PerIpThrottle - per-source-IP flood defense

Layer: L4 Transport · Build flags: PC_ENABLE_PER_IP_THROTTLE

What this example teaches

The global accept throttle caps total accepts but cannot tell one noisy client from many legitimate ones. This per-IP throttle closes that gap: the accept callback rejects a new connection once a single source IPv4 has opened more than PC_PER_IP_THROTTLE_MAX connections within PC_PER_IP_THROTTLE_WINDOW_MS, so one abusive host is throttled without affecting everyone else.

Bounded memory, no heap. A fixed BSS table of PC_PER_IP_THROTTLE_SLOTS buckets tracks the busiest recent addresses (an LRU-ish set, not one slot per possible IP), so the defense itself stays deterministic.

Build-time only. Like the global throttle, there is no runtime API - the handler is plain; enabling the flag activates the defense in the accept path:

server.on("/", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.send(id, 200, "text/plain", "per-IP throttled"); });
server.begin(80); // per-IP throttle is active automatically when the flag is built in

Tuning + pairing. Set the knobs as build flags (cap, window, table size), and pair it with the global accept throttle for layered defense:

build_flags = -DPC_ENABLE_PER_IP_THROTTLE=1 -DPC_PER_IP_THROTTLE_MAX=10 \
              -DPC_PER_IP_THROTTLE_WINDOW_MS=10000 -DPC_PER_IP_THROTTLE_SLOTS=16

Build and run

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_PER_IP_THROTTLE=1" \
  --lib="." examples/L4-Transport/PerIpThrottle/PerIpThrottle.ino

From one host, open many rapid connections and watch that host get refused while another host still connects.

Annotated source

The complete sketch (PerIpThrottle.ino), reproduced verbatim with added explanatory comments:

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
#define PC_ENABLE_PER_IP_THROTTLE 1
 
#include "protocore.h"
#include "network_drivers/physical/physical.h"
 
static const char *SSID = "YOUR_SSID";
static const char *PASSWORD = "YOUR_PASSWORD";
 
PC server;
 
void setup()
{
    Serial.begin(115200);
    init_wifi_physical(SSID, PASSWORD);
    Serial.print("Connecting to WiFi");
    while (!wifi_ready())
    {
        delay(250);
        Serial.print('.');
    }
    uint32_t ip = pc_net_egress_ip(); // library egress IP (network byte order), no Arduino WiFi
    Serial.printf("IP: %u.%u.%u.%u\n", (unsigned)(ip & 0xFF), (unsigned)((ip >> 8) & 0xFF),
                  (unsigned)((ip >> 16) & 0xFF), (unsigned)((ip >> 24) & 0xFF));
 
    server.on("/", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.send(id, 200, "text/plain", "per-IP throttled"); });
    server.begin(80); // per-IP throttle is active automatically when the flag is built in
}
 
void loop()
{
    server.handle();
}