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The rule in the previous line tests packets with any of the TCP flags and selects those packets with no flags set at all and DROPs them.
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Now you will look at some combinations of flags that you want to block with your iptables rules. Most of these are not actually attacks but rather more likely to be attempts by attackers to determine more information about the host with tools such as nmap.
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s You can see a fairly complete list of nmap scan forms at http://security.rbaumann.net/ Tip
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scans.php sel=1. Most other scanners use variations on this, and these rules should address most of these scan forms.
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For example, probably the best-known combination of illegal flags is SYN/FIN, which is used by a variety of network scanners to perform operating system detection. The SYN flag opens a connection, and the FIN flag closes a connection. In combination these flags make no sense in a single packet. Thus, any occurrence of this combination of flags will be malicious traffic, and you will start the TCP flag rules by blocking this traffic. But first I will start by adding a chain to hold the bad TCP flag rules. kitten# iptables -N BAD_FLAGS Then you place a rule toward the start of the bastion host rules to redirect all TCP traffic to the bad TCP flags rules to be processed. The traffic that does not match these rules and is not dropped will then proceed to be processed by the other rules. kitten# iptables -A INPUT -p tcp -j BAD_FLAGS Here you are putting all incoming TCP traffic through the BAD_FLAGS chain. As explained earlier, when traffic is redirected to a user chain by a rule, it will be processed against all the rules in the new chain and then return to the chain that redirected it to be processed by the next rule in sequence. Thus, all the TCP traffic will pass through the rules in the BAD_FLAGS user chain and then return to the INPUT chain. You can now add the first rules to handle bad flags. I have added a rule that logs and drops the SYN/FIN TCP flag combination, which you can see in Listing 2-38. Listing 2-38. Blocking SYN/FIN packets kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG --log-prefix "IPT: Bad SF Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP You start with a logging statement, which logs all packets with this combination of TCP flags to your log file. Unlike the ICMP traffic where you specified a single logging rule for the traffic, in this instance you will log each type of TCP flag combination with its own log prefix. This will aid you in determining from where particular types of attacks have originated. To further aid in this,
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you have added a log prefix that specifies exactly what sort of illegal packet you are seeing, with SF indicating SYN/FIN. Then after logging the packets, you have dropped them. Other variations on the SYN/FIN flag combination are used for similar purposes: SYN/RST, SYN/FIN/PSH, SYN/FIN/RST, and SYN/FIN/RST/PSH. Let s add some additional rules in Listing 2-39 to handle these variants. Listing 2-39. Rules for SYN/FIN Variations kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,RST SYN,RST -j LOG --log-prefix "IPT: Bad SR Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,RST SYN,RST -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,PSH SYN,FIN,PSH -j LOG --log-prefix "IPT: Bad SFP Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,PSH SYN,FIN,PSH -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST SYN,FIN,RST -j LOG --log-prefix "IPT: Bad SFR Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST SYN,FIN,RST -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST,PSH SYN,FIN,RST,PSH -j LOG --log-prefix "IPT: Bad SFRP Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags SYN,FIN,RST,PSH SYN,FIN,RST,PSH -j DROP Next in Listing 2-40 you add a rule to address single FIN flag packets. You will never find a packet that has only a FIN flag in normal TCP/IP connections; thus, any you do find are generally being used for port scans and network probing. Listing 2-40. Rules for FIN-Only Flag Packets kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags FIN FIN -j LOG --log-prefix "IPT: Bad F Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags FIN FIN -j DROP These rules in Listing 2-40 select only those packets with a FIN flag, and only those packets with a FIN flag set then log and drop them. Lastly you want to block so-called null packets, which have all flags present and set, and any other related Xmas-style scanning packets. These are generally used for other forms of network probing used by scanning tools such as nmap. Listing 2-41 shows how you can block these using the ALL and NONE special flag selectors. Listing 2-41. Rules for Null and Xmas Flag Packets kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags ALL NONE -j LOG --log-prefix "IPT: Null Flag " kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags ALL NONE -j DROP kitten# iptables -A BAD_FLAGS -p tcp --tcp-flags ALL ALL -j LOG --log-prefix "IPT: All Flags "
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