tcp_reset: Modify RST packet responses to comply with RFC 793.

[wenquan2015]

Summary

According to RFC793, the flags of TCP reply packet should be as follows:

1. RST if flags of request packet has ACK
2. RST|ACK if flags of request packet has no ACK, at the same time, the sequence should be set to zero.

Impact

tcp reset packet

Testing

python test code：
from scapy.all import *

syn_pkt = IP(dst="10.0.1.2")/TCP(
    dport=800,
    flags="S",
    seq= 123456,
    ack= 654321
)

send(syn_pkt, verbose=1)

syn_ack_pkt = IP(dst="10.0.1.2")/TCP(
    dport=800,
    flags="SA",
    seq= 123456,
    ack= 654321
)
send(syn_ack_pkt, verbose=1)

test log:

10:34:17.027825 ARP, Request who-has 10.0.1.2 tell 10.0.1.1, length 28
10:34:17.034566 ARP, Reply 10.0.1.2 is-at 00:e0:de:ad:be:ef, length 28
10:34:17.040816 IP 10.0.1.1.20 > 10.0.1.2.800: Flags [S], seq 123456, win 8192, length 0
10:34:17.044677 IP 10.0.1.2.800 > 10.0.1.1.20: Flags [R.], seq 0, ack 1, win 0, length 0
10:34:17.060826 IP 10.0.1.1.20 > 10.0.1.2.800: Flags [S.], seq 123456, ack 654321, win 8192, length 0
10:34:17.064675 IP 10.0.1.2.800 > 10.0.1.1.20: Flags [R], seq 654321, win 0, length 0
10:34:25.263222 IP 10.0.1.2 > 119.29.29.29: ICMP echo request, id 12, seq 0, length 88
10:34:25.271558 IP 119.29.29.29 > 10.0.1.2: ICMP echo reply, id 12, seq 0, length 88