Net-SNMP

Net-SNMP Tutorial -- 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 its 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 (and the only one we currently support). 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 Version-based Access Control Module and controls which users (and SNMPv1/v2c communities as well) are allowed to access and how they can access 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 the 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 users 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.

Authentication is performed by using a users privKey to encrypt the data portion the message being sent. The privProtocol can be either AES or DES.

Messages can be be sent unauthenticated, authenticated, or authenticated and encrypted 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, none the less):


% 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"

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Last modified: Wednesday, 01-Aug-2018 04:41:28 UTC
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