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SNMPUSM


Section: Net-SNMP (1)
Updated: 22 Oct 2005
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NAME

snmpusm – creates and maintains SNMPv3 users on a network entity
 

SYNOPSIS

snmpusm

[COMMON OPTIONS]
create

USER [CLONEFROM-USER]

snmpusm

[COMMON OPTIONS]
delete

USER

snmpusm

[COMMON OPTIONS]
cloneFrom

USER CLONEFROM-USER

snmpusm

[COMMON OPTIONS] [-Ca] [-Cx]
passwd

OLD-PASSPHRASE NEW-PASSPHRASE [USER]

snmpusm

[COMMON OPTIONS] <-Ca | -Cx> -Ck
passwd

OLD-KEY-OR-PASSPHRASE NEW-KEY-OR-PASSPHRASE [USER]

snmpusm

[COMMON OPTIONS] [-Ca] [-Cx]
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 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

[OPTIONS] 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.


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



snmpusm

[OPTIONS] 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

[OPTIONS] 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

[OPTIONS] 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

[OPTIONS] [-Ca] [-Cx] 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

[OPTIONS] [-Ca] [-Cx] 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

[OPTIONS] <-Ca | -Cx> -Ck 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.


 

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.


 

SEE ALSO

snmpd.conf(5), snmp.conf(5), RFC 3414



 

Index



NAME

SYNOPSIS

DESCRIPTION

OPTIONS

CREATING USERS

DELETING USERS

CHANGING PASS PHRASES

EXAMPLES


Create a new user

Change the user’s passphrase

Test the new user


WARNING

SEE ALSO



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