[COMMON OPTIONS] [-Ca] [-Cx]
OLD-PASSPHRASE NEW-PASSPHRASE [USER]
[COMMON OPTIONS] <-Ca | -Cx> -Ck
OLD-KEY-OR-PASSPHRASE NEW-KEY-OR-PASSPHRASE [USER]
[COMMON OPTIONS] [-Ca] [-Cx]
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.
Common options for all
Options for the
An unauthenticated SNMPv3 user can be created using the command
[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
[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
[OPTIONS] create USER CLONEFROM-USER
The two forms of the
sub-command require that the user being created does not already exist.
sub-command requires that the user being cloned to
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.
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.
manual page for details of the
A user can be deleted from the usmUserTable using the command
[OPTIONS] delete USER
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
[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
[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
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
sub-command is used:
[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
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
# lets add the new user we’ll create too:
# 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
snmpusm -v3 -u initial -n "" -l authNoPriv -a MD5 -A setup_passphrase
localhost create wes initial
snmpusm -v 3 -u wes -n "" -l authNoPriv -a MD5 -A setup_passphrase
localhost passwd setup_passphrase new_passphrase
snmpget -v 3 -u wes -n "" -l authNoPriv -a MD5 -A new_passphrase
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.
Manipulating the usmUserTable using this command can
be done using SNMPv3.
This command will not work with the community-based versions,
even if they have write access to the table.