如何通过Scapy发送FIN数据包关闭连接？

Question

I need to send a FIN packet to close a connection having IP and ports information (plus other informations from previus packets).

I've seen that is possbile to craft a new connection through a 3-way-handshake (see 3 way handshake in Scapy ), but nothing on connection closure.

Answer 1

As stated by the RFC you need to send a FIN segment then wait for the endpoint's acknowledgment (ACK) + FIN segment and then send a last ACK segment for it.

Here is a simple example using Scapy:

from scapy.all import *

conf.L3socket=L3RawSocket

sport=10000
dport=45000
pkt=IP(src="1.2.3.4", dst="4.5.6.7")

SYN=pkt/TCP(sport=sport, dport=dport, flags="S")
SYNACK=sr1(SYN)
ACK=pkt/TCP(sport=sport, dport=dport, flags="A", seq=SYNACK.ack, ack=SYNACK.seq + 1)
send(ACK)

# ...

FIN=pkt/TCP(sport=sport, dport=dport, flags="FA", seq=SYNACK.ack, ack=SYNACK.seq + 1)
FINACK=sr1(FIN)
LASTACK=pkt/TCP(sport=sport, dport=dport, flags="A", seq=FINACK.ack, ack=FINACK.seq + 1)
send(LASTACK)