如何通过Netfilter队列和Scapy更改TCP有效负载和长度

Question

using Netfilter Queue and scapy change the TCP payload, test client send a msg to test server:

iptables rule in test server: iptables -A OUTPUT -p TCP -d [test client ip] -j NFQUEUE --queue-num 1

after changing the TCP payload, if the length is same after changed, and just replace strings, all behave correctly.

but if the length is different, the program is abnormal:

if the payload length is shorter: client can receive the modified payload, but the socket can not close normally, the client send a RST connection to server

if the payload length is longer: client also can receive the modified payload, but the server repeatedly to send packet for several times, the socket can not close normally, server socket is 'CLOSING', and client socket is 'FIN_WAIT1' or 'TIME_WAIT' the system queue is 14 at last, normal is 6:

cat /proc/net/netfilter/nfnetlink_queue

1   7888     0 2  4016     0     0       14  1

from tcpdump display, there are many "tcp retransmission"

somebody help me? thanks, below is the test code:

from netfilterqueue import NetfilterQueue
from scapy.layers.inet import IP,TCP
from scapy.packet import Packet,Raw

def print_and_accept(pkt):
msg = IP(pkt.get_payload())
try:
    if msg.haslayer(TCP) and msg.haslayer(Raw):
        print msg[IP].show()
        _Data = 'for the hook test'
        msg[TCP].remove_payload()
        msg[TCP].add_payload(_Data)
        #msg[Raw].load = _Data
        msg[IP].len = len(msg)
        del msg[IP].chksum
        del msg[TCP].chksum
        msg = msg.__class__(str(msg))
        pkt.set_payload(str(msg))
        #new_msg = IP(pkt.get_payload())
        #print new_msg[TCP].payload
        #print new_msg[IP].show()
        print msg[IP].show()
        print 'End!!!'
        pkt.accept()
    else:
        pkt.accept()
except Exception, err:
    print err
    pkt.accept()

nfqueue = NetfilterQueue()
nfqueue.bind(1, print_and_accept)
try:
    nfqueue.run()
except KeyboardInterrupt:
    print('')

Answer 1

长度已更改，因此seq，ack num有所不同，它在关闭套接字时引起了异常，我仍然不知道如何解决此问题，但它不会影响有效负载传输