# SNMP
[TOC]
## What is SNMP?
- **Simple Network Management Protocol**
- An **application-level protocol** which is used to **communicate management information** between the managers and the agents in the network elements.
### Managers and Agents
- **Manager** (management station)
- A host runs the **SNMP client program**.
- **Agent** (managed station)
- A host or a router that runs **SNMP server program**.
> The agent keeps performance information in a database. The manager has access to the values in the database.
### Concepts
> What exactly does SNMP do?
1. A manager checks an agent by requesting information that reflects the behavior of the agent.
2. A manager forces an agent to perform a task by resetting values in the agent database.
3. An agent contributes to the management process by warning the manager of an unusual situation.

### UDP ports 161/162
- SNMP use the services of UDP on two well-known ports
- **161**
- The agent(server) issues a passive open on port 161, waits for a connection from a manager (client).
- **The request messages are sent from the client's ephemeral port to the server's port 161.**
> Also, the response messages will sent back through the same two ports(server's port 161 --> client's ephemeral port).
- **162**
- The manager(client) issues a passive open on port 162, waits for a connection from an agent (server).
- **The Trap messages are sent from the server's ephemeral port to the client's port 162.**
> This connection is only **one-way**, from the server to the client.
:::info
- **Trap Message**
- An unsolicited message sent from an agent to the the manager.
- **Allow the remote devices to alert the manager in case an important event happens.**
- Traps don't need a status request from the manager(client).
:::
:::warning
Notice that **request messages are initiated by a manager(client)**, but **Trap messages are initiated by an agent(server)**.
Here both the client and the server use well-known ports. In addition, both the client and the server are running infinitely.
:::

## Framework
- Actually, the management on the Internet is done through the cooperation of three protocols -- SNMP, SMI, MIB.
### Analogy of three protocols -- SNMP/SMI/MIB
### Role of SNMP
- **Defines the format of the packet** to be sent from a manager to an agent and vice versa.
- The packets exchanged contain **the object(variable) names and their status(values)**. SNMP is responsible for **reading and changing** these values.
### Role of SMI
- **Structure of Management Information**
- [RFC 1155](https://datatracker.ietf.org/doc/html/rfc1155)
- **Defines the rules** for
- naming objects
- the length, size, range of object types
- how to encode
- The objects in SNMP form a hierarchical structure(parent/child). So the relationship of inheritance is also defined in SMI.
:::warning
SMI only defines the rules, it's a collection of general rules to name objects and to list their types.
:::
### Role of MIB
- **Management Information Base**
- [RFC 1156](https://datatracker.ietf.org/doc/html/rfc1156)
- **Defines the number of objects, name them** according to the rules defined by SMI.
- Associate a type to each named object.
> MIB creates a set of objects defined for each entity similar to metadata in a database(names and types without values).
> ```c=
> int a;
> char b[10];
> int c = 2 * a;
> ```

#### OID

Cisco 1.3.6.1.4.1.9 
Juniper 1.3.6.1.4.1.2636

## Some Details
- The SNMP server must support a PDU with an upper limit of 484 bytes or greater.
### PDUs in SNMP
- SNMPv3 defines eight types of protocol data units
1. GetRequest
2. GetNextRequest
3. GetBulkRequest
4. SetRequest
5. Response
6. Trap
7. InformRequest
8. Report
### Format for SNMP PDUs
### Messages
- SNMP does not send only a PDU, it **embeds the PDU in a message**.
- A message is a sequence made of four elements
- Version
- GlobalData
- SecurityParameters
- ScopePDU(encoded PDU)
### SNMPv3
- Added two new features to the previous version
- Security levels
- Manager can choose one or more levels of security when accessing an agent.
- Remote administration
- Allows remote configuration of security aspects.

## Reference
### Official
- [RFC 1157 - A Simple Network Management Protocol (SNMP)](https://datatracker.ietf.org/doc/html/rfc1157)
### Article
- [SNMP Community Strings Tutorial and Monitoring Tool](https://www.dnsstuff.com/snmp-community-string)