Simple Network Management Protocol (SNMP) is an "Internet-standard protocol for managing devices on IP networks". Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks and more. SNMP is widely used in network management systems to monitor network-attached devices for conditions that warrant administrative attention. SNMP is a component of the Internet Protocol Suite as defined by the Internet Engineering Task Force (IETF). 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
Principle of SNMP Communication
In typical uses of SNMP one or more administrative computers, called managers, have the task of monitoring or managing a group of hosts or devices on a computer network. Each managed system executes, at all times, a software component called an agent which reports information via SNMP to the manager.
SNMP agents expose management data on the managed systems as variables. The protocol also permits active management tasks, such as modifying and applying a new configuration through remote modification of these variables. 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).
An SNMP-managed network consists of three key components:
- Managed device
- Agent — software which runs on managed devices
- Network management station (NMS) — software which runs on the manager
A managed device is a network node that implements an SNMP interface that allows unidirectional (read-only) or bidirectional (read and write) access to node-specific information. Managed devices exchange node-specific information with the NMSs. Sometimes called network elements, the managed devices can be any type of device, including, but not limited to, routers, access servers, switches, cable modems, bridges, hubs, IP telephones, IP video cameras, computer hosts, and printers.
An agent is a network-management software module that resides on a managed device. An agent has local knowledge of management information and translates that information to or from an SNMP-specific form.
A network management station (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.
Management information base (MIB)
Main article: Management information base
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 (MIBs). MIBs describe the structure of the management data of a device subsystem; they use a hierarchical namespace containing object identifiers (OID). Each OID identifies a variable that can be read or set via SNMP. MIBs use the notation defined by Structure of Management Information Version 2.0 (SMIv2, RFC 2578), a subset of ASN.1.
SNMP operates in the Application Layer of the Internet Protocol Suite (Layer 7 of the OSI model). The SNMP agent receives requests on UDP port 161. The manager may send requests from any available source port to port 161 in the agent. The agent response will be sent back to the source port on the manager. The manager receives notifications (Traps and InformRequests) on port 162. The agent may generate notifications from any available port. When used with Transport Layer Security or Datagram Transport Layer Security requests are received on port 10161 and traps are sent to port 10162.
SNMPv1 specifies five core protocol data units (PDUs). Two other PDUs, GetBulkRequest and InformRequest were added in SNMPv2 and the Report PDU was added in SNMPv3.
All SNMP PDUs are constructed as follows:
|IP header||UDP header||version||community||PDU-type||request-id||error-status||error-index||variable bindings|
The seven SNMP protocol data unit (PDU) types are as follows:
GetRequestA manager-to-agent request to retrieve the value of a variable or list of variables. Desired variables are specified in variable bindings (values are not used). Retrieval of the specified variable values is to be done as an atomic operation by the agent. A Response with current values is returned.SetRequestA manager-to-agent request to change the value of a variable or list of variables. Variable bindings are specified in the body of the request. Changes to all specified variables are to be made as an atomic operation by the agent. A Response with (current) new values for the variables is returned.GetNextRequestA manager-to-agent request to discover available variables and their values. Returns a Response with variable binding for the lexicographically next variable in the MIB. The entire MIB of an agent can be walked by iterative application of GetNextRequest starting at OID 0. Rows of a table can be read by specifying column OIDs in the variable bindings of the request.GetBulkRequestOptimized version of GetNextRequest. A manager-to-agent request for multiple iterations of GetNextRequest. Returns a Response with multiple variable bindings walked from the variable binding or bindings in the request. PDU specific non-repeaters and max-repetitions fields are used to control response behavior. GetBulkRequest was introduced in SNMPv2.ResponseReturns variable bindings and acknowledgement from agent to manager for GetRequest, SetRequest, GetNextRequest, GetBulkRequest and InformRequest. Error reporting is provided by error-status and error-index fields. Although it was used as a response to both gets and sets, this PDU was called GetResponse in SNMPv1.TrapAsynchronous notification from agent to manager. SNMP traps enable an agent to notify the management station of significant events by way of an unsolicited SNMP message. Includes current sysUpTime value, an OID identifying the type of trap and optional variable bindings. Destination addressing for traps is determined in an application-specific manner typically through trap configuration variables in the MIB. The format of the trap message was changed in SNMPv2 and the PDU was renamed SNMPv2-Trap. While in classic communication the client always actively requests information from the server, SNMP allows the additional use of so-called "traps". These are data packages that are sent from the SNMP client to the server without being explicitly requested.InformRequestAcknowledged asynchronous notification. This PDU was introduced in SNMPv2 and was originally defined as manager to manager communication. Later implementations have loosened the original definition to allow agent to manager communications. Manager-to-manager notifications were already possible in SNMPv1 (using a Trap), but as SNMP commonly runs over UDP where delivery is not assured and dropped packets are not reported, delivery of a Trap was not guaranteed. InformRequest fixes this by sending back an acknowledgement on receipt.
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 the 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 GetBulkRequest, an alternative to iterative GetNextRequests for retrieving large amounts of management data in a single request. However, the new party-based security system in SNMPv2, viewed by many as overly complex, was not widely accepted. This version of SNMP reached the Proposed Standard level of maturity, but was deemed obsoleted by later versions.
Community-Based Simple Network Management Protocol version 2, or SNMPv2c, is defined in RFC 1901–RFC 1908. SNMPv2c comprises SNMPv2 without the controversial new SNMP v2 security model, using instead the simple community-based security scheme of SNMPv1. This version is one of relatively few standards to meet the IETF's Draft Standard maturity level, and was widely considered the de facto SNMPv2 standard. It too was later obsoleted, by SNMPv3.
User-Based Simple Network Management Protocol version 2, or SNMPv2u, is defined in RFC 1909–RFC 1910. This is a compromise that attempts to offer greater security than SNMPv1, but without incurring the high complexity of SNMPv2. 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, SNMPv2c 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 2576 defines two possible SNMPv1/v2c coexistence strategies: proxy agents and bilingual network-management systems.
An SNMPv2 agent can act as a proxy agent on behalf of SNMPv1 managed devices, as follows:
- An SNMPv2 NMS issues a command intended for an SNMPv1 agent.
- The NMS sends the SNMP message to the SNMPv2 proxy agent.
- The proxy agent forwards
Setmessages to the SNMPv1 agent unchanged.
- GetBulk messages are converted by the proxy agent to
GetNextmessages 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.
Although SNMPv3 makes no changes to the protocol aside from the addition of cryptographic security, it looks much different due to new textual conventions, concepts, and terminology.
SNMPv3 primarily added security and remote configuration enhancements to SNMP. Due to lack of security with the use of SNMP, network administrators were using other means, such as telnet for configuration, accounting, and fault management.
SNMPv3 addresses issues related to the large-scale deployment of SNMP, accounting, and fault management. Currently, SNMP is predominantly used for monitoring and performance management.
SNMPv3 defines a secure version of SNMP and also facilitates remote configuration of the SNMP entities.
SNMPv3 provides a secure environment for the management of systems covering the following:
- Identification of SNMP entities to facilitate communication only between known SNMP entities - Each SNMP entity has an identifier called the SNMPEngineID, and SNMP communication is possible only if an SNMP entity knows the identity of its peer. Traps and Notifications are exceptions to this rule.
- Support for security models - A security model may define the security policy within an administrative domain or an intranet. SNMPv3 contains the specifications for USM (User-based Security Model).
- Definition of security goals where the goals of message authentication service include protection against the following:
- Modification of Information - Protection against some unauthorized SNMP entity altering in-transit messages generated by an authorized principal.
- Masquerade - Protection against attempting management operations not authorized for some principal by assuming the identity of another principal that has the appropriate authorizations.
- Message Stream Modification - Protection against messages getting maliciously re-ordered, delayed, or replayed to effect unauthorized management operations.
- Disclosure - Protection against eavesdropping on the exchanges between SNMP engines.
- Specification for USM - USM (User-based Security Model) consists of the general definition of the following communication mechanisms available:
- Communication without authentication and privacy (NoAuthNoPriv).
- Communication with authentication and without privacy (AuthNoPriv).
- Communication with authentication and privacy (AuthPriv).
- Definition of different authentication and privacy protocols - Currently, the MD5 and SHA authentication protocols and the CBC_DES and CFB_AES_128 privacy protocols are supported in the USM. Operations and Management Area Working Group (OpsAWG) of IETF is currently (March 2015) advancing HMAC-SHA-2 authentication protocols for USM.
- Definition of a discovery procedure - To find the SNMPEngineID of an SNMP entity for a given transport address and transport endpoint address.
- Definition of the time synchronization procedure - To facilitate authenticated communication between the SNMP entities.
- Definition of the SNMP framework MIB - To facilitate remote configuration and administration of the SNMP entity.
- Definition of the USM MIBs - To facilitate remote configuration and administration of the security module.
- Definition of the VACM MIBs - To facilitate remote configuration and administration of the access control module.
SNMPv3 focuses on two main aspects, namely security and administration. The security aspect is addressed by offering both strong authentication and data encryption for privacy. The administration aspect is focused on two parts, namely notification originators and proxy forwarders.
SNMPv3 defines a number of security-related capabilities. The initial specifications defined the USM and VACM, which were later followed by a transport security model that provided support for SNMPv3 over SSH and SNMPv3 over TLS and DTLS.
- USM (User-based Security Model) provides authentication and privacy (encryption) functions and operates at the message level.
- VACM (View-based Access Control Model) determines whether a given principal is allowed access to a particular MIB object to perform specific functions and operates at the PDU level.
- TSM (Transport Security Mode) provides a method for authenticating and encrypting messages over external security channels. Two transports, SSH and TLS/DTLS, have been defined that make use of the TSM specification.
Security has been the biggest weakness of SNMP since the beginning. Authentication in SNMP Versions 1 and 2 amounts to nothing more than a password (community string) sent in clear text between a manager and agent.
Each SNMPv3 message contains security parameters which are encoded as an octet string. The meaning of these security parameters depends on the security model being used.
SNMPv3 provides important security features:
- Confidentiality - Encryption of packets to prevent snooping by an unauthorized source.
- Integrity - Message integrity to ensure that a packet has not been tampered while in transit including an optional packet replay protection mechanism.
- Authentication - to verify that the message is from a valid source.
As of 2004 the IETF 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. 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 variously "Historic" or "Obsolete").
In practice, SNMP implementations often support multiple versions: typically SNMPv1, SNMPv2c, and SNMPv3.
SNMP implementations vary across platform vendors. In some cases, SNMP is 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. Consequently, processing SNMP queries 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.
Some SNMP values (especially tabular values) require specific knowledge of table indexing schemes, and these index values are not necessarily consistent across platforms. This can cause correlation issues when fetching information from multiple devices that may not employ the same table indexing scheme (for example fetching disk utilization metrics, where a specific disk identifier is different across platforms.)