Simple Network Management Protocol
) forms part of the internet protocol suite
as defined by the Internet Engineering Task Force
(IETF). SNMP is used in network management systems
network-attached devices for conditions that warrant administrative attention. It consists of a set of standards
for network management, including an Application Layer protocol
, a database schema
, and a set of data objects
SNMP exposes management data in the form of variables on the managed systems, which describe the system configuration. These variables can then be queried (and sometimes set) by managing applications.
Overview and basic concepts
In typical SNMP usage, there are a number of systems to be managed, and one or more systems managing them. A software component called an agent
(see below) runs on each managed system and reports information via SNMP to the managing systems.
Essentially, SNMP agents expose management data on the managed systems as variables (such as "free memory", "system name", "number of running processes", "default route"). But the protocol also permits active management tasks, such as modifying and applying a new configuration.
The managing system can retrieve the information through the GET, GETNEXT and GETBULK protocol operations or the agent will send data without being asked using TRAP or INFORM protocol operations. Management systems can also send configuration updates or controlling requests through the SET protocol operation to actively manage a system. Configuration and control operations are used only when changes are needed to the network infrastructure. The monitoring operations are usually performed on a regular basis.
The variables accessible via SNMP are organized in hierarchies. These hierarchies, and other metadata (such as type and description of the variable), are described by Management Information Bases (MIBs).
SNMP is part of the Internet network management architecture. This architecture is based on the interaction of many entities, as described in the following section.
The Internet Management Model
As specified in Internet RFCs and other documents, a network management system comprises:
- Network elements -- Sometimes called managed devices, network elements are hardware devices such as computers, routers, and terminal servers that are connected to networks.
- Agents -- Agents are software modules that reside in network elements. They collect and store management information such as the number of error packets received by a network element.
- Managed object -- A managed object is a characteristic of something that can be managed. For example, a list of currently active TCP circuits in a particular host computer is a managed object. Managed objects differ from variables, which are particular object instances. Using our example, an object instance is a single active TCP circuit in a particular host computer. Managed objects can be scalar (defining a single object instance) or tabular (defining multiple, related instances).
- Management information base (MIB) -- A MIB is a collection of managed objects residing in a virtual information store. Collections of related managed objects are defined in specific MIB modules.
- Syntax notation -- A syntax notation is a language used to describe a MIB's managed objects in a machine-independent format. Consistent use of a syntax notation allows different types of computers to share information. Internet management systems use a subset of the International Organization for Standardization's (ISO's) Open System Interconnection (OSI) Abstract Syntax Notation (ASN.1) to define both the packets exchanged by the management protocol and the objects that are to be managed.
- Structure of Management Information (SMI) -- The SMI defines the rules for describing management information. The SMI is defined using ASN.1.
- Network management stations (NMSs) -- Sometimes called consoles, these devices execute management applications that monitor and control network elements. Physically, NMSs are usually engineering workstation-caliber computers with fast CPUs, megapixel color displays, substantial memory, and abundant disk space. At least one NMS must be present in each managed environment.
- Parties -- Newly defined in SNMPv2, a party is a logical SNMPv2 entity that can initiate or receive SNMPv2 communication. Each SNMPv2 party comprises a single, unique party identity, a logical network location, a single authentication protocol, and a single privacy protocol. SNMPv2 messages are communicated between two parties. An SNMPv2 entity can define multiple parties, each with different parameters. For example, different parties can use different authentication and/or privacy protocols.
- Management protocol -- A management protocol is used to convey management information between agents and NMSs. SNMP is the Internet community's de facto standard management protocol.
Management Information Bases (MIBs)
SNMP itself does not define which information (which variables) a managed system should offer. Rather, SNMP uses an extensible design, where the available information is defined by management information bases
s). MIBs describe the structure of the management data of a device subsystem; they use a hierarchical namespace
containing object identifiers
). Each OID identifies a variable that can be read or set via SNMP. MIBs use the notation defined by ASN.1
The MIB hierarchy can be depicted as a tree with a nameless root, the levels of which are assigned by different organizations. The top-level MIB OIDs belong to different standards organizations, while lower-level object IDs are allocated by associated organizations. This model permits management across all layers of the OSI reference model, extending into applications such as databases, email, and the Java EE reference model, as MIBs can be defined for all such area-specific information and operations.
A managed object (sometimes called a MIB object, an object, or a MIB) is one of any number of specific characteristics of a managed device. Managed objects are made up of one or more object instances (identified by their OIDs), which are essentially variables.
Two types of managed objects exist:
- Scalar objects define a single object instance.
- Tabular objects define multiple related object instances that are grouped in MIB tables.
An example of a managed object is atInput, which is a scalar object that contains a single object instance, the integer value that indicates the total number of input AppleTalk packets on a router interface.
An object identifier (or object ID or OID) uniquely identifies a managed object in the MIB hierarchy.
Abstract Syntax Notation One (ASN.1)
In telecommunications and computer networking, Abstract Syntax Notation One (ASN.1
) is a standard and flexible notation that describes data structures
for representing, encoding
, transmitting, and decoding data. It provides a set of formal rules for describing the structure of objects that are independent of machine-specific encoding techniques and is a precise, formal notation that removes ambiguities.
ASN.1 is a joint ISO and ITU-T standard, originally defined in 1984 as part of 1984. ASN.1 moved to its own standard, X.208, in 1988 due to wide applicability. The substantially revised 1995 version is covered by the X.680 series.''
An adapted subset of ASN.1, Structure of Management Information (SMI), is specified in SNMP to define sets of related MIB objects; these sets are termed MIB modules.
SNMP basic components
An SNMP-managed network consists of three key components:
- Managed devices
- Network-management systems (NMSs)
A managed device is a network node that contains an SNMP agent and that resides on a managed network. Managed devices collect and store management information and make this information available to NMSs using SNMP. Managed devices, sometimes called network elements, can be any type of device including, but not limited to, routers, access servers, switches, bridges, hubs, IP telephones, computer hosts, and printers.
An agent is a network-management software module that resides in a managed device. An agent has local knowledge of management information and translates that information into a form compatible with SNMP.
A network management system (NMS) executes applications that monitor and control managed devices. NMSs provide the bulk of the processing and memory resources required for network management. One or more NMSs may exist on any managed network.
SNMPv1 and SMI-specific data types
The SNMPv1 SMI
specifies the use of a number of SMI-specific data types, which are divided into two categories:
- Simple data types
- Application-wide data types
Simple data types
Three simple data types are defined in the SNMPv1 SMI, all of which are unique values:
- The integer data type is a signed integer in the range of -231 to 231-1.
- Octet strings are ordered sequences of 0 to 65,535 octets.
- Object IDs come from the set of all object identifiers allocated according to the rules specified in ASN.1.
Application-wide data types
The following seven application-wide data types exist in the SNMPv1 SMI:
- Network addresses represent addresses from a particular protocol family. SMIv1 supports only 32-bit (IPv4) addresses (SMIv2 uses Octet Strings to represent addresses generically, and thus are usable in SMIv1 too. SMIv1 had an explicit IPv4 address datatype.)
- Counters are non-negative integers that increase until they reach a maximum value and then roll over to zero. SNMPv1 specifies a counter size of 32 bits.
- Gauges are non-negative integers that can increase or decrease between specified minimum and maximum values. Whenever the system property represented by the gauge is outside of that range, the value of the gauge itself will vary no further than the respective maximum or minimum, as specified in RFC 2578.
- Time ticks represent time since some event, measured in hundredths of a second.
- Opaques represent an arbitrary encoding that is used to pass arbitrary information strings that do not conform to the strict data typing used by the SMI.
- Integers represent signed integer-valued information. This data type redefines the integer data type, which has arbitrary precision in ASN.1 but bounded precision in the SMI.
- Unsigned integers represent unsigned integer-valued information, which is useful when values are always non-negative. This data type redefines the integer data type, which has arbitrary precision in ASN.1 but bounded precision in the SMI.
SNMPv1 MIB tables
The SNMPv1 SMI defines highly structured tables that are used to group the instances of a tabular object (that is, an object that contains multiple variables). Tables are composed of zero or more rows, which are indexed in a way that allows SNMP to retrieve or alter an entire row with a single Get, GetNext, or Set command.
SNMPv2 and structure of management information
The SNMPv2 SMI is described in RFC 2578. It makes certain additions and enhancements to the SNMPv1 SMI-specific data types, such as including bit strings, network addresses, and counters. Bit strings are defined only in SNMPv2 and comprise zero or more named bits that specify a value. Network addresses represent an address from a particular protocol family. Counters are non-negative integers that increase until they reach a maximum value and then return to zero. In SNMPv1, a 32-bit counter size is specified. In SNMPv2, 32-bit and 64-bit counters are defined.
The SNMP protocol operates at the application layer (layer 7) of the OSI model. It specifies (in version 1) five core protocol data units (PDUs):
- GET REQUEST - used to retrieve a piece of management information.
- GETNEXT REQUEST - used iteratively to retrieve sequences of management information.
- GET RESPONSE - used by the agent to respond with data to get and set requests from the manager.
- SET REQUEST - used to initialize and make a change to a value of the network element.
- TRAP - used to report an alert or other asynchronous event about a managed subsystem. In SNMPv1, asynchronous event reports are called traps while they are called notifications in later versions of SNMP. In SMIv1 MIB modules, traps are defined using the TRAP-TYPE macro; in SMIv2 MIB modules, traps are defined using the NOTIFICATION-TYPE macro.
Other PDUs were added in SNMPv2, including:
- GETBULK REQUEST - a faster iterator used to retrieve sequences of management information.
- INFORM - similar to a TRAP but the receiver must respond with an acknowledgement RESPONSE message.
- REPORT - definable by an administrative framework.
Typically, SNMP uses UDP ports 161 for the agent and 162 for the manager. The Manager may send Requests from any available ports (source port) to port 161 in the agent (destination port). The agent response will be given back to the source port. The Manager will receive traps on port 162. The agent may generate traps from any available port.
Many distributions change this, however, and this is not necessarily always true.
SNMPv2 SMI information modules
The SNMPv2 SMI also specifies information modules, which specify a group of related definitions. Three types of SMI information modules exist: MIB modules, compliance statements, and capability statements.
- MIB modules contain definitions of interrelated managed objects.
- Compliance statements provide a systematic way to describe a group of managed objects that must be implemented for conformance to a standard.
- Capability statements are used to indicate the precise level of support that an agent claims with respect to a MIB group. A NMS can adjust its behavior toward agents according to the capabilities statements associated with each agent.
SNMPv3 is defined by RFC 3411–RFC 3418 (also known as 'STD0062').
SNMPv3 primarily added security and remote configuration enhancements to SNMP. SNMPv3 is the current standard version of SNMP as of 2004
. The IETF
has designated SNMPv3 a full Internet Standard
, the highest maturity level
for an RFC. It considers earlier versions to be obsolete (designating them "Historic"). In December 1997 the "Simple Times" newsletter published several articles written by the SNMPv3 RFC editors explaining some of the ideas behind version 3 specifications.
SNMPv3 provides important security features:
- Message integrity to ensure that a packet has not been tampered with in transit.
- Authentication to verify that the message is from a valid source.
- Encryption of packets to prevent snooping by an unauthorized source.
Development and usage
SNMP version 1 (SNMPv1) is the initial implementation of the SNMP protocol. SNMPv1 operates over protocols such as User Datagram Protocol (UDP), Internet Protocol (IP), OSI Connectionless Network Service (CLNS), AppleTalk Datagram-Delivery Protocol (DDP), and Novell Internet Packet Exchange (IPX). SNMPv1 is widely used and is the de facto network-management protocol in the Internet community.
The first RFCs for SNMP, now known as SNMPv1, appeared in 1988:
- RFC 1065 — Structure and identification of management information for TCP/IP-based internets
- RFC 1066 — Management information base for network management of TCP/IP-based internets
- RFC 1067 — A simple network management protocol
These protocols were obsoleted by:
- RFC 1155 — Structure and identification of management information for TCP/IP-based internets
- RFC 1156 — Management information base for network management of TCP/IP-based internets
- RFC 1157 — A simple network management protocol
After a short time, RFC 1156 (MIB-1) was replaced by more often used:
- RFC 1213 — Version 2 of management information base (MIB-2) for network management of TCP/IP-based internets
Version 1 has been criticized for its poor security. Authentication of clients is performed only by a "community string", in effect a type of password, which is transmitted in cleartext. The '80s design of SNMP V1 was done by a group of collaborators who viewed the officially sponsored OSI/IETF/NSF (National Science Foundation) effort (HEMS/CMIS/CMIP) as both unimplementable in the computing platforms of the time as well as potentially unworkable. SNMP was approved based on a belief that it was an interim protocol needed for taking steps towards large scale deployment of the Internet and its commercialization. In that time period Internet-standard authentication/security was both a dream and discouraged by focused protocol design groups.
SNMPv2 (RFC 1441–RFC 1452), revises version 1 and includes improvements in the areas of performance, security, confidentiality, and manager-to-manager communications. It introduced GETBULK, an alternative to iterative GETNEXTs for retrieving large amounts of management data in a single request. However, the new party-based security system in SNMP v2, viewed by many as overly complex, was not widely accepted.
Community-Based Simple Network Management Protocol version 2, or SNMPv2c, is defined in RFC 1901–RFC 1908. In its initial stages, this was also informally known as SNMP v1.5. SNMP v2c comprises SNMP v2 without the controversial new SNMP v2 security model, using instead the simple community-based security scheme of SNMP v1. While officially only a "Draft Standard", this is widely considered the de facto SNMP v2 standard.
User-Based Simple Network Management Protocol version 2, or SNMP v2u, is defined in RFC 1909–RFC 1910. This is a compromise that attempts to offer greater security than SNMP v1, but without incurring the high complexity of SNMP v2. A variant of this was commercialized as SNMP v2*, and the mechanism was eventually adopted as one of two security frameworks in SNMP v3.
SNMPv1 & SNMPv2c interoperability
As presently specified, SNMPv2 is incompatible with SNMPv1 in two key areas: message formats and protocol operations. SNMPv2c messages use different header and protocol data unit (PDU) formats from SNMPv1 messages. SNMPv2c also uses two protocol operations that are not specified in SNMPv1. Furthermore, RFC 1908 defines two possible SNMPv1/v2c coexistence strategies: proxy agents and bilingual network-management systems.
A SNMPv2 agent can act as a proxy agent on behalf of SNMPv1 managed devices, as follows:
- A SNMPv2 NMS issues a command intended for a SNMPv1 agent.
- The NMS sends the SNMP message to the SNMPv2 proxy agent.
- The proxy agent forwards Get, GetNext, and Set messages to the SNMPv1 agent unchanged.
- GetBulk messages are converted by the proxy agent to GetNext messages and then are forwarded to the SNMPv1 agent.
The proxy agent maps SNMPv1 trap messages to SNMPv2 trap messages and then forwards them to the NMS.
Bilingual network-management system
Bilingual SNMPv2 network-management systems support both SNMPv1 and SNMPv2. To support this dual-management environment, a management application in the bilingual NMS must contact an agent. The NMS then examines information stored in a local database to determine whether the agent supports SNMPv1 or SNMPv2. Based on the information in the database, the NMS communicates with the agent using the appropriate version of SNMP.
recognizes Simple Network Management Protocol version 3
as defined by RFC 3411–RFC 3418 (also known as STD0062) as the current standard version of SNMP as of 2004
. The IETF considers earlier versions as "Obsolete" or "Historical".
In practice, SNMP implementations often support multiple versions: typically SNMPv1, SNMPv2c, and SNMPv3. See RFC 3584 "Coexistence between Version 1, Version 2, and Version 3 of the Internet-standard Network Management Framework".
SNMPv3 provides three important services: authentication, privacy and access control.
- Monitoring device uptimes (sysUpTimeInstance)
- Inventory of OS versions (sysDescr)
- Collect interface information (ifName, ifDescr, ifSpeed, ifType, ifPhysAddr)
- Measuring network interface throughput (ifInOctets, ifOutOctets)
- Querying a remote ARP cache (ipNetToMedia)
Other SNMP topics
SNMP by itself is simply a protocol for collecting and organizing information. Most toolsets implementing SNMP offer some form of discovery mechanism, a standardized collection of data common to most platforms and devices, to get a new user or implementor started. One of these features is often a form of automatic discovery, where new devices discovered in the network are polled automatically. For SNMPv1 and SNMPv2c, this presents a security risk, in that your SNMP read communities will be broadcast in cleartext to the target device. While security requirements and risk profiles vary from organization to organization, care should be taken when using a feature like this, with special regard to common environments such as mixed-tenant datacenters, server hosting and colocation facilities, and similar environments.
SNMP implementations vary across platform vendors. In some cases, SNMP is often an added feature, and is not taken seriously enough to be an element of the core design. Some major equipment vendors tend to over extend their proprietary Command Line Interface (CLI)
centric configuration and control systems .
SNMP's seemingly simple tree structure and linear indexing may not always be understood well enough within the internal data structures that are elements of a platform's basic design. As a result, processing SNMP query on certain data sets may result in higher CPU utilization than necessary. One example of this would be large routing tables, such as BGP or IGP .
Modular devices may dynamically increase or decrease their SNMP indexes (aka instances) whenever slotted hardware is added or removed. Although this is most common with hardware, virtual interfaces have the same effect. Index values are typically assigned at boot time and remain fixed until the next reboot. Hardware or virtual entities added while the device is 'live' may have their indexes assigned at the end of the existing range and possibly reassigned at the next reboot. Network inventory and monitoring tools need to account for this behavior and properly react to the cold start trap from the device reboot in order to avoid corruption and mismatch of polled data.
You cannot assume the index for a SNMP device instance is consistent poll to poll. If you need information for a particular interface, it is always prudent to determine the SNMP index before retrieving the data you need at each poll. Generally, a description table like ifDescr will map a user friendly name like Serial 0/1 (Blade 0, port 1) to a SNMP index. Once you know the index for Serial 0/1 you can then retrieve the SNMP data of interest.
Shifting SNMP indexes is a common issue with SNMP based management in general. Always build a programmatic routine to verify the SNMP instance you are monitoring is the physical/virtual instance you think it is.
Plants and data centers
and appliance operating temperatures and room humidity could be monitored remotely for SNMP enabled HVAC
- SNMP versions 1 and 2c are subject to packet sniffing of the clear text community string from the network traffic, because they do not implement encryption.
- All versions of SNMP are subject to brute force and dictionary attacks for guessing the community strings/authentication strings/authentication keys/encryption strings/encryption keys, because they do not implement a challenge-response handshake. Entropy is an important consideration when selecting keys, passwords and/or algorithms.
- Although SNMP works over TCP and other protocols, it is most commonly used over UDP which is connectionless and vulnerable to IP spoofing attacks. Thus, all versions are subject to bypassing device access lists that might have been implemented to restrict SNMP access, though SNMPv3's other security mechanisms should prevent a successful attack.
- SNMP's configuration (write) capabilities can be misconfigured and used to cause severe damage. These 'write' capabilities are very rarely used in practice, partly due to lack of security in SNMP versions before SNMPv3 and partly due to the fact that many devices do not implement SNMP configuration interfaces.
- SNMP tops the list of the SANS Institute's Common Default Configuration Issues with the issue of default SNMP community strings set to ‘public’ and ‘private’ and is number ten on the SANS The Top 10 Most Critical Internet Security Threats for the year 2000.
For more detail on SNMP security implications see the CERT SNMP Vulnerabilities FAQ
- RFC 1155 — Structure and Identification of Management Information for the TCP/IP-based Internets
- RFC 1156 — Management Information Base for Network Management of TCP/IP-based internets
- RFC 1157 — A Simple Network Management Protocol (SNMP)
- RFC 1441 — Introduction to version 2 of the Internet-standard Network Management Framework
- RFC 1213 — Management Information Base for Network Management of TCP/IP-based internets: MIB-II
- RFC 3410 (Informational) — Introduction and Applicability Statements for Internet Standard Management Framework
- RFC 3411 (Standard 62) — An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks
- RFC 3412 (Standard 62) — Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)
- RFC 3413 (Standard 62) — Simple Network Management Protocol (SNMP) Application
- RFC 3414 (Standard 62) — User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)
- RFC 3415 (Standard 62) — View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)
- RFC 3416 (Standard 62) — Version 2 of the Protocol Operations for the Simple Network Management Protocol (SNMP)
- RFC 3417 (Standard 62) — Transport Mappings for the Simple Network Management Protocol (SNMP)
- RFC 3418 (Standard 62) — Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)
- RFC 3584 (Best Current Practice) — Coexistence between Version 1, Version 2, and Version 3 of the Internet-standard Network Management Framework
- RFC 3826 (Proposed) — The Advanced Encryption Standard (AES) Cipher Algorithm in the SNMP User-based Security Model