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SNMPD.CONF(5) Net-SNMP SNMPD.CONF(5)
NAME
snmpd.conf - configuration file for the Net-SNMP SNMP agent
DESCRIPTION
The Net-SNMP agent uses one or more configuration files to control its operation and the management
information provided. These files (snmpd.conf and snmpd.local.conf) can be located in one of several
locations, as described in the snmp_config(5) manual page.
The (perl) application snmpconf can be used to generate configuration files for the most common agent
requirements. See the snmpconf(1) manual page for more information, or try running the command:
snmpconf -g basic_setup
There are a large number of directives that can be specified, but these mostly fall into four dis-tinct distinct
tinct categories:
• those controlling who can access the agent
• those configuring the information that is supplied by the agent
• those controlling active monitoring of the local system
• those concerned with extending the functionality of the agent.
Some directives don't fall naturally into any of these four categories, but this covers the majority
of the contents of a typical snmpd.conf file. A full list of recognised directives can be obtained
by running the command:
snmpd -H
AGENT BEHAVIOUR
Although most configuration directives are concerned with the MIB information supplied by the agent,
there are a handful of directives that control the behaviour of snmpd considered simply as a daemon
providing a network service.
agentaddress [<transport-specifier>:]<transport-address>[,...]
defines a list of listening addresses, on which to receive incoming SNMP requests. See the
section LISTENING ADDRESSES in the snmpd(8) manual page for more information about the format
of listening addresses.
The default behaviour is to listen on UDP port 161 on all IPv4 interfaces.
agentgroup {GROUP|#GID}
changes to the specified group after opening the listening port(s). This may refer to a group
by name (GROUP), or a numeric group ID starting with '#' (#GID).
agentuser {USER|#UID}
changes to the specified user after opening the listening port(s). This may refer to a user
by name (USER), or a numeric user ID starting with '#' (#UID).
leave_pidfile yes
instructs the agent to not remove its pid file on shutdown. Equivalent to specifying "-U" on
the command line.
maxGetbulkRepeats NUM
Sets the maximum number of responses allowed for a single variable in a getbulk request. Set
to 0 to enable the default and set it to -1 to enable unlimited. Because memory is allocated
ahead of time, sitting this to unlimited is not considered safe if your user population can
not be trusted. A repeat number greater than this will be truncated to this value.
This is set by default to -1.
maxGetbulkResponses NUM
Sets the maximum number of responses allowed for a getbulk request. This is set by default to
100. Set to 0 to enable the default and set it to -1 to enable unlimited. Because memory is
allocated ahead of time, sitting this to unlimited is not considered safe if your user popula-tion population
tion can not be trusted.
In general, the total number of responses will not be allowed to exceed the maxGetbulkRe-sponses maxGetbulkResponses
sponses number and the total number returned will be an integer multiple of the number of
variables requested times the calculated number of repeats allow to fit below this number.
Also not that processing of maxGetbulkRepeats is handled first.
SNMPv3 Configuration - Real Security
SNMPv3 is added flexible security models to the SNMP packet structure so that multiple security solu-tions solutions
tions could be used. SNMPv3 was original defined with a "User-based Security Model" (USM) [RFC3414]
that required maintaining a SNMP-specific user database. This was later determined to be troublesome
to maintain and had some minor security issues. The IETF has since added additional security models
to tunnel SNMP over SSH [RFC5592] and DTLS/TLS [RFC-to-be]. Net-SNMP contains robust support for
SNMPv3/USM, SNMPv3/TLS, and SNMPv3/DTLS. It contains partial support for SNMPv3/SSH as well but has
not been as extensively tested. It also contains code for support for an experimental Kerberos based
SNMPv3 that never got standardized.
Hopeful more SNMP software and devices will eventually support SNMP over (D)TLS or SSH, but it is
likely that devices with original support for SNMP will only contain support for SNMP over USM. If
your network manager supports SNMP over (D)TLS or SNMP over SSH we suggest you use one of these mech-anisms mechanisms
anisms instead of using USM, but as always with Net-SNMP we give you the options to pick from so you
can make the choice that is best for you.
SNMPv3 generic parameters
These parameters are generic to all the forms of SNMPv3. The SNMPv3 protocol defines "engineIDs"
that uniquely identify an agent. The string must be consistent through time and should not change or
conflict with another agent's engineID. Ever. Internally, Net-SNMP by default creates a unique
engineID that is based off of the current system time and a random number. This should be sufficient
for most users unless you're embedding our agent in a device where these numbers won't vary between
boxes on the devices initial boot.
EngineIDs are used both as a "context" for selecting information from the device and SNMPv3
with USM uses it to create unique entries for users in its user table.
The Net-SNMP agent offers the following mechanisms for setting the engineID, but again you
should only use them if you know what you're doing:
engineID STRING
specifies that the engineID should be built from the given text STRING.
engineIDType 1|2|3
specifies that the engineID should be built from the IPv4 address (1), IPv6 address (2) or MAC
address (3). Note that changing the IP address (or switching the network interface card) may
cause problems.
engineIDNic INTERFACE
defines which interface to use when determining the MAC address. If engineIDType 3 is not
specified, then this directive has no effect.
The default is to use eth0.
SNMPv3 over TLS
SNMPv3 may be tunneled over TLS and DTLS. TLS runs over TCP and DTLS is the UDP equivalent. Wes
Hardaker (the founder of Net-SNMP) performed a study and presented it at an IETF meeting that showed
that TCP based protocols are sufficient for stable networks but quickly becomes a problem in unstable
networks with even moderate levels of packet loss (~ 20-30%). If you are going to use TLS or DTLS,
you should use the one appropriate for your networking environment. You should potentially turn them
both on so your management system can access either the UDP or the TCP port as needed.
Many of the configuration tokens described below are prefixed with a '[snmp]' tag. If you place
these tokens in your snmpd.conf file, this take is required. See the snmp_config(5) manual page for
the meaning of this context switch.
[snmp] serverCert <specifier>
This token defines the default X.509 public key to use as the server's identity. It should
either be a fingerprint or a filename. To create a public key for use, please run the "net-snmp-cert" "netsnmp-cert"
snmp-cert" utility which will help you create the required certificate.
The default value for this is the certificate in the "snmpd" named certificate file.
[snmp] clientCert <specifier>
If outgoing TLS connections are to be opened, for example because notifications are to be sent
over TLS, then this will select the client-side public key to use for those outgoing connec-tions. connections.
tions. It can either be specified as a fingerprint or a filename. See the "net-snmp-cert"
utility for creating certificates.
[snmp] tlsAlgorithms <algorithms>
This string will select the algorithms to use when negotiating security during (D)TLS session
establishment. See the openssl manual page ciphers(1) for details on the format. Examples
strings include:
DEFAULT
ALL
HIGH
HIGH:!AES128-SHA
The default value is whatever openssl itself was configured with.
[snmp] x059CRLFile
If you are using a Certificate Authority (CA) that publishes a Certificate Revocation List
(CRL) then this token can be used to specify the location in the filesystem of a copy of the
CRL file. Note that Net-SNMP will not pull a CRL over http and this must be a file, not a
URL. Additionally, OpenSSL does not reload a CRL file when it has changed so modifications or
updates to the file will only be noticed upon a restart of the snmpd agent.
certSecName PRIORITY FINGERPRINT OPTIONS
OPTIONS can be one of <--sn SECNAME | --rfc822 | --dns | --ip | --cn | --any>.
The certSecName token will specify how to map a certificate field from the client's X.509 cer-tificate certificate
tificate to a SNMPv3 username. Use the --sn SECNAME flag to directly specify a securityName
for a given certificate. The other flags extract a particular component of the certificate
for use as a snmpv3 securityName. These fields are one of: A SubjectAltName containing an
rfc822 value (eg hardaker@net-snmp.org), A SubjectAltName containing a dns name value (eg
foo.net-snmp.org), an IP address (eg 192.0.2.1) or a common name "Wes Hardaker". The --any
flag specifies that any of the subjecAltName fields may be used. Make sure once a security-Name securityName
Name has been selected that it is given authorization via the VACM controls discussed later in
this manual page.
See the http://www.net-snmp.org/wiki/index.php/Using_DTLS web page for more detailed instruc-tions instructions
tions for setting up (D)TLS.
trustCert <specifier>
For X509 to properly verify a certificate, it should be verifiable up until a trust anchor for
it. This trust anchor is typically a CA certificate but it could also be a self-signed cer-tificate. certificate.
tificate. The "trustCert" token should be used to load specific trust anchors into the veri-fication verification
fication engine.
SNMP over (D)TLS requires the use of the Transport Security Model (TSM), so read the section on the
usage of the Transport Security Model as well. Make sure when you configure the VACM to accept con-nections connections
nections from (D)TLS that you use the "tsm" security model. E.G.:
rwuser -s tsm hardaker@net-snmp.org
SNMPv3 over SSH Support
To use SSH, you'll need to configure sshd to invoke the sshtosnmp program as well as configure the
access control settings to allow access through the tsm security model using the user name provided
to snmpd by the ssh transport.
SNMPv3 with the Transport Security Model (TSM)
The Transport Security Model [RFC5591] defines a SNMPv3 security system for use with "tunneled" secu-rity security
rity protocols like TLS, DTLS and SSH. It is a very simple security model that simply lets properly
protected packets to pass through into the snmp application. The transport is required to pass a
securityName to use to the TSM and the TSM may optionally prefix this with a transport string (see
below).
tsmUseTransportPrefix (1|yes|true|0|no|false)
If set to true, the TSM module will take every securityName passed to it from the transports
underneath and prefix it with a string that specifically identities the transport it came
from. This is useful to avoid securityName clashes with transports that generate identical
security names. For example, if the ssh security transport delivered the security name of
"hardaker" for a SSH connection and the TLS security transport also delivered the security
name of "hardaker" for a TLS connection then it would be impossible to separate out these two
users to provide separate access control rights. With the tsmUseTransportPrefix set to true,
however, the securityNames would be prefixed appropriately with one of: "tls:", "dtls:" or
"ssh:".
SNMPv3 with the User-based Security Model (USM)
SNMPv3 was originally defined using the User-Based Security Model (USM), which contains a private
list of users and keys specific to the SNMPv3 protocol. The operational community, however, declared
it a pain to manipulate yet another database and would prefer to use existing infrastructure. To
that end the IETF created the ISMS working group to battle that problem, and the ISMS working group
decided to tunnel SNMP over SSH and DTLS to make use existing user and authentication infrastruc-tures. infrastructures.
tures.
SNMPv3 USM Users
To use the USM based SNMPv3-specific users, you'll need to create them. It is recommended you use
the net-snmp-config command to do this, but you can also do it by directly specifying createUser
directives yourself instead:
createUser [-e ENGINEID] username (MD5|SHA) authpassphrase [DES|AES] [privpassphrase]
MD5 and SHA are the authentication types to use. DES and AES are the privacy protocols to
use. If the privacy passphrase is not specified, it is assumed to be the same as the authen-tication authentication
tication passphrase. Note that the users created will be useless unless they are also added
to the VACM access control tables described above.
SHA authentication and DES/AES privacy require OpenSSL to be installed and the agent to be
built with OpenSSL support. MD5 authentication may be used without OpenSSL.
Warning: the minimum pass phrase length is 8 characters.
SNMPv3 users can be created at runtime using the snmpusm(1) command.
Instead of figuring out how to use this directive and where to put it (see below), just run
"net-snmp-config --create-snmpv3-user" instead, which will add one of these lines to the right
place.
This directive should be placed into the /var/db/net-snmp/snmpd.conf file instead of the other
normal locations. The reason is that the information is read from the file and then the line
is removed (eliminating the storage of the master password for that user) and replaced with
the key that is derived from it. This key is a localized key, so that if it is stolen it can
not be used to access other agents. If the password is stolen, however, it can be.
If you need to localize the user to a particular EngineID (this is useful mostly in the simi-lar similar
lar snmptrapd.conf file), you can use the -e argument to specify an EngineID as a hex value
(EG, "0x01020304").
If you want to generate either your master or localized keys directly, replace the given pass-word password
word with a hexstring (preceeded by a "0x") and precede the hex string by a -m or -l token
(respectively). EGs:
[these keys are *not* secure but are easy to visually parse for
counting purposes. Please generate random keys instead of using
these examples]
createUser myuser SHA -l 0x0001020304050607080900010203040506070809 AES -l 0x00010203040506070809000102030405
createUser myuser SHA -m 0x0001020304050607080900010203040506070809 AES -m 0x0001020304050607080900010203040506070809
Due to the way localization happens, localized privacy keys are expected to be the length
needed by the algorithm (128 bits for all supported algorithms). Master encryption keys,
though, need to be the length required by the authentication algorithm not the length required
by the encrypting algorithm (MD5: 16 bytes, SHA: 20 bytes).
ACCESS CONTROL
snmpd supports the View-Based Access Control Model (VACM) as defined in RFC 2575, to control who can
retrieve or update information. To this end, it recognizes various directives relating to access
control.
Traditional Access Control
Most simple access control requirements can be specified using the directives rouser/rwuser (for
SNMPv3) or rocommunity/rwcommunity (for SNMPv1 or SNMPv2c).
rouser [-s SECMODEL] USER [noauth|auth|priv [OID | -V VIEW [CONTEXT]]]
rwuser [-s SECMODEL] USER [noauth|auth|priv [OID | -V VIEW [CONTEXT]]]
specify an SNMPv3 user that will be allowed read-only (GET and GETNEXT) or read-write (GET,
GETNEXT and SET) access respectively. By default, this will provide access to the full OID
tree for authenticated (including encrypted) SNMPv3 requests, using the default context. An
alternative minimum security level can be specified using noauth (to allow unauthenticated
requests), or priv (to enforce use of encryption). The OID field restricts access for that
user to the subtree rooted at the given OID, or the named view. An optional context can also
be specified, or "context*" to denote a context prefix. If no context field is specified (or
the token "*" is used), the directive will match all possible contexts.
If SECMODEL is specified then it will be the security model required for that user (note that
identical user names may come in over different security models and will be appropriately sep-arated separated
arated via the access control settings). The default security model is "usm" and the other
common security models are likely "tsm" when using (D)TLS or SSH support and "ksm" if the Ker-beros Kerberos
beros support has been compiled in.
rocommunity COMMUNITY [SOURCE [OID | -V VIEW [CONTEXT]]]
rwcommunity COMMUNITY [SOURCE [OID | -V VIEW [CONTEXT]]]
specify an SNMPv1 or SNMPv2c community that will be allowed read-only (GET and GETNEXT) or
read-write (GET, GETNEXT and SET) access respectively. By default, this will provide access
to the full OID tree for such requests, regardless of where they were sent from. The SOURCE
token can be used to restrict access to requests from the specified system(s) - see com2sec
for the full details. The OID field restricts access for that community to the subtree rooted
at the given OID, or named view. Contexts are typically less relevant to community-based SNMP
versions, but the same behaviour applies here.
rocommunity6 COMMUNITY [SOURCE [OID | -V VIEW [CONTEXT]]]
rwcommunity6 COMMUNITY [SOURCE [OID | -V VIEW [CONTEXT]]]
are directives relating to requests received using IPv6 (if the agent supports such transport
domains). The interpretation of the SOURCE, OID, VIEW and CONTEXT tokens are exactly the same
as for the IPv4 versions.
In each case, only one directive should be specified for a given SNMPv3 user, or community string.
It is not appropriate to specify both rouser and rwuser directives referring to the same SNMPv3 user
(or equivalent community settings). The rwuser directive provides all the access of rouser (as well
as allowing SET support). The same holds true for the community-based directives.
More complex access requirements (such as access to two or more distinct OID subtrees, or different
views for GET and SET requests) should use one of the other access control mechanisms. Note that if
several distinct communities or SNMPv3 users need to be granted the same level of access, it would
also be more efficient to use the main VACM configuration directives.
VACM Configuration
The full flexibility of the VACM is available using four configuration directives - com2sec, group,
view and access. These provide direct configuration of the underlying VACM tables.
com2sec [-Cn CONTEXT] SECNAME SOURCE COMMUNITY
com2sec6 [-Cn CONTEXT] SECNAME SOURCE COMMUNITY
map an SNMPv1 or SNMPv2c community string to a security name - either from a particular range
of source addresses, or globally ("default"). A restricted source can either be a specific
hostname (or address), or a subnet - represented as IP/MASK (e.g. 10.10.10.0/255.255.255.0),
or IP/BITS (e.g. 10.10.10.0/24), or the IPv6 equivalents.
The same community string can be specified in several separate directives (presumably with
different source tokens), and the first source/community combination that matches the incoming
request will be selected. Various source/community combinations can also map to the same
security name.
If a CONTEXT is specified (using -Cn), the community string will be mapped to a security name
in the named SNMPv3 context. Otherwise the default context ("") will be used.
com2secunix [-Cn CONTEXT] SECNAME SOCKPATH COMMUNITY
is the Unix domain sockets version of com2sec.
group GROUP {v1|v2c|usm|tsm|ksm} SECNAME
maps a security name (in the specified security model) into a named group. Several group
directives can specify the same group name, allowing a single access setting to apply to sev-eral several
eral users and/or community strings.
Note that groups must be set up for the two community-based models separately - a single
com2sec (or equivalent) directive will typically be accompanied by two group directives.
view VNAME TYPE OID [MASK]
defines a named "view" - a subset of the overall OID tree. This is most commonly a single sub-tree, subtree,
tree, but several view directives can be given with the same view name (VNAME), to build up a
more complex collection of OIDs. TYPE is either included or excluded, which can again define
a more complex view (e.g by excluding certain sensitive objects from an otherwise accessible
subtree).
MASK is a list of hex octets (optionally separated by '.' or ':') with the set bits indicating
which subidentifiers in the view OID to match against. If not specified, this defaults to
matching the OID exactly (all bits set), thus defining a simple OID subtree. So:
view iso1 included .iso 0xf0
view iso2 included .iso
view iso3 included .iso.org.dod.mgmt 0xf0
would all define the same view, covering the whole of the 'iso(1)' subtree (with the third
example ignoring the subidentifiers not covered by the mask).
More usefully, the mask can be used to define a view covering a particular row (or rows) in a
table, by matching against the appropriate table index value, but skipping the column subiden-tifier: subidentifier:
tifier:
view ifRow4 included .1.3.6.1.2.1.2.2.1.0.4 0xff:a0
Note that a mask longer than 8 bits must use ':' to separate the individual octets.
access GROUP CONTEXT {any|v1|v2c|usm|tsm|ksm} LEVEL PREFX READ WRITE NOTIFY
maps from a group of users/communities (with a particular security model and minimum security
level, and in a specific context) to one of three views, depending on the request being pro-cessed. processed.
cessed.
LEVEL is one of noauth, auth, or priv. PREFX specifies how CONTEXT should be matched against
the context of the incoming request, either exact or prefix. READ, WRITE and NOTIFY specifies
the view to be used for GET*, SET and TRAP/INFORM requests (althought the NOTIFY view is not
currently used). For v1 or v2c access, LEVEL will need to be noauth.
Typed-View Configuration
The final group of directives extend the VACM approach into a more flexible mechanism, which can be
applied to other access control requirements. Rather than the fixed three views of the standard VACM
mechanism, this can be used to configure various different view types. As far as the main SNMP agent
is concerned, the two main view types are read and write, corresponding to the READ and WRITE views
of the main access directive. See the 'snmptrapd.conf(5)' man page for discussion of other view
types.
authcommunity TYPES COMMUNITY [SOURCE [OID | -V VIEW [CONTEXT]]]
is an alternative to the rocommunity/rwcommunity directives. TYPES will usually be read or
read,write respectively. The view specification can either be an OID subtree (as before), or
a named view (defined using the view directive) for greater flexibility. If this is omitted,
then access will be allowed to the full OID tree. If CONTEXT is specified, access is config-ured configured
ured within this SNMPv3 context. Otherwise the default context ("") is used.
authuser TYPES [-s MODEL] USER [LEVEL [OID | -V VIEW [CONTEXT]]]
is an alternative to the rouser/rwuser directives. The fields TYPES, OID, VIEW and CONTEXT
have the same meaning as for authcommunity.
authgroup TYPES [-s MODEL] GROUP [LEVEL [OID | -V VIEW [CONTEXT]]]
is a companion to the authuser directive, specifying access for a particular group (defined
using the group directive as usual). Both authuser and authgroup default to authenticated
requests - LEVEL can also be specified as noauth or priv to allow unauthenticated requests, or
require encryption respectively. Both authuser and authgroup directives also default to con-figuring configuring
figuring access for SNMPv3/USM requests - use the '-s' flag to specify an alternative security
model (using the same values as for access above).
authaccess TYPES [-s MODEL] GROUP VIEW [LEVEL [CONTEXT]]
also configures the access for a particular group, specifying the name and type of view to
apply. The MODEL and LEVEL fields are interpreted in the same way as for authgroup. If CON-TEXT CONTEXT
TEXT is specified, access is configured within this SNMPv3 context (or contexts with this pre-fix prefix
fix if the CONTEXT field ends with '*'). Otherwise the default context ("") is used.
setaccess GROUP CONTEXT MODEL LEVEL PREFIX VIEW TYPES
is a direct equivalent to the original access directive, typically listing the view types as
read or read,write as appropriate. (or see 'snmptrapd.conf(5)' for other possibilities). All
other fields have the same interpretation as with access.
SYSTEM INFORMATION
Most of the information reported by the Net-SNMP agent is retrieved from the underlying system, or
dynamically configured via SNMP SET requests (and retained from one run of the agent to the next).
However, certain MIB objects can be configured or controlled via the snmpd.conf(5) file.
System Group
Most of the scalar objects in the 'system' group can be configured in this way:
sysLocation STRING
sysContact STRING
sysName STRING
set the system location, system contact or system name (sysLocation.0, sysContact.0 and sys-Name.0) sysName.0)
Name.0) for the agent respectively. Ordinarily these objects are writeable via suitably
authorized SNMP SET requests. However, specifying one of these directives makes the corre-sponding corresponding
sponding object read-only, and attempts to SET it will result in a notWritable error response.
sysServices NUMBER
sets the value of the sysServices.0 object. For a host system, a good value is 72 (applica-tion (application
tion + end-to-end layers). If this directive is not specified, then no value will be reported
for the sysServices.0 object.
sysDescr STRING
sysObjectID OID
sets the system description or object ID for the agent. Although these MIB objects are not
SNMP-writable, these directives can be used by a network administrator to configure suitable
values for them.
Interfaces Group
interface NAME TYPE SPEED
can be used to provide appropriate type and speed settings for interfaces where the agent
fails to determine this information correctly. TYPE is a type value as given in the
IANAifType-MIB, and can be specified numerically or by name (assuming this MIB is loaded).
Host Resources Group
This requires that the agent was built with support for the host module (which is now included as
part of the default build configuration on the major supported platforms).
ignoreDisk STRING
controls which disk devices are scanned as part of populating the hrDiskStorageTable (and
hrDeviceTable). The HostRes implementation code includes a list of disk device patterns
appropriate for the current operating system, some of which may cause the agent to block when
trying to open the corresponding disk devices. This might lead to a timeout when walking
these tables, possibly resulting in inconsistent behaviour. This directive can be used to
specify particular devices (either individually or wildcarded) that should not be checked.
Note: Please consult the source (host/hr_disk.c) and check for the Add_HR_Disk_entry calls
relevant for a particular O/S to determine the list of devices that will be scanned.
The pattern can include one or more wildcard expressions. See snmpd.examples(5) for illustra-tion illustration
tion of the wildcard syntax.
skipNFSInHostResources true
controls whether NFS and NFS-like file systems should be omitted from the hrStorageTable (true
or 1) or not (false or 0, which is the default). If the Net-SNMP agent gets hung on NFS-mounted NFSmounted
mounted filesystems, you can try setting this to '1'.
storageUseNFS [1|2]
controls how NFS and NFS-like file systems should be reported in the hrStorageTable. as 'Net-work 'Network
work Disks' (1) or 'Fixed Disks' (2) Historically, the Net-SNMP agent has reported such file
systems as 'Fixed Disks', and this is still the default behaviour. Setting this directive to
'1' reports such file systems as 'Network Disks', as required by the Host Resources MIB.
Process Monitoring
The hrSWRun group of the Host Resources MIB provides information about individual processes running
on the local system. The prTable of the UCD-SNMP-MIB complements this by reporting on selected ser-vices services
vices (which may involve multiple processes). This requires that the agent was built with support
for the ucd-snmp/proc module (which is included as part of the default build configuration).
proc NAME [MAX [MIN]]
monitors the number of processes called NAME (as reported by "/bin/ps -acx") running on the
local system.
If the number of NAMEd processes is less than MIN or greater than MAX, then the corresponding
prErrorFlag instance will be set to 1, and a suitable description message reported via the
prErrMessage instance.
Note: This situation will not automatically trigger a trap to report the problem - see the
DisMan Event MIB section later.
If neither MAX nor MIN are specified, they will default to infinity and 1 respectively ("at
least one"). If only MAX is specified, MIN will default to 0 ("no more than MAX"). If MAX is
0 (and MIN is not), this indicates infinity ("at least MIN"). If both MAX and MIN are 0, this
indicates a process that should not be running.
procfix NAME PROG ARGS
registers a command that can be run to fix errors with the given process NAME. This will be
invoked when the corresponding prErrFix instance is set to 1.
Note: This command will not be invoked automatically.
The procfix directive must be specified after the matching proc directive, and cannot be used
on its own.
If no proc directives are defined, then walking the prTable will fail (noSuchObject).
Disk Usage Monitoring
This requires that the agent was built with support for the ucd-snmp/disk module (which is included
as part of the default build configuration).
disk PATH [ MINSPACE | MINPERCENT% ]
monitors the disk mounted at PATH for available disk space.
The minimum threshold can either be specified in kB (MINSPACE) or as a percentage of the total
disk (MINPERCENT% with a '%' character), defaulting to 100kB if neither are specified. If the
free disk space falls below this threshold, then the corresponding dskErrorFlag instance will
be set to 1, and a suitable description message reported via the dskErrorMsg instance.
Note: This situation will not automatically trigger a trap to report the problem - see the
DisMan Event MIB section later.
includeAllDisks MINPERCENT%
configures monitoring of all disks found on the system, using the specified (percentage)
threshold. The threshold for individual disks can be adjusted using suitable disk directives
(which can come either before or after the includeAllDisks directive).
Note: Whether disk directives appears before or after includeAllDisks may affect the indexing
of the dskTable.
Only one includeAllDisks directive should be specified - any subsequent copies will be
ignored.
The list of mounted disks will be determined when the agent starts using the setmntent(3) and
getmntent(3), or fopen(3) and getmntent(3), or setfsent(3) and getfsent(3) system calls. If
none of the above system calls are available then the root partition "/" (which is assumed
to exist on any UNIX based system) will be monitored. Disks mounted after the agent has
started will not be monitored.
If neither any disk directives or includeAllDisks are defined, then walking the dskTable will fail
(noSuchObject).
System Load Monitoring
This requires that the agent was built with support for either the ucd-snmp/loadave module or the
ucd-snmp/memory module respectively (both of which are included as part of the default build configu-ration). configuration).
ration).
load MAX1 [MAX5 [MAX15]]
monitors the load average of the local system, specifying thresholds for the 1-minute,
5-minute and 15-minute averages. If any of these loads exceed the associated maximum value,
then the corresponding laErrorFlag instance will be set to 1, and a suitable description mes-sage message
sage reported via the laErrMessage instance.
Note: This situation will not automatically trigger a trap to report the problem - see the
DisMan Event MIB section later.
If the MAX15 threshold is omitted, it will default to the MAX5 value. If both MAX5 and MAX15
are omitted, they will default to the MAX1 value. If this directive is not specified, all
three thresholds will default to a value of DEFMAXLOADAVE.
If a threshold value of 0 is given, the agent will not report errors via the relevant laError-Flag laErrorFlag
Flag or laErrMessage instances, regardless of the current load.
Unlike the proc and disk directives, walking the walking the laTable will succeed (assuming the ucd-snmp/loadave ucdsnmp/loadave
snmp/loadave module was configured into the agent), even if the load directive is not present.
swap MIN
monitors the amount of swap space available on the local system. If this falls below the
specified threshold (MIN kB), then the memErrorSwap object will be set to 1, and a suitable
description message reported via memSwapErrorMsg.
Note: This situation will not automatically trigger a trap to report the problem - see the
DisMan Event MIB section later.
If this directive is not specified, the default threshold is 16 MB.
Log File Monitoring
This requires that the agent was built with support for either the ucd-snmp/file or ucd-snmp/logmatch
modules respectively (both of which are included as part of the default build configuration).
file FILE [MAXSIZE]
monitors the size of the specified file (in kB). If MAXSIZE is specified, and the size of the
file exceeds this threshold, then the corresponding fileErrorFlag instance will be set to 1,
and a suitable description message reported via the fileErrorMsg instance.
Note: This situation will not automatically trigger a trap to report the problem - see the
DisMan Event MIB section later.
Note: A maximum of 20 files can be monitored.
Note: If no file directives are defined, then walking the fileTable will fail (noSuchObject).
logmatch NAME FILE CYCLETIME REGEX
monitors the specified file for occurances of the specified pattern REGEX. The file position
is stored internally so the entire file is only read initially, every subsequent pass will
only read the new lines added to the file since the last read.
NAME name of the logmatch instance (will appear as logMatchName under logMatch/log-MatchTable/logMatchEntry/logMatchName logMatch/logMatchTable/logMatchEntry/logMatchName
MatchTable/logMatchEntry/logMatchName in the ucd-snmp MIB tree)
FILE absolute path to the logfile to be monitored. Note that this path can contain date/time
directives (like in the UNIX 'date' command). See the manual page for 'strftime' for
the various directives accepted.
CYCLETIME
time interval for each logfile read and internal variable update in seconds. Note: an
SNMPGET* operation will also trigger an immediate logfile read and variable update.
REGEX the regular expression to be used. Note: DO NOT enclose the regular expression in
quotes even if there are spaces in the expression as the quotes will also become part
of the pattern to be matched!
Example:
logmatch apache-GETs /usr/local/apache/logs/access.log-%Y-%m-%d 60 GET.*HTTP.*
This logmatch instance is named 'apache-GETs', uses 'GET.*HTTP.*' as its regular
expression and it will monitor the file named (assuming today is May 6th 2009):
'/usr/local/apache/logs/access.log-2009-05-06', tomorrow it will look for
'access.log-2009-05-07'. The logfile is read every 60 seconds.
Note: A maximum of 250 logmatch directives can be specified.
Note: If no logmatch directives are defined, then walking the logMatchTable will fail (noSu-chObject). (noSuchObject).
chObject).
ACTIVE MONITORING
The usual behaviour of an SNMP agent is to wait for incoming SNMP requests and respond to them - if
no requests are received, an agent will typically not initiate any actions. This section describes
various directives that can configure snmpd to take a more active role.
Notification Handling
trapcommunity STRING
defines the default community string to be used when sending traps. Note that this directive
must be used prior to any community-based trap destination directives that need to use it.
trapsink HOST [COMMUNITY [PORT]]
trap2sink HOST [COMMUNITY [PORT]]
informsink HOST [COMMUNITY [PORT]]
define the address of a notification receiver that should be sent SNMPv1 TRAPs, SNMPv2c
TRAP2s, or SNMPv2 INFORM notifications respectively. See the section LISTENING ADDRESSES in
the snmpd(8) manual page for more information about the format of listening addresses. If
COMMUNITY is not specified, the most recent trapcommunity string will be used.
If the transport address does not include an explicit port specification, then PORT will be
used. If this is not specified, the well known SNMP trap port (162) will be used.
Note: This mechanism is being deprecated, and the listening port should be specified via the
transport specification HOST instead.
If several sink directives are specified, multiple copies of each notification (in the appro-priate appropriate
priate formats) will be generated.
Note: It is not normally appropriate to list two (or all three) sink directives with the same
destination.
trapsess [SNMPCMD_ARGS] HOST
provides a more generic mechanism for defining notification destinations. SNMPCMD_ARGS should
be the command-line options required for an equivalent snmptrap (or snmpinform) command to
send the desired notification. The option -Ci can be used (with -v2c or -v3) to generate an
INFORM notification rather than an unacknowledged TRAP.
This is the appropriate directive for defining SNMPv3 trap receivers. See http://www .net-
snmp.org/tutorial/tutorial-5/commands/snmptrap-v3.html for more information about SNMPv3 noti-fication notification
fication behaviour.
authtrapenable {1|2}
determines whether to generate authentication failure traps (enabled(1)) or not (disabled(2) -the (disabled(2)the
the default). Ordinarily the corresponding MIB object (snmpEnableAuthenTraps.0) is read-write, readwrite,
write, but specifying this directive makes this object read-only, and attempts to set the
value via SET requests will result in a notWritable error response.
v1trapaddress HOST
defines the agent address, which is inserted into SNMPv1 TRAPs. Arbitrary local IPv4 address
is chosen if this option is ommited. This option is useful mainly when the agent is visible
from outside world by specific address only (e.g. because of network address translation or
firewall).
DisMan Event MIB
The previous directives can be used to configure where traps should be sent, but are not concerned
with when to send such traps (or what traps should be generated). This is the domain of the Event
MIB - developed by the Distributed Management (DisMan) working group of the IETF.
This requires that the agent was built with support for the disman/event module (which is now
included as part of the default build configuration for the most recent distribution).
Note: The behaviour of the latest implementation differs in some minor respects from the pre-vious previous
vious code - nothing too significant, but existing scripts may possibly need some minor
adjustments.
iquerySecName NAME
agentSecName NAME
specifies the default SNMPv3 username, to be used when making internal queries to retrieve any
necessary information (either for evaluating the monitored expression, or building a notifica-tion notification
tion payload). These internal queries always use SNMPv3, even if normal querying of the agent
is done using SNMPv1 or SNMPv2c.
Note that this user must also be explicitly created (createUser) and given appropriate access
rights (e.g. rouser). This directive is purely concerned with defining which user should be
used - not with actually setting this user up.
monitor [OPTIONS] NAME EXPRESSION
defines a MIB object to monitor. If the EXPRESSION condition holds (see below), then this
will trigger the corresponding event, and either send a notification or apply a SET assignment
(or both). Note that the event will only be triggered once, when the expression first
matches. This monitor entry will not fire again until the monitored condition first becomes
false, and then matches again. NAME is an administrative name for this expression, and is
used for indexing the mteTriggerTable (and related tables). Note also that such monitors use
an internal SNMPv3 request to retrieve the values being monitored (even if normal agent
queries typically use SNMPv1 or SNMPv2c). See the iquerySecName token described above.
EXPRESSION
There are three types of monitor expression supported by the Event MIB - existence, boolean
and threshold tests.
OID | ! OID | != OID
defines an existence(_) monitor test. A bare OID specifies a present(_) test, which
will fire when (an instance of) the monitored OID is created. An expression of the
form ! OID specifies an absent(1) test, which will fire when the monitored OID is
delected. An expression of the form != OID specifies a changed(2) test, which will
fire whenever the monitored value(s) change. Note that there must be whitespace before
the OID token.
OID OP VALUE
defines a boolean(1) monitor test. OP should be one of the defined comparison opera-tors operators
tors (!=, ==, <, <=, >, >=) and VALUE should be an integer value to compare against.
Note that there must be whitespace around the OP token. A comparison such as OID !=0
will not be handled correctly.
OID MIN MAX [DMIN DMAX]
defines a threshold(2) monitor test. MIN and MAX are integer values, specifying lower
and upper thresholds. If the value of the monitored OID falls below the lower thresh-old threshold
old (MIN) or rises above the upper threshold (MAX), then the monitor entry will trigger
the corresponding event.
Note that the rising threshold event will only be re-armed when the monitored value
falls below the lower threshold (MIN). Similarly, the falling threshold event will be
re-armed by the upper threshold (MAX).
The optional parameters DMIN and DMAX configure a pair of similar threshold tests, but
working with the delta differences between successive sample values.
OPTIONS
There are various options to control the behaviour of the monitored expression. These
include:
-D indicates that the expression should be evaluated using delta differences between sam-ple sample
ple values (rather than the values themselves).
-d OID
-di OID
specifies a discontinuity marker for validating delta differences. A -di object
instance will be used exactly as given. A -d object will have the instance subidenti-fiers subidentifiers
fiers from the corresponding (wildcarded) expression object appended. If the -I flag
is specified, then there is no difference between these two options.
This option also implies -D.
-e EVENT
specifies the event to be invoked when this monitor entry is triggered. If this option
is not given, the monitor entry will generate one of the standard notifications defined
in the DISMAN-EVENT-MIB.
-I indicates that the monitored expression should be applied to the specified OID as a
single instance. By default, the OID will be treated as a wildcarded object, and the
monitor expanded to cover all matching instances.
-i OID
-o OID define additional varbinds to be added to the notification payload when this monitor
trigger fires. For a wildcarded expression, the suffix of the matched instance will be
added to any OIDs specified using -o, while OIDs specified using -i will be treated as
exact instances. If the -I flag is specified, then there is no difference between
these two options.
See strictDisman for details of the ordering of notification payloads.
-r FREQUENCY
monitors the given expression every FREQUENCY seconds. By default, the expression will
be evaluated every 600s (10 minutes).
-S indicates that the monitor expression should not be evaluated when the agent first
starts up. The first evaluation will be done once the first repeat interval has
expired.
-s indicates that the monitor expression should be evaluated when the agent first starts
up. This is the default behaviour.
Note: Notifications triggered by this initial evaluation will be sent before the cold-Start coldStart
Start trap.
-u SECNAME
specifies a security name to use for scanning the local host, instead of the default
iquerySecName. Once again, this user must be explicitly created and given suitable
access rights.
notificationEvent ENAME NOTIFICATION [-m] [-i OID | -o OID ]*
defines a notification event named ENAME. This can be triggered from a given monitor entry by
specifying the option -e ENAME (see above). NOTIFICATION should be the OID of the NOTIFICA-TION-TYPE NOTIFICATION-TYPE
TION-TYPE definition for the notification to be generated.
If the -m option is given, the notification payload will include the standard varbinds as
specified in the OBJECTS clause of the notification MIB definition. This option must come
after the NOTIFICATION OID (and the relevant MIB file must be available and loaded by the
agent). Otherwise, these varbinds must be listed explicitly (either here or in the corre-sponding corresponding
sponding monitor directive).
The -i OID and -o OID options specify additional varbinds to be appended to the notification
payload, after the standard list. If the monitor entry that triggered this event involved a
wildcarded expression, the suffix of the matched instance will be added to any OIDs specified
using -o, while OIDs specified using -i will be treated as exact instances. If the -I flag
was specified to the monitor directive, then there is no difference between these two options.
setEvent ENAME [-I] OID = VALUE
defines a set event named ENAME, assigning the (integer) VALUE to the specified OID. This can
be triggered from a given monitor entry by specifying the option -e ENAME (see above).
If the monitor entry that triggered this event involved a wildcarded expression, the suffix of
the matched instance will normally be added to the OID. If the -I flag was specified to
either of the monitor or setEvent directives, the specified OID will be regarded as an exact
single instance.
strictDisman yes
The definition of SNMP notifications states that the varbinds defined in the OBJECT clause
should come first (in the order specified), followed by any "extra" varbinds that the notifi-cation notification
cation generator feels might be useful. The most natural approach would be to associate these
mandatory varbinds with the notificationEvent entry, and append any varbinds associated with
the monitor entry that triggered the notification to the end of this list. This is the
default behaviour of the Net-SNMP Event MIB implementation.
Unfortunately, the DisMan Event MIB specifications actually state that the trigger-related
varbinds should come first, followed by the event-related ones. This directive can be used to
restore this strictly-correct (but inappropriate) behaviour.
Note: Strict DisMan ordering may result in generating invalid notifications payload lists if
the notificationEvent -n flag is used together with monitor -o (or -i) varbind options.
If no monitor entries specify payload varbinds, then the setting of this directive is irrele-vant. irrelevant.
vant.
linkUpDownNotifications yes
will configure the Event MIB tables to monitor the ifTable for network interfaces being taken
up or down, and triggering a linkUp or linkDown notification as appropriate.
This is exactly equivalent to the configuration:
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
defaultMonitors yes
will configure the Event MIB tables to monitor the various UCD-SNMP-MIB tables for problems
(as indicated by the appropriate xxErrFlag column objects).
This is exactly equivalent to the configuration:
monitor -o prNames -o prErrMessage "process table" 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
In both these latter cases, the snmpd.conf must also contain a iquerySecName directive, together with
a corresponding createUser entry and suitable access control configuration.
DisMan Schedule MIB
The DisMan working group also produced a mechanism for scheduling particular actions (a specified SET
assignment) at given times. This requires that the agent was built with support for the dis-man/schedule disman/schedule
man/schedule module (which is included as part of the default build configuration for the most recent
distribution).
There are three ways of specifying the scheduled action:
repeat FREQUENCY OID = VALUE
configures a SET assignment of the (integer) VALUE to the MIB instance OID, to be run every
FREQUENCY seconds.
cron MINUTE HOUR DAY MONTH WEEKDAY OID = VALUE
configures a SET assignment of the (integer) VALUE to the MIB instance OID, to be run at the
times specified by the fields MINUTE to WEEKDAY. These follow the same pattern as the equiva-lent equivalent
lent crontab(5) fields.
Note: These fields should be specified as a (comma-separated) list of numeric values. Named
values for the MONTH and WEEKDAY fields are not supported, and neither are value
ranges. A wildcard match can be specified as '*'.
The DAY field can also accept negative values, to indicate days counting backwards from the
end of the month.
at MINUTE HOUR DAY MONTH WEEKDAY OID = VALUE
configures a one-shot SET assignment, to be run at the first matching time as specified by the
fields MINUTE to WEEKDAY. The interpretation of these fields is exactly the same as for the
cron directive.
EXTENDING AGENT FUNCTIONALITY
One of the first distinguishing features of the original UCD suite was the ability to extend the
functionality of the agent - not just by recompiling with code for new MIB modules, but also by con-figuring configuring
figuring the running agent to report additional information. There are a number of techniques to sup-port support
port this, including:
• running external commands (exec, extend, pass)
• loading new code dynamically (embedded perl, dlmod)
• communicating with other agents (proxy, SMUX, AgentX)
Arbitrary Extension Commands
The earliest extension mechanism was the ability to run arbitrary commands or shell scripts. Such
commands do not need to be aware of SNMP operations, or conform to any particular behaviour - the MIB
structures are designed to accommodate any form of command output. Use of this mechanism requires
that the agent was built with support for the ucd-snmp/extensible and/or agent/extend modules (which
are both included as part of the default build configuration).
exec [MIBOID] NAME PROG ARGS
sh [MIBOID] NAME PROG ARGS
invoke the named PROG with arguments of ARGS. By default the exit status and first line of
output from the command will be reported via the extTable, discarding any additional output.
Note: Entries in this table appear in the order they are read from the configuration file.
This means that adding new exec (or sh) directives and restarting the agent, may affect
the indexing of other entries.
The PROG argument for exec directives must be a full path to a real binary, as it is executed
via the exec() system call. To invoke a shell script, use the sh directive instead.
If MIBOID is specified, then the results will be rooted at this point in the OID tree, return-ing returning
ing the exit statement as MIBOID.100.0 and the entire command output in a pseudo-table based
at MIBNUM.101 - with one 'row' for each line of output.
Note: The layout of this "relocatable" form of exec (or sh) output does not strictly form a
valid MIB structure. This mechanism is being deprecated - please see the extend direc-tive directive
tive (described below) instead.
The agent does not cache the exit status or output of the executed program.
execfix NAME PROG ARGS
registers a command that can be invoked on demand - typically to respond to or fix errors with
the corresponding exec or sh entry. When the extErrFix instance for a given NAMEd entry is
set to the integer value of 1, this command will be called.
Note: This directive can only be used in combination with a corresponding exec or sh direc-tive, directive,
tive, which must be defined first. Attempting to define an unaccompanied execfix
directive will fail.
exec and sh extensions can only be configured via the snmpd.conf file. They cannot be set up via
SNMP SET requests.
extend [MIBOID] NAME PROG ARGS
works in a similar manner to the exec directive, but with a number of improvements. The MIB
tables (nsExtendConfigTable etc) are indexed by the NAME token, so are unaffected by the order
in which entries are read from the configuration files. There are two result tables - one
(nsExtendOutput1Table) containing the exit status, the first line and full output (as a single
string) for each extend entry, and the other (nsExtendOutput2Table) containing the complete
output as a series of separate lines.
If MIBOID is specified, then the configuration and result tables will be rooted at this point
in the OID tree, but are otherwise structured in exactly the same way. This means that several
separate extend directives can specify the same MIBOID root, without conflicting.
The exit status and output is cached for each entry individually, and can be cleared (and the
caching behaviour configured) using the nsCacheTable.
extendfix NAME PROG ARGS
registers a command that can be invoked on demand, by setting the appropriate nsExtendRunType
instance to the value run-command(3). Unlike the equivalent execfix, this directive does not
need to be paired with a corresponding extend entry, and can appear on its own.
Both extend and extendfix directives can be configured dynamically, using SNMP SET requests to the
NET-SNMP-EXTEND-MIB.
MIB-Specific Extension Commands
The first group of extension directives invoke arbitrary commands, and rely on the MIB structure (and
management applications) having the flexibility to accommodate and interpret the output. This is a
convenient way to make information available quickly and simply, but is of no use when implementing
specific MIB objects, where the extension must conform to the structure of the MIB (rather than vice
versa). The remaining extension mechanisms are all concerned with such MIB-specific situations -starting situationsstarting
starting with "pass-through" scripts. Use of this mechanism requires that the agent was built with
support for the ucd-snmp/pass and ucd-snmp/pass_persist modules (which are both included as part of
the default build configuration).
pass [-p priority] MIBOID PROG
will pass control of the subtree rooted at MIBOID to the specified PROG command. GET and GET-NEXT GETNEXT
NEXT requests for OIDs within this tree will trigger this command, called as:
PROG -g OID
PROG -n OID
respectively, where OID is the requested OID. The PROG command should return the response
varbind as three separate lines printed to stdout - the first line should be the OID of the
returned value, the second should be its TYPE (one of the text strings integer, gauge,
counter, timeticks, ipaddress, objectid, or string ), and the third should be the value
itself.
If the command cannot return an appropriate varbind - e.g the specified OID did not correspond
to a valid instance for a GET request, or there were no following instances for a GETNEXT -then GETNEXTthen
then it should exit without producing any output. This will result in an SNMP noSuchName
error, or a noSuchInstance exception.
Note: The SMIv2 type counter64 and SNMPv2 noSuchObject exception are not supported.
A SET request will result in the command being called as:
PROG -s OID TYPE VALUE
where TYPE is one of the tokens listed above, indicating the type of the value passed as the
third parameter.
If the assignment is successful, the PROG command should exit without producing any output.
Errors should be indicated by writing one of the strings not-writable, or wrong-type to std-out, stdout,
out, and the agent will generate the appropriate error response.
Note: The other SNMPv2 errors are not supported.
In either case, the command should exit once it has finished processing. Each request (and
each varbind within a single request) will trigger a separate invocation of the command.
The default registration priority is 127. This can be changed by supplying the optional -p
flag, with lower priority registrations being used in preference to higher priority values.
pass_persist [-p priority] MIBOID PROG
will also pass control of the subtree rooted at MIBOID to the specified PROG command. However
this command will continue to run after the initial request has been answered, so subsequent
requests can be processed without the startup overheads.
Upon initialization, PROG will be passed the string "PING\n" on stdin, and should respond by
printing "PONG\n" to stdout.
For GET and GETNEXT requests, PROG will be passed two lines on stdin, the command (get or get-next) getnext)
next) and the requested OID. It should respond by printing three lines to stdout - the OID
for the result varbind, the TYPE and the VALUE itself - exactly as for the pass directive
above. If the command cannot return an appropriate varbind, it should print print "NONE\n" to
stdout (but continue running).
For SET requests, PROG will be passed three lines on stdin, the command (set) and the
requested OID, followed by the type and value (both on the same line). If the assignment is
successful, the command should print "DONE\n" to stdout. Errors should be indicated by writ-ing writing
ing one of the strings not-writable, wrong-type, wrong-length, wrong-value or inconsistent-value inconsistentvalue
value to stdout, and the agent will generate the appropriate error response. In either case,
the command should continue running.
The registration priority can be changed using the optional -p flag, just as for the pass
directive.
pass and pass_persist extensions can only be configured via the snmpd.conf file. They cannot be set
up via SNMP SET requests.
Embedded Perl Support
Programs using the previous extension mechanisms can be written in any convenient programming lan-guage language
guage - including perl, which is a common choice for pass-through extensions in particular. However
the Net-SNMP agent also includes support for embedded perl technology (similar to mod_perl for the
Apache web server). This allows the agent to interpret perl scripts directly, thus avoiding the
overhead of spawning processes and initializing the perl system when a request is received.
Use of this mechanism requires that the agent was built with support for the embedded perl mechanism,
which is not part of the default build environment. It must be explicitly included by specifying the
'--enable-embedded-perl' option to the configure script when the package is first built.
If enabled, the following directives will be recognised:
disablePerl true
will turn off embedded perl support entirely (e.g. if there are problems with the perl instal-lation). installation).
lation).
perlInitFile FILE
loads the specified initialisation file (if present) immediately before the first perl direc-tive directive
tive is parsed. If not explicitly specified, the agent will look for the default initialisa-tion initialisation
tion file /usr/share/snmp/snmp_perl.pl.
The default initialisation file creates an instance of a NetSNMP::agent object - a variable
$agent which can be used to register perl-based MIB handler routines.
perl EXPRESSION
evaluates the given expression. This would typically register a handler routine to be called
when a section of the OID tree was requested:
perl use Data::Dumper;
perl sub myroutine { print "got called: ",Dumper(@_),"\n"; }
perl $agent->register('mylink', '.1.3.6.1.8765', \&myroutine);
This expression could also source an external file:
perl 'do /path/to/file.pl';
or perform any other perl-based processing that might be required.
Dynamically Loadable Modules
Most of the MIBs supported by the Net-SNMP agent are implemented as C code modules, which were com-piled compiled
piled and linked into the agent libraries when the suite was first built. Such implementation mod-ules modules
ules can also be compiled independently and loaded into the running agent once it has started. Use
of this mechanism requires that the agent was built with support for the ucd-snmp/dlmod module (which
is included as part of the default build configuration).
dlmod NAME PATH
will load the shared object module from the file PATH (an absolute filename), and call the
initialisation routine init_NAME.
Note: If the specified PATH is not a fully qualified filename, it will be interpreted rela-tive relative
tive to /usr/lib/snmp/dlmod, and .so will be appended to the filename.
This functionality can also be configured using SNMP SET requests to the UCD-DLMOD-MIB.
Proxy Support
Another mechanism for extending the functionality of the agent is to pass selected requests (or
selected varbinds) to another SNMP agent, which can be running on the same host (presumably listening
on a different port), or on a remote system. This can be viewed either as the main agent delegating
requests to the remote one, or acting as a proxy for it. Use of this mechanism requires that the
agent was built with support for the ucd-snmp/proxy module (which is included as part of the default
build configuration).
proxy [-Cn CONTEXTNAME] [SNMPCMD_ARGS] HOST OID [REMOTEOID]
will pass any incoming requests under OID to the agent listening on the port specified by the
transport address HOST. See the section LISTENING ADDRESSES in the snmpd(8) manual page for
more information about the format of listening addresses.
Note: To proxy the entire MIB tree, use the OID .1.3 (not the top-level .1)
The SNMPCMD_ARGS should provide sufficient version and administrative information to generate a valid
SNMP request (see snmpcmd(1)).
Note: The proxied request will not use the administrative settings from the original request.
If a CONTEXTNAME is specified, this will register the proxy delegation within the named context in
the local agent. Defining multiple proxy directives for the same OID but different contexts can be
used to query several remote agents through a single proxy, by specifying the appropriate SNMPv3 con-text context
text in the incoming request (or using suitable configured community strings - see the com2sec direc-tive). directive).
tive).
Specifying the REMOID parameter will map the local MIB tree rooted at OID to an equivalent subtree
rooted at REMOID on the remote agent.
SMUX Sub-Agents
The Net-SNMP agent supports the SMUX protocol (RFC 1227) to communicate with SMUX-based subagents
(such as gated, zebra or quagga). Use of this mechanism requires that the agent was built with sup-port support
port for the smux module, which is not part of the default build environment, and must be explicitly
included by specifying the '--with-mib-modules=smux' option to the configure script when the package
is first built.
Note: This extension protocol has been officially deprecated in favour of AgentX (see below).
smuxpeer OID PASS
will register a subtree for SMUX-based processing, to be authenticated using the password
PASS. If a subagent (or "peer") connects to the agent and registers this subtree then
requests for OIDs within it will be passed to that SMUX subagent for processing.
A suitable entry for an OSPF routing daemon (such as gated, zebra or quagga) might be some-thing something
thing like
smuxpeer .1.3.6.1.2.1.14 ospf_pass
smuxsocket <IPv4-address>
defines the IPv4 address for SMUX peers to communicate with the Net-SNMP agent. The default
is to listen on all IPv4 interfaces ("0.0.0.0"), unless the package has been configured with
"--enable-local-smux" at build time, which causes it to only listen on 127.0.0.1 by default.
SMUX uses the well-known TCP port 199.
Note the Net-SNMP agent will only operate as a SMUX master agent. It does not support acting in a
SMUX subagent role.
AgentX Sub-Agents
The Net-SNMP agent supports the AgentX protocol (RFC 2741) in both master and subagent roles. Use of
this mechanism requires that the agent was built with support for the agentx module (which is
included as part of the default build configuration), and also that this support is explicitly
enabled (e.g. via the snmpd.conf file).
There are two directives specifically relevant to running as an AgentX master agent:
master agentx
will enable the AgentX functionality and cause the agent to start listening for incoming
AgentX registrations. This can also be activated by specifying the '-x' command-line option
(to specify an alternative listening socket).
agentXPerms SOCKPERMS [DIRPERMS [USER|UID [GROUP|GID]]]
Defines the permissions and ownership of the AgentX Unix Domain socket, and the parent direc-tories directories
tories of this socket. SOCKPERMS and DIRPERMS must be octal digits (see chmod(1) ). By
default this socket will only be accessible to subagents which have the same userid as the
agent.
There is one directive specifically relevant to running as an AgentX sub-agent:
agentXPingInterval NUM
will make the subagent try and reconnect every NUM seconds to the master if it ever becomes
(or starts) disconnected.
The remaining directives are relevant to both AgentX master and sub-agents:
agentXSocket [<transport-specifier>:]<transport-address>[,...]
defines the address the master agent listens at, or the subagent should connect to. The
default is the Unix Domain socket AGENTX_SOCKET. Another common alternative is tcp:local-host:705. tcp:localhost:705.
host:705. See the section LISTENING ADDRESSES in the snmpd(8) manual page for more informa-tion information
tion about the format of addresses.
Note: Specifying an AgentX socket does not automatically enable AgentX functionality (unlike
the '-x' command-line option).
agentXTimeout NUM
defines the timeout period (NUM seconds) for an AgentX request. Default is 1 second.
agentXRetries NUM
defines the number of retries for an AgentX request. Default is 5 retries.
net-snmp ships with both C and Perl APIs to develop your own AgentX subagent.
OTHER CONFIGURATION
override [-rw] OID TYPE VALUE
This directive allows you to override a particular OID with a different value (and possibly a
different type of value). The -rw flag will allow snmp SETs to modify it's value as well.
(note that if you're overriding original functionality, that functionality will be entirely
lost. Thus SETS will do nothing more than modify the internal overridden value and will not
perform any of the original functionality intended to be provided by the MIB object. It's an
emulation only.) An example:
override sysDescr.0 octet_str "my own sysDescr"
That line will set the sysDescr.0 value to "my own sysDescr" as well as make it modifiable
with SNMP SETs as well (which is actually illegal according to the MIB specifications).
Note that care must be taken when using this. For example, if you try to override a property
of the 3rd interface in the ifTable with a new value and later the numbering within the
ifTable changes it's index ordering you'll end up with problems and your modified value won't
appear in the right place in the table.
Valid TYPEs are: integer, uinteger, octet_str, object_id, counter, null (for gauges, use "uin-teger"; "uinteger";
teger"; for bit strings, use "octet_str"). Note that setting an object to "null" effectively
delete's it as being accessible. No VALUE needs to be given if the object type is null.
More types should be available in the future.
If you're trying to figure out aspects of the various mib modules (possibly some that you've added
yourself), the following may help you spit out some useful debugging information. First off, please
read the snmpd manual page on the -D flag. Then the following configuration snmpd.conf token, com-bined combined
bined with the -D flag, can produce useful output:
injectHandler HANDLER modulename
This will insert new handlers into the section of the mib tree referenced by "modulename".
The types of handlers available for insertion are:
stash_cache
Caches information returned from the lower level. This greatly help the performance of
the agent, at the cost of caching the data such that its no longer "live" for 30 sec-onds seconds
onds (in this future, this will be configurable). Note that this means snmpd will use
more memory as well while the information is cached. Currently this only works for
handlers registered using the table_iterator support, which is only a few mib tables.
To use it, you need to make sure to install it before the table_iterator point in the
chain, so to do this:
injectHandler stash_cache NAME table_iterator
If you want a table to play with, try walking the nsModuleTable with and without this
injected.
debug Prints out lots of debugging information when the -Dhelper:debug flag is passed to the
snmpd application.
read_only
Forces turning off write support for the given module.
serialize
If a module is failing to handle multiple requests properly (using the new 5.0 module
API), this will force the module to only receive one request at a time.
bulk_to_next
If a module registers to handle getbulk support, but for some reason is failing to
implement it properly, this module will convert all getbulk requests to getnext
requests before the final module receives it.
dontLogTCPWrappersConnects
If the snmpd was compiled with TCP Wrapper support, it logs every connection made to the
agent. This setting disables the log messages for accepted connections. Denied connections
will still be logged.
Figuring out module names
To figure out which modules you can inject things into, run snmpwalk on the nsModuleTable
which will give a list of all named modules registered within the agent.
Internal Data tables
table NAME
add_row NAME INDEX(ES) VALUE(S)
NOTES
o The Net-SNMP agent can be instructed to re-read the various configuration files, either via an
snmpset assignment of integer(1) to UCD-SNMP-MIB::versionUpdateConfig.0
(.1.3.6.1.4.1.2021.100.11.0), or by sending a kill -HUP signal to the agent process.
o All directives listed with a value of "yes" actually accept a range of boolean values. These
will accept any of 1, yes or true to enable the corresponding behaviour, or any of _, no or
false to disable it. The default in each case is for the feature to be turned off, so these
directives are typically only used to enable the appropriate behaviour.
EXAMPLE CONFIGURATION FILE
See the EXAMPLE.CONF file in the top level source directory for a more detailed example of how the
above information is used in real examples.
FILES
/etc/snmp/snmpd.conf
SEE ALSO
snmpconf(1), snmpusm(1), snmp.conf(5), snmp_config(5), snmpd(8), EXAMPLE.conf, netsnmp_config_api(3).
V5.6 30 Jun 2010 SNMPD.CONF(5)
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