nahamcon "Contemporaneous Open" solution

server.py

# sudo ufw disable
# sudo iptables -A INPUT -p tcp --destination-port 80 -j DROP
import random
from queue import Queue

recvq = Queue()
saddr = "10.138.0.11"
sport = 80
caddr = None
cport = None

def my_send(pkt):
    print("SEND", repr(pkt))
    send(pkt)

def recv_callback(pkt):
    if IP not in pkt:
        return
    if pkt[IP].dst != saddr:
        return
    if TCP not in pkt:
        return
    if pkt[TCP].dport != sport:
        return
    print("RECV", repr(pkt))
    recvq.put(pkt)

def my_recv():
    while 1:
        pkt = recvq.get()
        if caddr is not None and pkt[IP].src != caddr:
            continue
        if cport is not None and pkt[TCP].sport != cport:
            continue
        return pkt

sniffer = AsyncSniffer(prn=recv_callback, lfilter=lambda pkt: TCP in pkt)
sniffer.start()

def wrap(x):
    return x & 0xffffffff

try:
    while 1:
        # wait for SYN
        p = my_recv()
        if p[TCP].flags == "S":
            break

    caddr = p[IP].src
    cport = p[TCP].sport
    cseq = wrap(p[TCP].seq + 1)
    sseq = random.getrandbits(32)

    # send SYN
    my_send(IP(src=saddr, dst=caddr, proto="tcp") /
        TCP(sport=sport, dport=cport, flags="S", seq=sseq))
    sseq = wrap(sseq + 1)

    # send ACK
    my_send(IP(src=saddr, dst=caddr, proto="tcp") /
        TCP(sport=sport, dport=cport, flags="A", seq=sseq, ack=cseq))

    # read request
    data = bytearray()
    while 1:
        p = my_recv()
        print(repr(p))
        if "S" in p[TCP].flags:
            # ignore further SYNs
            continue
        cseq = p[TCP].seq + len(p[TCP].payload)
        my_send(IP(src=saddr, dst=caddr, proto="tcp") /
            TCP(sport=sport, dport=cport, flags="A", seq=sseq, ack=cseq))
        if p[TCP].payload:
            data += p[TCP].payload.load
            if data.endswith(b"\r\n\r\n"):
                break

    # reply
    msg = b"HTTP/1.1 200 OK\r\nContent-Type: text/html\r\nContent-Length: 6\r\nConnection: close\r\n\r\nHello!"
    my_send(IP(src=saddr, dst=caddr, proto="tcp") /
        TCP(sport=sport, dport=cport, flags="FPA", seq=sseq, ack=cseq) / 
        msg)
    sseq = wrap(sseq + len(msg))

    # wait for client to FIN
    while 1:
        p = my_recv()
        print(repr(p))
        if "F" in p[TCP].flags:
            break

    # ack FIN to complete TCP connection
    my_send(IP(src=saddr, dst=caddr, proto="tcp") /
        TCP(sport=sport, dport=cport, flags="A", seq=sseq, ack=cseq))

finally:
    sniffer.stop()