SNMP suite of tools. Nearly all the example commands in these tutorials works if you try it yourself, as they're all examples that talk to our online Net-SNMP test agent. Given them a shot!

Application Configuration

All of our applications support configuration to allow you to customize how they behave.

Net-SNMP Daemons

Net-SNMP comes with two long-running daemons: a SNMP agent (snmpd) for responding to management requests and a notification receiver (snmptrapd) for receiving SNMP notifications.

Coding Tutorials

Net-SNMP comes with a highly flexible and extensible API. The API allows you to create your own commands, add extensions to the agent to support your own MIBs and perform specialized processing of notifications.

Debugging SNMP Applications and Agents

All our tools and applications have extensive debugging output. These tutorials talk about how the debugging system works and how you can add your own debugging statements to you code:

Operating System Specific Tutorials

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