Where USM Security Information Is Contained

In an SNMPv3 packet string, USM information is contained in the following flags:

msgFlags

A single octet to indicate how the message is to be processed. For example, two bits of the msgFlags octet specify whether the packet has been encrypted and whether the packet has been authenticated. This flag is used to determine the security level of the message. Security levels, which are indicated in the main snmpd.conf file, are as follows:

noAuthNoPriv

Represented by an integer: 1.

Least access.

authNoPriv

Represented by an integer: 2.

More access than noAuthNoPriv but lower than authPriv.

authPriv

Represented by an integer: 3.

Most access, most secure.

msgSecurityModel

Specifies the security model used to generate the message, enabling the receiving entity to employ the appropriate model for security processing. In the System Management Agent, USM is the only supported security model.

msgSecurityParameters

An octet string containing data about the security model. This data is defined by the security model or models you are using. This data is used only by the security model or models you are using. The security model or models are specified in msgSecurityModel. USM uses this field to authenticate, encrypt, and decrypt SNMPv3 messages.

scopedPDU

Contains the normal Protocol Data Unit (PDU) and information for identifying the administratively unique context for processing the PDU. SNMPv2 and SNMPv3 messages both use the same PDU format. This scopedPDU format is encrypted if privacy was enabled for the transaction.

The MIB definitions for USM can be found at /etc/sma/snmp/mibs/SNMP-USER-BASED-SM-MIB.txt.

For more information about USM, see RFC 3414 at http://www.ietf.org/rfc/rfc3414.txt.