SNMP

Table of Contents

• Compatibility
• Configuration
• Usage
  • Agent
  • Manager
• Limitations
• Migrate to version 2.0.0
• Acknowledgements

Simple Network Management Protocol library to write agent or manager for Arduino boards.

The library supports:

• SNMP protocol version
  • v1
  • v2c
• SNMP messages
  • GETREQUEST
  • GETNEXTREQUEST
  • GETRESPONSE
  • SETREQUEST
  • TRAP
  • GETBULKREQUEST
  • INFORMREQUEST
  • SNMPV2TRAP
• SNMP objects
  • Boolean
  • Integer
  • OctetString
  • Null
  • ObjectIdentifier
  • Sequence
  • IPAddress
  • Counter32
  • Gauge32
  • TimeTicks
  • Opaque
  • Counter64
  • Float
  • OpaqueFloat

Compatibility

The library is developed and tested on several boards and architectures:

• Adafruit Huzzah32 and Olimex ESP32-PoE (ESP32)
• Arduino Uno and Arduino Mega 2560 (AVR)
• NUCLEO-F429ZI and NUCLEO-F767ZI (STM32)

Configuration

Some conditional defines are used to set options at compile time.

• SNMP_STREAM
  If set to 1, packets are parsed and built with stream functions, with low memory usage.
  If set to 0 or undefined, a buffer in RAM is allocated to read or write the entire packet, with high memory requirement.
  It must be set to 1 for Arduino Uno board. The default is 1.
• SNMP_VECTOR
  If set to 1, vectors are used.
  If set to 0 or undefined, fixed size arrays are used.
  As vectors are part of the STL, if you want to use them with Arduino Mega board the ArduinoSTL library is required.
  It must not be used with Arduino Uno board. The default is 0.
• SNMP_CAPACITY
  If arrays are used, this symbol defines the maximum number of items contained in a Sequence object.
  The default is 6.

A convenient way to configure the library is to use an optional SNMPcfg.h file at sketch level. The library will include it automatically and apply the configuration. This is an example of such a file.

#ifndef SNMPCFG_H_
#define SNMPCFG_H_
 
#define SNMP_STREAM 0 // Buffers are used.
 
#define SNMP_VECTOR 1 // Vectors are used.
 
#define SNMP_CAPACITY 6 // Ignored, as vectors are used.
 
#endif /* SNMPCFG_H_ */

Usage

There is only one header to include to use the library.

#include <SNMP.h>

Agent

An SNMP agent receives request from and sends response to an SNMP manager.

Declare an SNMP agent like this. An UDP client is also needed.

EthernetUDP udp; // STM32, use your own platform UDP class
SNMP::Agent snmp;

In Arduino setup() function, add code to setup agent.

void setup() {
    Serial.begin(115200);
    // Ethernet
    Ethernet.begin(IPAddress(192, 168, 2, 2), IPAddress(255, 255, 255, 0),
            IPAddress(192, 168, 2, 1), IPAddress(192, 168, 2, 1));
    // SNMP
    snmp.begin(udp);
    snmp.onMessage(onMessage);
}

The user function onMessage() will be called every time an SNMP message is received.

void onMessage(const SNMP::Message *message, const IPAddress remote, const uint16_t port) {
    // Get the variable binding list from the message.
    VarBindList *varbindlist = message->getVarBindList();
    for (unsigned int index = 0; index < varbindlist->count(); ++index) {
        // Each variable binding is a sequence of 2 objects:
        // - First one is and ObjectIdentifierBER. It holds the OID
        // - Second is the value of any type
        VarBind *varbind = (*varbindlist)[index];
        // There is a convenient function to get the OID as a const char*
        const char *name = varbind->getName();
        if (strcmp(SYSNAME_OID, name) == 0) {
            // System name is requested. We have to send a response.
            // Create an SNMP message for response
            SNMP::Message *response = new SNMP::Message(SNMP::Version::V2C, "public", SNMP::Type::GetResponse);
            // The response must have the same request-id as the request
            response->setRequestID(message->getRequestID());
            // SYSNAME
            // Create an OctetStringBER to hold the variable binding value
            OctetStringBER* value = new OctetStringBER(SYSNAME_VALUE);
            // Add the variable binding to the message
            response->add(SYSNAME_OID, value);
            // Send the response to remote IP and port
            snmp.send(response, remote, port);
            // Avoid memory leak
            delete response;
       }
    }
}

In Arduino loop() function, the agent loop() function must be called frequently.

void loop() {
    // Agent loop function must be called to process incoming messages
    snmp.loop();
}

Agent.ino is a complete example of an SNMP agent implementation.

Manager

An SNMP manager sends request to and receives response from an SNMP agent.

Declare an SNMP manager like this. An UDP client is also needed.

EthernetUDP udp; // STM32, use your own platform UDP class
SNMP::Manager snmp;

In Arduino setup() function, add code to setup manager.

void setup() {
    Serial.begin(115200);
    // Ethernet
    Ethernet.begin(IPAddress(192, 168, 2, 2), IPAddress(255, 255, 255, 0),
            IPAddress(192, 168, 2, 1), IPAddress(192, 168, 2, 1));
    // SNMP
    snmp.begin(udp);
    snmp.onMessage(onMessage);
}

The user function onMessage() will be called every time an SNMP message is received.

In Arduino loop() function, the manager loop() function must be called frequently.

void loop() {
    // Manager loop function must be called to process incoming messages
    snmp.loop();
}

Manager.ino is a complete example of an SNMP manager implementation.

MPOD.ino is another example of an SNMP manager implementation with use of SETREQUEST.

Advanced.ino is the more complete example. It shows how to write an SNMP agent able to handle GETREQUEST, GETNEXTREQUEST, GETBULKREQUEST and SETREQUEST and to generate TRAP, INFORMREQUEST and SNMPV2TRAP.

It uses PROGMEM to save enough RAM to allow the sketch to be run even on a basic Arduino Uno board.

Limitations

Limitations depend on library configuration and available RAM.

If STL vectors are used, there is virtually no limit to the size of messages and the number of items in Sequence objects.

The values ​​of OctetString and ObjectIdentifier objects are now dynamically allocated. The size is limited only by the available memory.

To achieve compatibility with low-end boards like Arduino Uno, the code has been stripped down and almost no checking is done.

For example, when creating an OpaqueBER, the library assumes that the embedded BER parameter passed to the constructor is not null. If null, code will crash as soon as embedded BER members or functions are accessed.

Migrate to version 2.0.0

Minor changes are required when migrating existing code to version 2.0.0 of the library.

This is mainly due to the change from C-like enum to C++ syntax.

• All version constants like VERSION2C are now defined in struct Version.
  Replace by Version::V2C.
• All error constants like NO_ERROR are now defined in struct Error.
  Replace by Error::NoError.
• All trap constants like COLDSTART are now defined in struct Trap.
  Replace by Trap::ColdStart.
• All type constants like TYPE_GETRESPONSE are now defined in class Type.
  Replace by Type::GetResponse.
• Class PORT is replaced by struct Port. Port constant TRAP is renamed as Trap.
  Replace PORT::SNMP by Port::SNMP and PORT::TRAP by Port::Trap.

// Version 1.4.1
SNMP::Message *response = new SNMP::Message(SNMP::VERSION2C, "public", SNMP::TYPE_GETRESPONSE);
 
// Version 2.0.0
SNMP::Message *response = new SNMP::Message(SNMP::Version::V2C, "public", SNMP::Type::GetResponse);

The API changed for SNMP::begin(). The UDP parameter is now a reference and no more a pointer.

// Version 1.4.1
snmp.begin(&udp);
 
// Version 2.0.0
snmp.begin(udp);

Acknowledgements

This library is inspired by SNMP Manager For ESP8266/ESP32/Arduino (and more) authored by Martin Rowan.