Demonstration: This code hangs on all arduino versions after some time...

EthernetPentest.ino

#include <SPI.h>
#include <Ethernet.h>

#define GREEN_PIN 41


byte mac[] = { ... };

// google.de
IPAddress server(173, 194, 69, 94);
int       port = 80;

EthernetClient client;
boolean        ethernetAvailable;
char           testCharacter = ' ';

void setup() {
  Ethernet.begin(mac);

  pinMode(GREEN_PIN, OUTPUT);
  digitalWrite(GREEN_PIN, HIGH);

  delay(1000);
}

void loop() {
  if (client.connected()) {
    if (client.available()) {
      char c         = client.read();
      testCharacter += c;
    } // no else
    
    digitalWrite(GREEN_PIN, LOW);
  } else {
    client.stop();

    digitalWrite(GREEN_PIN, HIGH);
    delay(1000);
  
    if (client.connect(server, port)) {
      client.println("GET / HTTP/1.0");
      client.println();
    } // no else
  }
}