linux – Detect SYN flood attack in python

I have a piece of python code as follow

s = socket.socket()
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind((IP, PORT))
while True:
    revdata = ""
       c, addr = s.accept()
       t = threading.Thread(target=handletcp, args=(c,addr))
    except socket.error as exc:
        log("Error: " + str(exc))

Currently, my server, which handles 64 IPs, usually gets SYN flood attack. The python service has maximum ~200 TCP connections normally. I want to detect SYN flood attack and suspend the service within python code. Any idea of how to do that efficiently?

Thanks in advance!

c – Does syn flooding affect attacking machine?

I am a beginner in pen testing. I tried syn flooding port 80 of victim but intrestingly attacking machine froze few mins after the attack. I tried it twice and both the times attacking machine froze.Why did it happen? Also I want to know if this has something to do with ram(my attacking machine has 4gb ram)?

I used libnet to code the syn flooding program. Attacking machine is Kali linux(inside virtual box). I targeted my mobile on port 80

    int main()
      libnet_t *l;
      char errbuf(LIBNET_ERRBUF_SIZE),des_str(16);
      u_char packet()="HELLO";
      u_int32_t des_addr;
      int des_port,i=0;
      int bytes;
      printf("Enter destination IP and port : n");
        printf("Error intialising libnet : %sn",errbuf);
      printf("SYN FLOODING port %d",des_port);
          printf("Error building tcp : %sn",libnet_geterror(l));
          printf("Error building ip : %sn",libnet_geterror(l));
          printf("Error writing packet : %sn",libnet_geterror(l));
          printf("Packet %d : %d bytes writtenn",i++,bytes);

IP Table Rules for SYN Flood

IP Table Rules for SYN Flood | Web Hosting Talk

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  1. IP Table Rules for SYN Flood

    Can anyone share some good IP Table rules for SYN flood? Randomly, I am getting one, out of blue. So would like to stop it.

    OS: Debian.


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ddos – SYN Flood Test Platform

I am a new programmer, i’m learning. I made a SYN Flood program. I observed with Wireshark, i saw how many packets. But I don’t know how many megabytes of attack were. I don’t know how many packages are going to the target in a healthy way. Is there a test platform that you would recommend for SYN Flood(Online/Offline)? I don’t want to attack real sites. I just want to see the damage done by the program.

TCP self-regulating Synack timeout under Linux if the syn backlog exceeds the threshold. Why?

I am studying Dos attacks and especially the Syn flood.
I replicate the attack on local Debian VMs.

Attack VM:

Target VM:

  • Python TCP server on the port 8080, listen(Arrears = 13)
  • sysctl -w net.ipv4.tcp_syncookies=0 (deactivated)
  • sysctl -w net.ipv4.tcp_max_syn_backlog=128 (Default value)

To monitor the half-open connections now, I use:

watch -n 0.1 "ss -o state syn-recv sport = :8080"

I'm starting to flood the destination with lots of syn packages.

  • What I originally expected: up to 128 half-open connections (size of the syn backlog).
  • What I see instead: up to 13 half-open connections.

Question 1 / curiosity:
If the acceptance queue is empty, why should you add only 13 connections to the syn backlog? Should not be the variable tcp_max_syn_backlog, which decides the size of the syn backlog and the synchronous backlog listen Argument that decides the size of the acceptor?
(Verified by Wireshark, only 13 connections receive the Synack response)

On with my problem,

  • What I then expected: 13 half-open connections during the entire timeout (5 Synack retries with exponential timeout with a total timeout of approx. 60 seconds).
  • What I see instead: 6 connections (half of the backlog) go through the entire time limit of 61 seconds, the other 7 connections are interrupted after the second retry, with a total time limit of only about 2 seconds.

I tried different values ​​of the backlog and the connections that go through the whole process are always half, at least 4.
I have received this information from RFC4987 that talks about syn flood protection:

3.3. Reduction of the SYN-RECEIVED timer

Another defense that can be implemented quickly shortens the timeout
Period between receipt of a SYN and harvesting the created TCB due to lack
of progress. Decrease the timer that limits the lifespan of TCBs
in SYN-RECEIVED is also incorrect. While a shorter timer stops
Attempted connection attempts remain in the backlog for as long as possible
and make room for legitimate connections earlier, it can
Prevent some of the legitimate connections from being fully established
established. This is the only reason that this tactic is ineffective
requires the attacker to linearly increase the blocking frequency
proportional amount.
This timer reduction is sometimes implemented
in response to exceeding a threshold in the backlog or
a certain rate of SYN reception.

Now everything is clearer. However,
Question (s) 2

Why is the RFC the only source that talks about this feature I found? And is there a way to disable it? Is there any way to determine the threshold?

Finally I tried to sit down

  • Python TCP server on the port 8080, Listen(Arrears = 1000)
  • sysctl -w net.ipv4.tcp_syncookies=0 (deactivated)
  • sysctl -w net.ipv4.tcp_max_syn_backlog=13 (low value)

And I found that in this case, all 13 half-open connections go through the entire timeout without the reduction going beyond half. Why is there such a difference?


Firewalls – block SYN, ACK response with iptables

I have a virtual environment and use Kali 2020 to make a SYN flood attack on port 53 of an Ubuntu server.

I realized that a countermeasure for this attack is to limit or block the responses to the SYN packets that are SYN, ACK.

But how can I do that with iptables?

What else should be done to prevent such attacks?

Any help would be appreciated.

ddos – What's the difference between a SYN Flood attack and a SYN-ACK Flood attack?

In the syn-flood attack, the sender sends many syn packets of IP addresses to the destination that are not online or never respond to the syn-ack packets returned from the destination to the attacker, so that the destination TCP / The IP stack is so exhausted by only two open connections that can never be fully executed: the third way of the tree handshake will never happen.
In Reflection Attack (, the target responds to its own challenge (the reflection), leaving the attacker with a fully authenticated channel connection, without the attacker responding to the first challenge he sent responds goal.

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