SNMP_Agent

(Previously Arduino_SNMP)

SNMP Agent built with Arduino

This is a fully-compliant SNMPv2c Agent built for Arduino's, but will work on any OS, providing API code is written for packet serialization (See tests/mock.cpp for an example)

Current Version: 3.1.5

Features

• Full SNMPv2c Data Type support:
  • INTEGER int
  • STRING char[] / const char* (C-style strings, no std::string or Arduino String)
  • NULLTYPE
  • OIDTYPE const char* (dotted-decimal, e.g. ".1.3.6.1.4.1.5.0")
• Complex data type support:
  • NETWORK ADDRESS
  • COUNTER32 uint32_t
  • GAUGE32 uint32_t
  • TIMESTAMP uint32_t
  • OPAQUE uint8_t*
  • CONTER64 uint64_t
• SNMP PDU Support
  • GetRequest
  • GetNextRequest
  • GetResponse (For SNMPv2c INFORM Responses only for now)
  • SetRequest
  • SNMPv2 Trap
  • GetBulkRequest
  • InformRequest
  • SNMPv2 Trap

It was designed and tested around an ESP32, but will work with any Arduino-based devied that has a UDP object available. Optimized for ESP-01 (ESP8266) and other memory-constrained embedded targets.

The example goes into detail around how to use, or look at src/SNMP_Agent.h for the API.

If you're coming from v1, most, but not all APIs are drop-in replaceable. Some of the API's, especially around strings have changed. Look in SNMP_Agent.h for details.

If you're upgrading from any prior release (including the original Arduino_SNMP v2.1.x, or this fork's v2.2.0 / v3.0.0 / v3.1.0 / v3.1.1 / v3.1.2 / v3.1.3 / v3.1.4), read "What's New in v3.1.5" immediately below. v3.1.5 is a single cumulative release that folds: (1) the v2.2.0 C-style string-model refactor, (2) the v3.0.0 critical BER TLV bug fixes (PR #60), (3) the 4-phase zero-heap deterministic-memory refactor, (4) v3.1.1 sketch-overridable tuning + SNMP_Sensor bug fixes, (5) v3.1.2 snmpTrapOID.0 RFC-3416 fix, (6) v3.1.3 ESP8266 auto-tune profile, (7) v3.1.4 startup-heap ASNPool + narrowed SortableOID + universal OCTET=256 defaults, and (8) v3.1.5 arduino-lint LD003 extras/demos compliance.

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What's New in v3.1.5 (Cumulative: ALL changes since v2.1.0)

v3.1.5 is a single release combining every earlier in-tree milestone category plus the latest patch-level housekeeping fixes. If you are upgrading from the historical Arduino_SNMP v2.1.x (or from any prior v2.2.0 / v3.0.0 / v3.1.x fork release) you are getting everything at once in this tag. Eight broad buckets of change rolled into v3.1.5:

1. v2.2.0 (embedded string model): all std::string replaced with fixed C-style char[] / const char*. No heap fragmentation from string reallocs; 3–8 KB Flash saved on ESP8266.
2. v3.0.0 (BER TLV hardening, upstream PR #60): 3 critical real on-the-wire BER bugs fixed (long-form header off-by-one, length == 256 silently encoded as 0 which broke snmpbulkwalk, and a double-store undefined-behavior sign-extend). 101/101 tests green.
3. Zero-Heap Deterministic-Memory Refactor (4-phase): hot-path packet processing (agent.loop(), GET/GETNEXT/GETBULK/SET decode + build, TRAP/INFORM send) now performs zero malloc/new/calloc/realloc. All ASN.1 BER objects come from a compile-time-sized global placement pool; all VarBind/PDU/agent callback lists use fixed C-arrays with explicit constexpr capacity caps. No mid-packet heap-fragmentation panics after 30+ days of polling.
4. v3.1.1 (sketch-overridable tuning + SNMP_Sensor bug fixes): every size/pool/buffer constant wrapped with #ifndef … #endif so sketch-side defines or build-flags win, no patching library sources required; SNMP_Sensor char* → const char* OID const-correctness; critical SNMP_Sensor addReadWriteStringHandler(&sysContact, 25, true) hardcoded 25-byte SET cap fixed → sizeof(sysContactValue).
5. v3.1.2 (RFC-3416 snmpTrapOID.0 fix, issue #64): SNMPv2 Trap/Inform VarBind #2 name sysObjectID.0 → correct snmpTrapOID.0 (.1.3.6.1.6.3.1.1.4.1.0) per RFC 3416 §3.1, so net-snmp snmptrapd can look up the NOTIFICATION-TYPE.
6. v3.1.3 (ESP8266 auto-tune profile): on ESP8266, unless SNMP_SKIP_ESP8266_AUTOTUNE=1 is set, automatically shrinks all pool/buffer constants (ASNPool 64→24, callbacks 64→24, VarBinds 16→6, OCTET max 500→256, packet 1400→1024, slot size 768→640, etc.) saving ~28 KB BSS vs v3.1.2 so WiFi + LittleFS + ArduinoJson + SNMPAgent fit on the 80 KB-DRAM D1 mini / ESP-01.
7. v3.1.4 (startup-heap ASNPool + narrowed SortableOID + universal OCTET 256): the single biggest static BSS sink — ASNPool slots[N] — moves out of .bss into a one-shot startup-time new Slot[N]() allocation done exactly once on the first asn_new<T>() call; never deallocated, never reallocated, never grows, count fixed at compile-time (opt-out back to static with SNMP_POOLS_IN_BSS 1). Effect: ESP8266 tiny gains a further ~15.5 KB BSS, ESP32 gains ~24.8 KB BSS free, and the ASNPool no longer occupies linker-reported global/static RAM on any target. Additionally: SortableOIDType::sortingMap narrows unsigned long[32] → uint32_t[32] (SMIv2 sub-IDs fit in 32 bits; saves 128 B/instantiation on 64-bit hosts + width matches encoder math); OCTET_TYPE_MAX_LENGTH universal default 500 → 256, still sketch-overridable.
8. v3.1.5 (arduino-lint compliance, patch-level housekeeping): (a) Rule LD003 fix: demos/ folder contained 3 .ino files → moved whole tree to extras/demos/ (Arduino library spec allows sketches only under examples/ or extras/). 3 .ino files are pure 100% renames; platformio.ini lib_extra_dirs updated ../../.. → ../../../.. (+1 nesting level). Fixes the arduino/arduino-lint-action@v1.0.0 CI failure: ERROR: Sketch(es) found outside examples and extras folders. (b) README absolute-path sanitization sweep: maintainer-local deep-home-folder file:///... references across README files stripped to GitHub-native repo-root-relative links. No code or API change; 100% wire and consumer compatible on top of v3.1.4.

User-visible API changes since v2.1.0 (only 2, both from v2.2.0)

Everything else — addXxxHandler / sortHandlers / sendTrapTo / setUDP / begin / stop / loop — is 100% source + wire compatible back to v2.2.0.

Symbol	Change since v2.1.0
GETSTRING_FUNC typedef	const std::string (*)() → const char* (*)()
OIDType::string() return type	const std::string& → const char* (zero-copy, returns a const char* into a fixed backing buffer)

String model change (v2.2.0 → present) — copy-paste migration

Static string handlers (no std::string anymore; use a static char[] literal):

// OLD, v2.1:
std::string sysDescr = "ESP32 SNMP Agent";
snmp.addReadOnlyStaticStringHandler(".1.3.6.1.2.1.1.1.0", sysDescr);

// NEW, v3.1:
char sysDescr[] = "ESP32 SNMP Agent";     // or const char* PROGMEM literal
snmp.addReadOnlyStaticStringHandler(".1.3.6.1.2.1.1.1.0", sysDescr);

Read-write string buffers (static storage, no malloc):

char _sysContactBuf[255];
char* sysContact = _sysContactBuf;
snprintf(sysContact, sizeof(_sysContactBuf), "admin@example.com");
snmp.addReadWriteStringHandler(".1.3.6.1.2.1.1.4.0", &sysContact, sizeof(_sysContactBuf), true);

Dynamic-string callbacks (GETSTRING_FUNC now returns const char*):

const char* getFirmwareVersion(void) { return LIBRARY_VERSION; }  // "3.1.5" from defs.h
snmp.addReadOnlyStringHandler(".1.3.6.1.4.1.99.0", getFirmwareVersion);

Critical BER TLV bug fixes (v3.0.0 → present)

These are real on-the-wire failures. Upgrade if you use snmpbulkwalk, responses ≥ 128 bytes, or SNMP Set with 3-byte signed integer payloads. All five are integrated from upstream PR #60 plus defensive boundary hardening grown out of the audit.

1. Hardcoded _length + 2 return bug (OIDType / Counter64 / ComplexType / BER_CONTAINER fromBuffer). BER length fields ≥ 128 bytes use long-form headers (3+ bytes instead of 2). Before: returned a hardcoded + 2 regardless, walked off-structure. After: returns the actual TLV header bytes consumed.
2. length == 256 encoded as 0 (catastrophic). encode_ber_length_integer used if(integer > 256) (off-by-one). Exactly 256-byte response PDUs serialized as 0x81 0x00 (= length 0 per ASN.1 BER), which net-snmp/pysnmp silently dropped. Fixed in both encode_ber_length_integer and its paired byte-counter encode_ber_length_integer_count.
3. Undefined behavior tempVal = tempVal |= 0xFF000000 in IntegerType 3-byte signed decode (double-store, -Wsequence-point error). Reduced to tempVal |= 0xFF000000;.
4. Stack buffer overread in test harness memcpy(&buffer[i], &randomLong, 10) → memcpy(…, sizeof(randomLong)) (2–6 bytes past stack end on 64-bit hosts).
5. Defensive overflow pre-checks added at BER_CONTAINER / OIDType / Counter64 / ComplexType fromBuffer entry. ComplexType::fromBuffer child-walk replaced buggy dual-condition loop with a descending remaining counter.

Deterministic zero-heap (4-phase) — what this means for your firmware

Before the zero-heap refactor, a single decoded SNMP PDU did ≥24 new/delete pairs (one per ASN.1 field, two per shared_ptr refcount block). Under sustained 1 Hz polling this fragmented the ESP-01 heap so badly that after ≈30 days, the next incoming 512-byte UDP packet could not be allocated contiguously → panic reboot.

v3.1.0 replaces every hot-path allocation, and v3.1.4 extends the model with optional startup-heap pool allocation, using one of two storage strategies:

• Compile-time-sized global placement pool for all BER_CONTAINER subclass objects (IntegerType, OctetType, OIDType, ComplexType, …). Generic default: SNMP_POOL_ASN_OBJECTS = 32 slots × SNMP_POOL_SLOT_SIZE = 768 B = ~24,576 B; ESP8266 tiny auto-profile drops that to 24 × 640 B = 15,360 B. v3.1.4 allocates the slot storage once at startup via new Slot[N]() (opt-out SNMP_POOLS_IN_BSS 1 returns the old static .bss layout). If all N slots are ever simultaneously occupied (pathological trap storm), the code gracefully falls back to a regular ::new T — defensive, never triggers in steady state (decode tree + serialise + free all return to pool before the next packet). ASAN is clean on both paths.
• Fixed C-arrays with explicit count member for every library list/queue: VarBinds per packet, OID handlers per agent, UDPs per agent, concurrent SNMPAgent instances, INFORM retry queue, callback list per SNMPTrap object, child-values inside ComplexType. Every such buffer has a compile-time constexpr maximum. Overflow returns a well-defined error code — no OOM panic.

14 Compile-Time Sizing Constants (tune before #include <SNMP_Agent.h>)

Declared in defs.h:

Constant	Default (non-ESP8266)	_SNMP_ESP8266_TINY auto (ESP8266, on by default)	Purpose
MAX_SNMP_PACKET_LENGTH	1400	1024	Incoming/outgoing UDP packet scratch buffer
OCTET_TYPE_MAX_LENGTH	256	256	OctetType / OpaqueType internal fixed buffer
SNMP_MAX_COMMUNITY_LEN	64	64	Community-string buffer
SNMP_MAX_OID_STR_LEN	256	192	Dotted-decimal OID storage (e.g. "1.3.6.1.4.1.…")
SNMP_MAX_STRING_LEN	= OCTET_TYPE_MAX_LENGTH	same	String column SET upper bound (alias)
SNMP_MAX_OID_SUBIDENTIFIERS	32	32	BER-encoded OID sub-ID count (realistic max ~17)
SNMP_MAX_COMPLEX_CHILDREN	16	8	Children per decoded PDU/VarBind-list ComplexType
SNMP_MAX_VARBINDS	16	6	VarBinds per request/response (snmpbulkwalk default = 10)
SNMP_MAX_CALLBACKS_PER_AGENT	64	24	Registered OID handlers per SNMPAgent instance
SNMP_MAX_AGENTS	2	2	Concurrent SNMPAgent instances (usually just 1)
SNMP_MAX_UDP_PER_AGENT	2	2	UDP transport interfaces per agent (WiFi + ETH fallback)
SNMP_MAX_TRAPS_INFLIGHT	8	4	INFORM retry queue + pending trap depth
SNMP_MAX_CALLBACKS_PER_TRAP	16	8	OID pointers embedded in a single SNMPTrap object
SNMP_POOL_ASN_OBJECTS	32	24	ASNPool slots — global BER_CONTAINER placement pool (decode + build + trap + clone)
SNMP_POOL_VARBIND_OBJECTS	12	8	VarBind placement pool (packet build/trap path)
SNMP_POOL_SLOT_SIZE	768	640	Byte payload size per ASNPool slot (must fit SortableOIDType largest subclass)

Two opt-out #define switches (place BEFORE #include <SNMP_Agent.h>):

• #define SNMP_SKIP_ESP8266_AUTOTUNE 1 — disable the ESP8266 _SNMP_ESP8266_TINY shrink profile on ESP8266, use generic defaults above.
• #define SNMP_POOLS_IN_BSS 1 — force the ASNPool back into static .bss arrays (v3.1.3 behavior). Without it (default, v3.1.4+ → still current in v3.1.5), ASNPool storage is allocated once at startup via new Slot[N]() so it does not count against linker-reported globals — ~20–50 KB more headroom for your code.

ESP-01 1 MB / 80 KB-DRAM tuning (headroom: ~50 KB globals FREE / 80 KB)

Since v3.1.4 (still current in v3.1.5), on ESP8266 the _SNMP_ESP8266_TINY profile is automatic (no user defines needed) and ASNPool storage lives by default in the startup heap, not BSS. So most small sketches compile + link at 38–41% globals, well under the 80 KB limit. Only re-tune if you need to serve >24 OIDs, >6 VarBinds/bulkwalk, or >4 in-flight traps.

Important ordering: put overrides BEFORE #include <SNMP_Agent.h> in your sketch.ino.

/* Optional: increase capacity on D1 mini / bigger ESP8266 modules. */
#define SNMP_MAX_CALLBACKS_PER_AGENT   64
#define SNMP_POOL_ASN_OBJECTS          32
#define SNMP_MAX_VARBINDS               16
#define SNMP_MAX_COMPLEX_CHILDREN       16
#include <SNMP_Agent.h>

→ Saves ≈ (default v3.1.2 pool of 64 × 768 B) − (tiny 24 × 640 B) = ~33,792 B BSS + pool-on-heap = an additional ~15,360 B / slot versus v3.1.2 on ESP8266, measured in real builds.

Release verification matrix (all green)

Check	Result
Host catch2 (native clang 14 / g++)	✅ 101/101 assertions in 10 test cases
AddressSanitizer (memory + leak)	✅ 0 errors / 0 leaks (ASNPool path + heap-fallback both clean)
Arduino-CLI esp8266:esp8266:d1_mini + examples/ESP32_SNMP	✅ Flash 261,340 B (24%), globals 30,668 / 80,192 B (38%) — 49,524 B FREE
Arduino-CLI esp8266:esp8266:d1_mini + examples/SNMP_Sensor	✅ Flash 296,392 B (28%), globals 33,128 / 80,192 B (41%) — 47,064 B FREE
Arduino-CLI esp32:esp32:esp32 + examples/ESP32_SNMP	✅ Flash 918,531 B (70%), globals 49,856 / 327,680 B (15%)
Arduino-CLI esp32:esp32:esp32 + examples/SNMP_Sensor	✅ Flash 964,843 B (73%), globals 50,984 / 327,680 B (15%)
Build flags across all 4 targets	✅ -Wall -Wextra -Werror clean
src/ standard-container header audit	✅ 0 <vector> / 0 <deque> / 0 <list> / 0 <functional>. Only 2 <memory> retained purely for backwards-compat public shared_ptr ctor signatures (OIDType::cloneOID + legacy VarBind ctors).

Net footprint vs v3.1.2 pre-optimization

Deterministic zero-hot-path heap unchanged (same deterministic pool sizing). Global BSS shrinks per build:

• ESP8266 d1_mini + ESP32_SNMP.ino: from 101% overflow (linker OOM) (v3.1.2) → 30,668 B / 80,192 B (38%) = −~49 KB globals.
• ESP32 DevKit + ESP32_SNMP.ino: from 74,680 B / 327,680 B (22%) (v3.1.3) → 49,856 B / 327,680 B (15%) = −24,824 B globals, almost exactly the ASNPool slots[32] moved out of .bss into the startup heap.
• Flash: ≈ ±0.1% vs v3.1.3 (essentially unchanged; the startup new Slot[] code path is a handful of instructions).

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2.2.0 Embedded Optimization: C-Style Strings Only

As of v2.2.0, the entire library uses fixed-size C-style strings (char[] + const char* + explicit length fields) exclusively. The C++ std::string type and Arduino String class have been completely removed from all library code, examples, and callback APIs.

Why this change

• Zero heap fragmentation — no dynamic malloc/new for string storage
• Smaller binary — eliminates <string> template instantiation bloat (~3–8 KB Flash on ESP8266)
• Deterministic memory — all buffers are compile-time sized, no surprise OOM at runtime
• Faster — no SBO/COW indirection; fixed memcpy/strcmp paths that the compiler can heavily optimize

Buffer Sizing

Fixed maximum sizes are declared in defs.h:

Constant	Default	ESP8266 tiny auto	Purpose
SNMP_MAX_COMMUNITY_LEN	64	64	Community string (RO/RW)
SNMP_MAX_OID_STR_LEN	256	192	OID dotted-decimal representation
OCTET_TYPE_MAX_LENGTH / SNMP_MAX_STRING_LEN	256	256	OctetString (OID value payload)

These are sensible defaults; tune them from your sketch with #define BEFORE #include <SNMP_Agent.h> if you need a smaller RAM footprint on the ESP-01.

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It you need a STRING OID that can be written to/updated, be very sure that you need to update it, because you will be dealing with raw pointers into fixed-size buffers. Always pass the true buffer size as the maxLength parameter of addReadWriteStringHandler to prevent overflow. It's safer to use addReadOnlyStaticStringHandler() whenever possible.

This library does not support the Arduino String class and does not use the C++ std::string class — all string handling uses C <cstring> primitives (strncpy, memcpy, strcmp, strlen, strchr, strtol) with explicit bounds checking.

Getting Started

To setup a simple SNMP Agent, include the required libraries and declare an instance of the SNMPAgent class;

#include <SNMP_Agent.h>

/* Can declare read-write, or both read-only and read-write community strings */
SNMPAgent snmp("public", "private");

Depending on what arduino you are using, you will have to setup the wifi/internet conection for the device. For ESP32, you can use WiFi.begin(), you will then need to supply a UDP object to the snmp library.

#include <WiFi.h>
#include <WiFiUdp.h>
WiFiUDP udp;

... later in setup()

WiFi.begin(ssid, password);

// Give snmp a pointer to the UDP object
snmp.setUDP(&udp);

// Add OID Handlers (see below)
...

snmp.begin();

... later in loop()
snmp.loop();

Setting OID callbacks

If you want the Arduino to response to an SNMP server at some specified OIDs, you need to implement a ValueCallback for each OID, attached to a variable to respond with. Whenever an OID is requested by sn SNMP manager, the ValueCallback for that OID is found, and the latest value of that variable is used to respond.

For example, to respones to the OID ".1.3.6.1.4.1.5.0" with the number: 5.

int testNumber = 5;
snmp.addIntegerHandler(".1.3.6.1.4.1.5.0", &testNumber);

Yopu can enable SNMPSet requests, by setting isSettable = true as a parameter when adding the handler, for example:

int settableNumber = 0;
snmp.addIntegerHandler(".1.3.6.1.4.1.5.1", &settableNumber, true);
// snmpset -v 2c -c private <IP> 1.3.6.1.4.1.5.1 i 24

You can store the return value of the handler calls in a variable ValueCallback*, and use them later for things like SNMP Traps, or for removing the handler later.

Be sure to call snmp.sortHandlers() after adding any OID handlers, to ensure functions like SNMP Walk work correctly.

The full list of ValueCallback handlers you can specify can be found in SNMP_Agent.h

SNMP Traps

You can send SNMP v1 traps, as well as SNMPv2 Trap and INFORMS with this library.

Therre are a few requirements in setting up a trap in order to comply with the SNMP RFC.

// Setup a trap object for later use, specify the SNMP version to use 
// SNMP_VERSION_1 or SNMP_VERSION_2C

SNMPTrap* testTrap = new SNMPTrap("public", SNMP_VERSION_2C);

// SNMP Traps MUST send a timestamp value when sent. This timestamp doesn't have to be valid, but we have to create one anyway. The timestamp value is stored as "tens of milliseconds"
TimestampCallback* timestampCallback;
int tensOfMillisCounter = 0;

In setup():

// The SNMP Trap spec requires an uptime counter to be sent along with the trap.
timestampCallback = (TimestampCallback*)snmp.addTimestampHandler(".1.3.6.1.2.1.1.3.0", &tensOfMillisCounter);

// Set UDP Object for trap to be sent on
testTrap->setUDP(&udp);

// OID of the trap (C-style string)
testTrap->setTrapOID(new OIDType(".1.3.6.1.2.1.33.2"));

// Specific Number of the trap
testTrap->setSpecificTrap(1);

// Set the uptime counter to use in the trap (required)
testTrap->setUptimeCallback(timestampCallback);

// Set some previously set OID Callbacks to send these values with the trap (optional)
testTrap->addOIDPointer(previouslySetValueCallback);

// Set our Source IP so the receiver knows where this is coming from
testTrap->setIP(WiFi.localIP());

// Set INFORM to be true or false (only works for SNMPV2 traps)
testTrap->setInform(true);

in loop()

// must be called as often as possible
snmp.loop();

// Update our timestamp value
tensOfMillisCounter = millis()/10;

// Send the trap to the specified IP address

IPAddress destinationIP = IPAddress(192, 168, 1, 243);

if(snmp.sendTrapTo(testTrap, destinationIP, true, 2, 5000) != INVALID_SNMP_REQUEST_ID){
    Serial.println("Sent SNMP Trap");
} else {
    Serial.println("Couldn't send SNMP Trap");
}

The snmp.sendTrapTo() values of true, 2, 5000 indicate that if this is an INFORM request, it will try to send the INFORM up to 2 times, with a Timeout of 5000 milliseconds before it gives up, if it receives no response from the other end. The snmp.loop() will keep trying to resend the trap until the timeout or retry limit is reached.

There is currencly no mechanism to know (with code) if an SNMP INFORM request has been responded to. I hope to work on this in the future.

SNMP Manager

I am working on adding the functionality to act as an SNMP Server or Manager. In the meantime, if you need to do this, look at the library here: https://github.com/shortbloke/Arduino_SNMP_Manager

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Version History

Version	Changes
3.1.5	Patch: arduino-lint LD003 compliance + README absolute-path (zero code/API/wire changes, on top of v3.1.4). (1) Arduino library spec Rule LD003 fix: the demos/ folder contained 3 .ino files (arduino_cli_esp32, arduino_cli_esp8266, platformio_minimal/src/main) that triggered arduino/arduino-lint-action@v1.0.0 CI error Sketch(es) found outside examples and extras folders. Relocated entire demos/ tree to extras/demos/ (Arduino spec allows sketches under extras/ or examples/ only). 3 .ino files pure renames (git 100% match); platformio.ini lib_extra_dirs bumped ../../.. → ../../../.. plus cd demos/platformio_minimal → cd extras/demos/platformio_minimal comment banner. (2) README local-path sanitization: maintainer-local absolute paths stripped to bare repo-root-relative links, which GitHub renders natively as correct jump-to-line links. Version bump 3.1.4 → 3.1.5 in library.properties and src/include/defs.h. Host Catch2 101/101 green. No functional, API, or on-the-wire changes: 100% consumer compatible.
3.1.4	Public milestone — v3.1.2 + v3.1.3 + v3.1.4 collapsed into one release (100% source & wire compatible, zero API breaks). Delivered on top of v3.1.1, folding every change from the zero-heap baseline. (A) v3.1.2 RFC-3416 trap fix: SNMPv2c Trap/Inform VarBind #2 name sysObjectID.0 → correct snmpTrapOID.0 (.1.3.6.1.6.3.1.1.4.1.0), new named constant SNMPv2_SNMPTRAP_OID_0 so net-snmp snmptrapd can look up NOTIFICATION-TYPE definitions (was logging "Cannot find TrapOID in TRAP2 PDU"). (B) v3.1.3 ESP8266 auto-tune + smaller generic defaults: on ESP8266, _SNMP_ESP8266_TINY profile activates automatically (opt-out SNMP_SKIP_ESP8266_AUTOTUNE 1) shrinking ASNPool/packet/VarBind/OCTET pools to safe small-sensor sizes; generic defaults also reduce ASNPool 64→32, VarBindPool 32→12; all constants now sketch-overridable via #ifndef…#endif; removes need for per-sketch shrink blocks. (C) v3.1.4 startup-heap ASNPool + narrower types: the single-biggest static BSS sink ASNPool slots[N] moves out of .bss into one-shot startup new Slot[N]() done exactly once on first asn_new<T>() (default; opt-out back to static .bss via SNMP_POOLS_IN_BSS 1); SortableOIDType::sortingMap unsigned long[32] → uint32_t[32] (saves 128 B/instantiation on 64-bit hosts; SMIv2 sub-IDs fit in 32 bits exactly); OCTET_TYPE_MAX_LENGTH universal default 500→256 (sketch-overridable); SNMP_Sensor.ino portability fixes (LittleFS header, ESP8266 FS_BEGIN() + os_random() rng, timestamp uint32_t, removal of the 25-byte sysContact sysName sysLocation SET length cap). Net measured footprint: ESP8266:d1_mini + SNMP_Sensor globals dropped from v3.1.2 101% OVERFLOW (linker OOM) → v3.1.4 33,128 / 80,192 B (41%) with 47,064 B FREE; ESP32 + ESP32_SNMP globals dropped 74,680→49,856 B = −24,824 B. Full 4-target Arduino CLI matrix (2 sketches × esp8266+esp32) links clean. Host Catch2 101/101 green; src/ header audit confirms zero <vector>/<deque>/<list>/<functional>.
3.1.1	Minor patch release — user tuning + example corrections. Four targeted, zero-API-breakage changes on top of v3.1.0. (1) defs.h user-overridable sizing: every tuneable size/pool/buffer constant wrapped with #ifndef … #endif (15 total: MAX_SNMP_PACKET_LENGTH, OCTET_TYPE_MAX_LENGTH, 3 string SNMP_MAX_*_LEN, 10 SNMP_MAX_*/SNMP_POOL_*, DEBUG). Sketch-side #define placed BEFORE #include <SNMP_Agent.h> or compiler -D flags now win over library defaults, no patch to headers needed. Large banner comment in defs.h documents the override order + ESP-01 clawback recipe. (2) Examples teach tuning: both ESP32_SNMP.ino and SNMP_Sensor.ino gain a top-of-sketch COMPILE-TIME TUNING banner showing commented-out 6-constant halved-ASNPool recipe (~24,576 B BSS saved). SNMP_Sensor banner additionally reminds ESP8266 users to swap LITTLEFS → LittleFS + install ESP8266LittleFS/ArduinoJson libraries. (3) SNMP_Sensor.ino const-correct OIDs: ~33 char* oidFoo = ".1.3.6.1…" variables → const char* oidFoo, matching the const string literals they bind. Eliminates deprecated -Wwrite-strings warnings on modern ESP32/ESP8266 toolchains. (4) CRITICAL SNMP_Sensor.ino SET length fix: addReadWriteStringHandler(&sysContact, 25, true) → sizeof(sysContactValue) (actual buffer size 255). Previously a 100-byte snmpset of sysContact/sysName/sysLocation was incorrectly rejected by the 25-byte artificial cap even though loadSNMPValues() used strlcpy(…, sizeof(buf)=255); now the three paths (SNMP SET cap, declared C buffer, flash load) use a single consistent 255-byte maximum. Verification: host catch2 101/101 green; examples/ESP32_SNMP compiles under Arduino-CLI esp32:esp32:esp32 with 0 warnings/errors. Net footprint delta vs v3.1.0: 0.
3.1.0	Public release — cumulative of ALL changes since v2.1.0 (string model v2.2.0 + BER v3.0.0 + zero-heap refactor). Delivered as one tested, backwards-compatible tag for upstream/public repos. 2 API signatures changed only (both from v2.2.0 string model, see above); addXxxHandler/sendTrapTo/begin/loop/setUDP all 100% unchanged since v2.2.0. (a) String model: all std::string removed library/examples/test-wide → fixed char[] + const char* + explicit length; GETSTRING_FUNC typedef → const char*(*)(); OIDType::string() → const char*; example sketch 3 mallocs → static buffers; 3 size constants: SNMP_MAX_COMMUNITY_LEN=64, SNMP_MAX_OID_STR_LEN=256, SNMP_MAX_STRING_LEN=500. (b) BER TLV critical fixes (PR #60 upstream + defensive): (1) 4× fromBuffer return _length+2 hardcode → actual consumed bytes (fixes long-form header ≥128 B). (2) encode_ber_length_integer* off-by-one >256 → >=256 (fixes length==256 → 0x8100 zero-length; broke snmpbulkwalk). (3) IntegerType 3-byte signed extend `tempVal = tempVal
2.2.0	(Internal precursor — absorbed into the cumulative v3.1.0 entry above for public release)
2.1.0	(Previous / original Arduino_SNMP) API cleanups, ESP-01 / ESP8266 support.
2.0.x	Rewrite from Arduino_SNMP v1. RFC-compliant SNMPv2c engine.
1.x	Original Arduino_SNMP project.

Pull requests/comments are welcome