Snmpd Conf Example Simple Essay


Section: Net-SNMP (5)
Updated: 05 Dec 2005
IndexReturn to Main Contents


snmpd.examples - example configuration for the Net-SNMP agent  


The snmpd.conf(5) 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 encryption:

createUser onering SHA "to rule them all" AES "to bind them"
Remember that these createUser directives should be defined in the /var/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 sysName 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/local/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/local/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

smuxsocket 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:
netsnmp_ds_set_string(NETSNMP_DS_APPLICATION_ID, NETSNMP_DS_AGENT_X_SOCKET, "/tmp/agentx/master");

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






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

This document was created by man2html, using the manual pages.
Time: 19:05:39 GMT, September 28, 2009

Net-SNMP Tutorial -- snmpd

This is a basic tutorial for configuring the Net-SNMP snmpd daemon. Note the word basic. This tutorial covers the use of snmpconf to create a configuration file (snmpd.conf) for snmpd. Note that snmpconf only understands a subset of the tokens that snmpd offers, so it is possible that some tokens will remain undocumented. Consult the snmpd.conf man page for more information on those tokens.

The following sections are available:



Location, location, location

One of the most important things you need to know is where to find or create the snmpd.conf configuration file. The best way to find out is by using the debug output of the agent:
Now that you know what the search path is, you should check for any existing configuration files. Again, the best way is to ask snmpd:

If no existing file exists, snmpconf will help create one for you.



Understanding an existing configuration file

If you discover an existing configuration file, open it with your favorite text editor. If it appears well commented, then you can use the snmpd.conf man page as a reference to understand the existing configuration. If it does not appear to be well commented, you can use snmpconf to comment it for you, like so:

This command will create a copy of the existing configuration file FILENAME as snmpd.conf in the current working directory. The copy will have comments documenting the tokens that snmpconf understands.



Basic setup with snmpconf

The Net-SNMP agent comes with a perl script to aid with configuration. This script, snmpconf, helps configure some of the basic configuration options offered by snmpd. The options are grouped together by functionality. The -G option to snmpconf will report the available groups.

$ $ snmpconf -r none -g basic_setup ************************************************ *** Beginning basic system information setup *** ************************************************ Do you want to configure the information returned in the system MIB group (contact info, etc)? (default = y):[RETURN] Notes:
This step is optional, but recommended.
Configuring: syslocation Description: The [typically physical] location of the system. Note that setting this value here means that when trying to perform an snmp SET operation to the sysLocation.0 variable will make the agent return the "notWritable" error code. IE, including this token in the snmpd.conf file will disable write access to the variable. arguments: location_string The location of the system: telephone closet, 3rd floor Finished Output: syslocation telephone closet, 3rd floor Notes:
This one is pretty self-explanatory.
Configuring: syscontact Description: The contact information for the administrator Note that setting this value here means that when trying to perform an snmp SET operation to the sysContact.0 variable will make the agent return the "notWritable" error code. IE, including this token in the snmpd.conf file will disable write access to the variable. arguments: contact_string The contact information: bill x704 Finished Output: syscontact bill x704 Notes:
The conctact information can be just about whatever you want: a phone number, extension, mail box or email address.
Do you want to properly set the value of the sysServices.0 OID (if you don't know, just say no)? (default = y): n Notes:
Unless you already know what the sysServices OID does, it is probably best to skip it and use the default.
Do you want to allow SNMPv3 read-write user based access (default = y):[RETURN]
Configuring: rwuser Description: a SNMPv3 read-write user arguments: user [noauth|auth|priv] [restriction_oid] The SNMPv3 user that should have read-write access: fred The minimum security level required for that user [noauth|auth|priv, default = auth]: auth The OID that this community should be restricted to [if appropriate]:[RETURN] Finished Output: rwuser fred ? Do another rwuser line? (default = y):n Notes:
fred can read or write any object that the agent supports
Do you want to allow SNMPv3 read-only user based access (default = y): Configuring: rouser Description: a SNMPv3 read-only user arguments: user [noauth|auth|priv] [restriction_oid] Enter the SNMPv3 user that should have read-only access to the system: sally The minimum security level required for that user [noauth|auth|priv, default = auth]: auth The OID that this community should be restricted to [if appropriate]: . Finished Output: rouser sally auth . Do another rouser line? (default = y): n Notes:
sally can only read objects in the netSnmp branch of the agent
Do you want to allow SNMPv1/v2c read-write community access (default = y): Configuring: rwcommunity Description: a SNMPv1/SNMPv2c read-write access community name arguments: community [default|hostname|network/bits] [oid] Enter the community name to add read-write access for: private The hostname or network address to accept this community name from [RETURN for all]: The OID that this community should be restricted to [RETURN for no-restriction]: RETURN Finished Output: rwcommunity private . Do another rwcommunity line? (default = y): n Notes:
Anyone with shell access on the host can use the community string private to write to any object in the system tree. (Not recommended)
Do you want to allow SNMPv1/v2c read-only community access (default = y): y Configuring: rocommunity Description: a SNMPv1/SNMPv2c read-only access community name arguments: community [default|hostname|network/bits] [oid] The community name to add read-only access for: public The hostname or network address to accept this community name from [RETURN for all]: [RETURN] The OID that this community should be restricted to [RETURN for no-restriction]: . Finished Output: rocommunity public . . Do another rocommunity line? (default = y): n Notes:
Anyone using the commonly used community string public can read any object in the system branch of the tree
The following files were created: snmpd.conf These files should be moved to /usr/local/share/snmp if you want them used by everyone on the system. In the future, if you add the -i option to the command line I'll copy them there automatically for you. Or, if you want them for your personal use only, copy them to /home/rks/.snmp . In the future, if you add the -p option to the command line I'll copy them there automatically for you. Notes:
This file is not automatically installed. You must copy it in to the correct location.

That last notes is worth repeating, as is is often overlooked:

  • This file is not automatically installed. You must copy it in to the correct location.

See the location section to determine the correct location.

Another important note: the rouser, rwuser, rwcommunity and rocommunity options found in the snmpd.conf configuration file are only half of the setup required for SNMPv3 users. The other half of the setup invovles setting up the SNMPv3 authentication and password parameters, which go in the persistent snmpd.conf file (usually found in /var/net-snmp/snmpd.conf). This file must be edited while the agent is not running, or any changes will be overwritten when the agent exits. See the next section for more details.



SNMPv3 user setup

If you choose not to set up any SNMPv3 users, you can skip this section.

This section under contruction. Please see the snmpusm man page for information on setting up SNMPv3 users.



Common options not covered by snmpconf

Some common tokens that you might want to add to the configuration file generated by snmpconf include:

  • VACM access control:
    The rouser, rwuser, rwcommunity and rocommunity access control options are simply wrappers around the more flexible (and thus complex) VACM access control tokens com2sec, view, group, and access. More information about these tokens can be found in the snmpd.conf man page and the VACM page.
  • ... TBD ...



Simple testing

If you want to simplify configuration (eg while testing), you can ignore the default configuration files and specify your configuration on the command line. Here is an example that can be used to run a freshly build agent from the source directory, without root access:



Quick and dirty, for the impatient

If you just want a cut-and-paste answer to get your agent up and running as quick a possible, the simplest configuration file that we can recommned is this:



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