snmpusm - Online in the Cloud

PROGRAM:

NAME

snmpusm - creates and maintains SNMPv3 users on a network entity

SYNOPSIS

snmpusm [COMMON OPTIONS] [-Cw] AGENT create USER [CLONEFROM-USER]
snmpusm [COMMON OPTIONS] AGENT delete USER
snmpusm [COMMON OPTIONS] AGENT cloneFrom USER CLONEFROM-USER
snmpusm [COMMON OPTIONS] [-Ca] [-Cx] AGENT passwd OLD-PASSPHRASE NEW-PASSPHRASE [USER]
snmpusm [COMMON OPTIONS] <-Ca | -Cx> -Ck AGENT passwd OLD-KEY-OR-PASSPHRASE NEW-KEY-OR-
PASSPHRASE [USER]
snmpusm [COMMON OPTIONS] [-Ca] [-Cx] AGENT changekey [USER]

DESCRIPTION

snmpusm is an SNMP application that can be used to do simple maintenance on the users
known to an SNMP agent, by manipulating the agent's User-based Security Module (USM)
table. The user needs write access to the usmUserTable MIB table. This tool can be used
to create, delete, clone, and change the passphrase of users configured on a running SNMP
agent.

OPTIONS

Common options for all snmpusm commands:

-CE ENGINE-ID
Set usmUserEngineID to be used as part of the index of the usmUserTable. Default
is to use the contextEngineID (set via -E or probed) as the usmUserEngineID.

-Cp STRING
Set the usmUserPublic value of the (new) user to the specified STRING.

Options for the passwd and changekey commands:

-Ca Change the authentication key.

-Cx Change the privacy key.

-Ck Allows one to use localized key (must start with 0x) instead of passphrase. When
this option is used, either the -Ca or -Cx option (but not both) must also be used.

CREATING USERS

An unauthenticated SNMPv3 user can be created using the command

snmpusm [COMMON OPTIONS] AGENT create USER

This constructs an (inactive) entry in the usmUserTable, with no authentication or privacy
settings. In principle, this user should be useable for 'noAuthNoPriv' requests, but in
practise the Net-SNMP agent will not allow such an entry to be made active. The user can
be created via the createAndWait operation instead by using the -Ca flag. This will
prevent the user from being marked as active in any agent until explicitly activated later
via the activate command.

In order to activate this entry, it is necessary to "clone" an existing user, using the
command

snmpusm [COMMON OPTIONS] AGENT cloneFrom USER CLONEFROM-USER

The USER entry then inherits the same authentication and privacy settings (including pass
phrases) as the CLONEFROM user.

These two steps can be combined into one, by using the command

snmpusm [COMMON OPTIONS] AGENT create USER CLONEFROM-USER

The two forms of the create sub-command require that the user being created does not
already exist. The cloneFrom sub-command requires that the user being cloned to does
already exist.

Cloning is the only way to specify which authentication and privacy protocols to use for a
given user, and it is only possible to do this once. Subsequent attempts to reclone onto
the same user will appear to succeed, but will be silently ignored. This (somewhat
unexpected) behaviour is mandated by the SNMPv3 USM specifications (RFC 3414). To change
the authentication and privacy settings for a given user, it is necessary to delete and
recreate the user entry. This is not necessary for simply changing the pass phrases (see
below). This means that the agent must be initialized with at least one user for each
combination of authentication and privacy protocols. See the snmpd.conf(5) manual page
for details of the createUser configuration directive.

DELETING USERS

A user can be deleted from the usmUserTable using the command

snmpusm [COMMON OPTIONS] AGENT delete USER

CHANGING PASS PHRASES

User profiles contain private keys that are never transmitted over the wire in clear text
(regardless of whether the administration requests are encrypted or not). To change the
secret key for a user, it is necessary to specify the user's old passphrase as well as the
new one. This uses the command

snmpusm [COMMON OPTIONS] [-Ca] [-Cx] AGENT passwd OLD-PASSPHRASE NEW-PASSPHRASE
[USER]

After cloning a new user entry from the appropriate template, you should immediately
change the new user's passphrase.

If USER is not specified, this command will change the passphrase of the (SNMPv3) user
issuing the command. If the -Ca or -Cx options are specified, then only the
authentication or privacy keys are changed. If these options are not specified, then both
the authentication and privacy keys are changed.

snmpusm [COMMON OPTIONS] [-Ca] [-Cx] AGENT changekey [USER]

This command changes the key in a perfect-forward-secrecy compliant way through a diffie-
helman exchange. The remote agent must support the SNMP-USM-DH-OBJECTS-MIB for this
command to work. The resulting keys are printed to the console and may be then set in
future command invocations using the --defAuthLocalizedKey and --defPrivLocalizedKey
options or in your snmp.conf file using the defAuthLocalizedKey and defPrivLocalizedKey
keywords.

Note that since these keys are randomly generated based on a diffie helman exchange, they
are no longer derived from a more easily typed password. They are, however, much more
secure.

To change from a localized key back to a password, the following variant of the passwd
sub-command is used:

snmpusm [COMMON OPTIONS] <-Ca | -Cx> -Ck AGENT passwd OLD-KEY-OR-PASSPHRASE NEW-
KEY-OR-PASSPHRASE [USER]

Either the -Ca or the -Cx option must be specified. The OLD-KEY-OR-PASSPHRASE and/or NEW-
KEY-OR-PASSPHRASE arguments can either be a passphrase or a localized key starting with
"0x", e.g. as printed out by the changekey sub-command.

Note that snmpusm REQUIRES an argument specifying the agent to query as described in the
.I snmpcmd(1) manual page.

EXAMPLES

Let's assume for our examples that the following VACM and USM configurations lines were in
the snmpd.conf file for a Net-SNMP agent. These lines set up a default user called
"initial" with the authentication passphrase "setup_passphrase" so that we can perform the
initial setup of an agent:

# VACM configuration entries
rwuser initial
# lets add the new user we'll create too:
rwuser wes
# USM configuration entries
createUser initial MD5 setup_passphrase DES

Note: the "initial" user's setup should be removed after creating a real user that you
grant administrative privileges to (like the user "wes" we'll be creating in this example.

Note: passphrases must be 8 characters minimum in length.

Create a new user
snmpusm -v3 -u initial -n "" -l authNoPriv -a MD5 -A setup_passphrase localhost create wes
initial

Creates a new user, here named "wes" using the user "initial" to do it. "wes" is
cloned from "initial" in the process, so he inherits that user's passphrase
("setup_passphrase").

Change the user's passphrase
snmpusm -v 3 -u wes -n "" -l authNoPriv -a MD5 -A setup_passphrase localhost passwd
setup_passphrase new_passphrase

After creating the user "wes" with the same passphrase as the "initial" user, we
need to change his passphrase for him. The above command changes it from
"setup_passphrase", which was inherited from the initial user, to "new_passphrase".

Test the new user
snmpget -v 3 -u wes -n "" -l authNoPriv -a MD5 -A new_passphrase localhost sysUpTime.0

If the above commands were successful, this command should have properly performed
an authenticated SNMPv3 GET request to the agent.

Now, go remove the vacm "group" snmpd.conf entry for the "initial" user and you have a
valid user 'wes' that you can use for future transactions instead of initial.

WARNING

Manipulating the usmUserTable using this command can only be done using SNMPv3. This
command will not work with the community-based versions, even if they have write access to
the table.

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