Provided by: snmpd_5.7.2~dfsg-8.1ubuntu3.3_amd64 
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 distinct
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 population
can not be trusted.
In general, the total number of responses will not be allowed to exceed the maxGetbulkResponses
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 solutions
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.
Hopefully 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 USM users. If your
network manager supports SNMP over (D)TLS or SNMP over SSH we suggest you use one of these mechanisms
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] localCert <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"
utility which will help you create the required certificate.
The default value for this is the certificate in the "snmpd" named certificate file.
[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
certificate 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 securityName 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 instructions
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 certificate.
The "trustCert" token should be used to load specific trust anchors into the verification 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 connections
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" security
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 infrastructures.
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 authentication
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/lib/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 similar
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 password
with a hexstring (preceded 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
separated 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 Kerberos
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 several
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 subtree,
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
subidentifier:
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 processed.
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 configured 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 configuring
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 CONTEXT is
specified, access is configured within this SNMPv3 context (or contexts with this prefix 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 sysName.0)
for the agent respectively. Ordinarily these objects are writable via suitably authorized SNMP
SET requests. However, specifying one of these directives makes the corresponding 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 (application +
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).
interface_fadeout TIMEOUT
specifies, for how long the agent keeps entries in ifTable after appropriate interfaces have been
removed from system (typically various ppp, tap or tun interfaces). Timeout value is in seconds.
Default value is 300 (=5 minutes).
interface_replace_old yes
can be used to remove already existing entries in ifTable when an interface with the same name
appears on the system. E.g. when ppp0 interface is removed, it is still listed in the table for
interface_fadeout seconds. This option ensures, that the old ppp0 interface is removed even before
the interface_fadeout timeour when new ppp0 (with different ifIndex) shows up.
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 illustration
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
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 'Network
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.
realStorageUnits
controlls how the agent reports hrStorageAllocationUnits, hrStorageSize and hrStorageUsed in
hrStorageTable. For big storage drives with small allocation units the agent re-calculates these
values so they all fit Integer32 and hrStorageAllocationUnits x hrStorageSize gives real size of
the storage.
Example:
Linux xfs 16TB filesystem with 4096 bytes large blocks will be reported as
hrStorageAllocationUnits = 8192 and hrStorageSize = 2147483647, so 8192 x 2147483647 gives
real size of the filesystem (=16 TB).
Setting this directive to '1' turns off this calculation and the agent reports real
hrStorageAllocationUnits, but it might report wrong hrStorageSize for big drives because the value
won't fit into Integer32. In this case, hrStorageAllocationUnits x hrStorageSize won't give real
size of the storage.
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 services
(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 -e") 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).
Disk I/O Monitoring
This requires that the agent was built with support for the ucd-snmp/diskio module (which is not included
as part of the default build configuration).
By default, all block devices known to the operating system are included in the diskIOTable. On platforms
other than Linux, this module has no configuration directives.
On Linux systems, it is possible to exclude several classes of block devices from the diskIOTable in
order to avoid cluttering the table with useless zero statistics for pseudo-devices that often are not in
use but are configured by default to exist in most recent Linux distributions.
diskio_exclude_fd yes
Excludes all Linux floppy disk block devices, whose names start with "fd", e.g. "fd0"
diskio_exclude_loop yes
Excludes all Linux loopback block devices, whose names start with "loop", e.g. "loop0"
diskio_exclude_ram yes
Excludes all LInux ramdisk block devices, whose names start with "ram", e.g. "ram0"
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
configuration).
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 message 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 laErrorFlag
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 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/logMatchTable/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
(noSuchObject).
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 appropriate
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 notification behaviour.
authtrapenable {1|2}
determines whether to generate authentication failure traps (enabled(1)) or not (disabled(2) - the
default). Ordinarily the corresponding MIB object (snmpEnableAuthenTraps.0) is read-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 previous
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 notification
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(0) monitor test. A bare OID specifies a present(0) 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 operators
(!=, ==, <, <=, >, >=) 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 threshold (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 sample
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 subidentifiers 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, where FREQUENCY is in seconds or optionally
suffixed by one of s (for seconds), m (for minutes), h (for hours), d (for days), or w (for
weeks). 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 coldStart
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
NOTIFICATION-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 corresponding 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 notification
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 irrelevant.
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 disman/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, where FREQUENCY is in seconds or optionally suffixed by one of s (for seconds),
m (for minutes), h (for hours), d (for days), or w (for weeks).
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 equivalent
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.
Data Delivery via Notfiications
Note: this functionality is only available if the deliver/deliverByNotify mib module was complied in to
the agent
In some situations it may be advantageous to deliver SNMP data over SNMP Notifications (TRAPs and
INFORMs) rather than the typical process of having the manager issue requests for the data (via GETs and
GETNEXTs). Reasons for doing this are numerous, but frequently corner cases. The most common reason for
wanting this behaviour might be to monitor devices that reside behind NATs or Firewalls that prevent
incoming SNMP traffic.
It should be noted that although most management software is capable of logging notifications, very
little (if any) management software will updated their "knowledge database" based on the contents of SNMP
notifications. IE, it won't (for example) update the interface traffic counter history that is used to
produce graphs. Most larger network management packages have a separate database for storing data
received via SNMP requests (GETs and GETNEXTs) vs those received from notifications. Researching the
capabilities of your management station software is required before assuming this functionality will
solve your data delivery requirements.
Notifications generated via this mechanism will be sent to the standard set of configured notification
targets. See the "Notification Handling" section of this document for further information.
deliverByNotify [-p] [-m] [-s MAXSIZE] FREQUENCY OID
This directive tells the SNMP agent to self-walk the OID, collect all the data and send it out
every FREQUENCY seconds, where FREQUENCY is in seconds or optionally suffixed by one of s (for
seconds), m (for minutes), h (for hours), d (for days), or w (for weeks). By default scalars are
included in the notification that specify the how often the notification will be sent (unless the
-p option is specified) and which message number of how many messages a particular notification is
(unless -m is specified). To break the notifications into manageable packet sizes, use the -s
flag to specify the approximate maximum number of bytes that a notification message should be
limited to. If more than MAXSIZE of bytes is needed then multiple notifications will be sent to
deliver the data. Note that the calculations for ensuring the maximum size is met are
approximations and thus it can be absolutely guaranteed they'll be under that size, so leave a
padding buffer if it is critical that you avoid fragmentation. A value of -1 indicates force
everything into a single message no matter how big it is.
Example usage: the following will deliver the contents of the ifTable once an hour and the
contents of the system group once every 2 hours:
deliverByNotify 3600 ifTable
deliverByNotify 7200 system
deliverByNotifyMaxPacketSize SIZEINBYTES
Sets the default notification size limit (see the -s flag above).
deliverByNotifyOid OID
deliverByNotifyFrequencyOid OID
deliverByNotifyMessageNumberOid OID
deliverByNotifyMaxMessageNumberOid OID
These set the data OID that the notification will be sent under, the scalar OID, the message
number OID, and the maximum message number OID. These default to objects in the
NET-SNMP-PERIODIC-NOTIFY-MIB.
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
configuring the running agent to report additional information. There are a number of techniques to
support 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, returning
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 directive
(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 directive,
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 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 GETNEXT
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 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 stdout, 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 getnext)
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 writing one of the strings
not-writable, wrong-type, wrong-length, wrong-value or inconsistent-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 language -
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
installation).
perlInitFile FILE
loads the specified initialisation file (if present) immediately before the first perl directive
is parsed. If not explicitly specified, the agent will look for the default initialisation 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 compiled
and linked into the agent libraries when the suite was first built. Such implementation modules 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 relative to
/usr/lib/x86_64-linux-gnu/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 context in the
incoming request (or using suitable configured community strings - see the com2sec directive).
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 support 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 something
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 directories
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 "/var/agentx/master". Another common alternative is tcp:localhost:705.
See the section LISTENING ADDRESSES in the snmpd(8) manual page for more information 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. NUM also be
specified with a suffix of one of s (for seconds), m (for minutes), h (for hours), d (for days),
or w (for weeks).
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
"uinteger"; 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, combined with
the -D flag, can produce useful output:
injectHandler HANDLER modulename [beforeThis]
This will insert new handlers into the section of the mib tree referenced by "modulename". If
"beforeThis" is specified then the module will be injected before the named module. This is
useful for getting a handler into the exact right position in the chain.
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 seconds (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 0, 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).