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One of the rules to prevent unexpected attack is to prevent SYN & FIN being used together.

SYN and FIN are both set

$IPT -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP

How do I use hping to test if this iptable rule works? hping3 192.168.7.0 --keep -S -F??? Is this complete?

When just type iptables -L, it shows:

Chain INPUT (policy DROP) target prot opt source destination
ACCEPT all -- anywhere anywhere
DROP tcp -- anywhere anywhere tcp flags:FIN,SYN/FIN,SYN

WHY do I need to put the "SYN,FIN" twice before drop? Is it because one for source and one for destination?

  • It's not a prevention unless your TCP/IP stack is lousy; merely a detection (only if you check counters/logs). – poige Feb 4 '11 at 5:06
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I use nmap:

# nmap --scanflags SYN,FIN HOSTNAME
# iptables -nv -L
Chain INPUT (policy ACCEPT 866K packets, 457M bytes)
 pkts bytes target     prot opt in     out     source               destination         
  120  5280 DROP       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp flags:0x03/0x03 

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