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Section: Net-SNMP (5)
Updated: 05 Dec 2005
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snmpd.examples – example configuration for the Net-SNMP agent



man page defines the syntax and behaviour of the various
configuration directives that can be used to control the
operation of the Net-SNMP agent, and the management information
it provides.

This companion man page illustrates these directives, showing
some practical examples of how they might be used.



Listening addresses

The default agent behaviour (listing on the standard SNMP UDP port on
all interfaces) is equivalent to the directive:

agentaddress udp:161

or simply

agentaddress 161

The agent can be configured to only accept requests sent to the
local loopback interface (again listening on the SNMP UDP port), using:

agentaddress localhost:161 # (udp implicit)


agentaddress # (udp and standard port implicit)

It can be configured to accept both UDP and TCP requests (over both IPv4
and IPv6), using:

agentaddress udp:161,tcp:161,udp6:161,tcp6:161

Other combinations are also valid.

Run-time privileges

The agent can be configured to relinquish any privileged access once it
has opened the initial listening ports. Given a suitable "snmp" group
(defined in /etc/group), this could be done using the directives:

agentuser nobody
agentgroup snmp

A similar effect could be achieved using numeric UID and/or GID values:

agentuser #10
agentgroup #10


SNMPv3 Configuration

Rather than being generated pseudo-randomly,
the engine ID for the agent could be calculated based on the MAC address
of the second network interface (eth1), using the directives:

engineIDType 3
engineIDNic eth1

or it could be calculated from the (first) IP address, using:

engineIDType 1

or it could be specified explicitly, using:





SNMPv3 Users

The following directives will create three users, all using exactly
the same authentication and encryption settings:

createUser me MD5 "single pass phrase"
createUser myself MD5 "single pass phrase" DES
createUser andI MD5 "single pass phrase" DES "single pass phrase"

Note that this defines three distinct users, who could be granted
different levels of access. Changing the passphrase for any one of
these would not affect the other two.

Separate pass phrases can be specified for authentication and

createUser onering SHA "to rule them all" AES "to bind them"

Remember that these createUser directives should be defined in the
/var/lib/net-snmp/snmpd.conf file, rather than the usual location.


Traditional Access Control

The SNMPv3 users defined above can be granted access to the full
MIB tree using the directives:

rouser me
rwuser onering

Or selective access to individual subtrees using:

rouser myself .
rwuser andI system

Note that a combination repeating the same user, such as:

rouser onering
rwuser onering

should not be used. This would configure the user onering
with read-only access (and ignore the rwuser entry altogether).
The same holds for the community-based directives.

The directives:

rocommunity public
rwcommunity private

would define the commonly-expected read and write community strings
for SNMPv1 and SNMPv2c requests. This behaviour is not
configured by default, and would need to be set up explicitly.


It would also be a very good idea to change private to something
a little less predictable!

A slightly less vulnerable configuration might restrict what information
could be retrieved:

rocommunity public default system

or the management systems that settings could be manipulated from:

rwcommunity private

or a combination of the two.

VACM Configuration

This last pair of settings are equivalent to the full VACM definitions:

# source community
com2sec public default public
com2sec mynet private
com2sec6 mynet fec0::/64 private

# sec.model
group worldGroup v1 public
group worldGroup v2c public
group myGroup v1 mynet
group myGroup v2c mynet

# incl/excl subtree [mask]
view all included .1
view sysView included system

# context model level prefix read write notify (unused)
access worldGroup "" any noauth exact system none none
access myGroup "" any noauth exact all all none

There are several points to note in this example:

The group directives must be repeated for
both SNMPv1 and SNMPv2c requests.

The com2sec security name is distinct from the community
string that is mapped to it. They can be the same ("public")
or different ("mynet"/"private") – but what appears in the
group directive is the security name, regardless of
the original community string.

Both of the view directives are defining simple OID
subtrees, so neither of these require an explicit mask.
The same holds for the "combined subtree2 view defined below.
In fact, a mask field is only needed when defining row slices
across a table (or similar views), and can almost always be omitted.

In general, it is advisible not to mix traditional and VACM-based
access configuration settings, as these can sometimes interfere
with each other in unexpected ways. Choose a particular style
of access configuration, and stick to it.


Typed-View Configuration

A similar configuration could also be configured as follows:

view sys2View included system
view sys2View included .

authcommunity read public default -v sys2View
authcommunity read,write private

This mechanism allows multi-subtree (or other non-simple) views to
be used with the one-line rocommunity style of configuration.

It would also support configuring "write-only" access, should this
be required.




System Group

The full contents of the ‘system’ group (with the exception of sysUpTime)
can be explicitly configured using:

# Override ‘uname -a’ and hardcoded system OID – inherently read-only values
sysDescr Universal Turing Machine mk I
sysObjectID .

# Override default values from ‘configure’ – makes these objects read-only
sysContact [email protected]
sysLocation An idea in the mind of AT

# Standard end-host behaviour
sysServices 72


Host Resources Group

The list of devices probed for potential inclusion in the
hrDiskStorageTable (and hrDeviceTable) can be amended using
any of the following directives:

ignoredisk /dev/rdsk/c0t2d0

which prevents the device /dev/rdsk/c0t2d0 from being scanned,

ignoredisk /dev/rdsk/c0t[!6]d0
ignoredisk /dev/rdsk/c0t[0-57-9a-f]d0

either of which prevents all devices /dev/rdsk/c0tXd0
(except …/c0t6d0) from being scanned,

ignoredisk /dev/rdsk/c1*

which prevents all devices whose device names start with /dev/rdsk/c1
from being scanned, or

ignoredisk /dev/rdsk/c?t0d0

which prevents all devices /dev/rdsk/cXt0d0
(where ‘X’ is any single character) from being scanned.

Process Monitoring

The list of services running on a system can be monitored
(and provision made for correcting any problems), using:

# At least one web server process must be running at all times
proc httpd
procfix httpd /etc/rc.d/init.d/httpd restart

# There should never be more than 10 mail processes running
# (more implies a probable mail storm, so shut down the mail system)

proc sendmail 10
procfix sendmail /etc/rc.d/init.d/sendmail stop

# There should be a single network management agent running
# ("There can be only one")

proc snmpd 1 1

Also see the "DisMan Event MIB" section later on.

Disk Usage Monitoring

The state of disk storage can be monitored using:

includeAllDisks 10%
disk /var 20%
disk /usr 3%
# Keep 100 MB free for crash dumps
disk /mnt/crash 100000


System Load Monitoring

A simple check for an overloaded system might be:

load 10

A more refined check (to allow brief periods of heavy use,
but recognise sustained medium-heavy load) might be:

load 30 10 5


Log File Monitoring







Notification Handling

Configuring the agent to report invalid access attempts might be done by:

authtrapenable 1
trapcommunity public
trap2sink localhost

Alternatively, the second and third directives could be combined
(and an acknowledgement requested) using:

informsink localhost public

A configuration with repeated sink destinations, such as:

trapsink localhost
trap2sink localhost
informsink localhost

should NOT be used, as this will cause multiple copies
of each trap to be sent to the same trap receiver.

TODO – discuss SNMPv3 traps

trapsess snmpv3 options localhost:162

TODO – mention trapd access configuration


DisMan Event MIB

The simplest configuration for active self-monitoring of
the agent, by the agent, for the agent, is probably:

# Set up the credentials to retrieve monitored values
createUser _internal MD5 "the first sign of madness"
iquerySecName _internal
rouser _internal

# Active the standard monitoring entries
defaultMonitors yes
linkUpDownNotifications yes

# If there’s a problem, then tell someone!
trap2sink localhost

The first block sets up a suitable user for retrieving the
information to by monitored, while the following pair of
directives activates various built-in monitoring entries.

Note that the DisMan directives are not themselves sufficient to
actively report problems – there also needs to be a suitable
destination configured to actually send the resulting notifications to.

A more detailed monitor example is given by:

monitor -u me -o hrSWRunName "high process memory" hrSWRunPerfMem > 10000

This defines an explicit boolean monitor entry, looking for any process
using more than 10MB of active memory. Such processes will be reported
using the (standard) DisMan trap mteTriggerFired,
but adding an extra (wildcarded) varbind hrSWRunName.

This entry also specifies an explicit user (me, as defined
earlier) for retrieving the monitored values, and building the trap.

Objects that could potentially fluctuate around the specified level
are better monitored using a threshold monitor entry:

monitor -D -r 10 "network traffic" ifInOctets 1000000 5000000

This will send a mteTriggerRising trap whenever the incoming
traffic rises above (roughly) 500 kB/s on any network interface,
and a corresponding mteTriggerFalling trap when it falls below
100 kB/s again.

Note that this monitors the deltas between successive samples (-D)
rather than the actual sample values themselves. The same effect
could be obtained using:

monitor -r 10 "network traffic" ifInOctets – – 1000000 5000000

The linkUpDownNotifications directive above is broadly
equivalent to:

notificationEvent linkUpTrap linkUp ifIndex ifAdminStatus ifOperStatus
notificationEvent linkDownTrap linkDown ifIndex ifAdminStatus ifOperStatus

monitor -r 60 -e linkUpTrap "Generate linkUp" ifOperStatus != 2
monitor -r 60 -e linkDownTrap "Generate linkDown" ifOperStatus == 2

This defines the traps to be sent (using notificationEvent),
and explicitly references the relevant notification in the corresponding
monitor entry (rather than using the default DisMan traps).

The defaultMonitors directive above is equivalent to a series
of (boolean) monitor entries:

monitor -o prNames -o prErrMessage "procTable" prErrorFlag != 0
monitor -o memErrorName -o memSwapErrorMsg "memory" memSwapError != 0
monitor -o extNames -o extOutput "extTable" extResult != 0
monitor -o dskPath -o dskErrorMsg "dskTable" dskErrorFlag != 0
monitor -o laNames -o laErrMessage "laTable" laErrorFlag != 0
monitor -o fileName -o fileErrorMsg "fileTable" fileErrorFlag != 0

and will send a trap whenever any of these entries indicate a problem.

An alternative approach would be to automatically invoke the corresponding
"fix" action:

setEvent prFixIt prErrFix = 1
monitor -e prFixIt "procTable" prErrorFlag != 0

(and similarly for any of the other defaultMonitor entries).

DisMan Schedule MIB

The agent could be configured to reload its configuration
once an hour, using:

repeat 3600 versionUpdateConfig.0 = 1

Alternatively this could be configured to be run at specific
times of day (perhaps following rotation of the logs):

cron 10 0 * * * versionUpdateConfig.0 = 1

The one-shot style of scheduling is rather less common, but the
secret SNMP virus could be activated on the next occurance of Friday 13th using:

at 13 13 13 * 5 snmpVirus.0 = 1




Arbitrary Extension Commands

Old Style


New Style



MIB-Specific Extension Commands


"pass [-p priority] MIBOID PROG"


"pass_persist [-p priority] MIBOID PROG"


Embedded Perl Support

If embedded perl support is enabled in the agent, the default
initialisation is equivalent to the directives:

disablePerl false
perlInitFile /usr/share/snmp/

The main mechanism for defining embedded perl scripts is the
perl directive. A very simple (if somewhat pointless)
MIB handler could be registered using:

perl use Data::Dumper;
perl sub myroutine { print "got called: ",Dumper(@_),"n"; }
perl $agent->register(‘mylink’, ‘.′, &myroutine);

This relies on the $agent object, defined in the example file.

A more realistic MIB handler might be:

XXX – WHAT ???

Alternatively, this code could be stored in an external file,
and loaded using:

perl ‘do /usr/share/snmp/';


Dynamically Loadable Modules


dlmod NAME PATH"


Proxy Support

A configuration for acting as a simple proxy for two other
SNMP agents (running on remote systems) might be:

com2sec -Cn rem1context rem1user default remotehost1
com2sec -Cn rem2context rem2user default remotehost2

proxy -Cn rem1context -v 1 -c public remotehost1 .1.3
proxy -Cn rem2context -v 1 -c public remotehost2 .1.3

(plus suitable access control entries).

The same proxy directives would also work with
(incoming) SNMPv3 requests, which can specify a context directly.
It would probably be more sensible to use contexts of
remotehost1 and remotehost2 – the names above were
chosen to indicate how these directives work together.

Note that the administrative settings for the proxied request
are specified explicitly, and are independent of the settings
from the incoming request.

An alternative use for the proxy directive is to pass
part of the OID tree to another agent (either on a remote host
or listening on a different port on the same system),
while handling the rest internally:

proxy -v 1 -c public localhost:6161 .

This mechanism can be used to link together two separate SNMP agents.

A less usual approach is to map one subtree into a different area
of the overall MIB tree (either locally or on a remote system):

# uses SNMPv3 to access the MIB tree . on ‘remotehost’
# and maps this to the local tree .

proxy -v 3 -l noAuthNoPriv -u user remotehost . .


SMUX Sub-Agents

smuxpeer . ospf_pass


AgentX Sub-Agents

The Net-SNMP agent could be configured to operate as an AgentX master
agent (listening on a non-standard named socket, and running using
the access privileges defined earlier), using:

master agentx
agentXSocket /tmp/agentx/master
agentXPerms 0660 0550 nobody snmp

A sub-agent wishing to connect to this master agent would need
the same agentXSocket directive, or the equivalent code:


A loopback networked AgentX configuration could be set up using:

agentXSocket tcp:localhost:705
agentXTimeout 5
agentXRetries 2

on the master side, and:

agentXSocket tcp:localhost:705
agentXTimeout 10
agentXRetries 1
agentXPingInterval 600

on the client.

Note that the timeout and retry settings can be asymmetric
for the two directions, and the sub-agent can poll the master agent
at regular intervals (600s = every 10 minutes), to ensure the
connection is still working.


override sysDescr.0 octet_str "my own sysDescr"

injectHandler stash_cache NAME table_iterator





snmpconf(1), snmpd.conf(5), snmp.conf(5), snmp_config(5), snmpd(8), EXAMPLE.conf, read_config(3).






Listening addresses

Run-time privileges

SNMPv3 Configuration


SNMPv3 Users

Traditional Access Control

VACM Configuration

Typed-View Configuration


System Group

Host Resources Group

Process Monitoring

Disk Usage Monitoring

System Load Monitoring

Log File Monitoring


Notification Handling

DisMan Event MIB

DisMan Schedule MIB


Arbitrary Extension Commands

MIB-Specific Extension Commands

Embedded Perl Support

Dynamically Loadable Modules

Proxy Support

SMUX Sub-Agents

AgentX Sub-Agents




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